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4-1
Genetics and Cellular Function
• DNA and RNA – the nucleic acids
• Genes and their action
• DNA replication and the cell cycle
• Chromosomes and heredity
4-2
Modern Genetics
• Mendelian genetics
– Gregor Mendel
– investigates family patterns of inheritance
• Cytogenetics
– uses techniques of cytology and microscopy to study
chromosomes and their relationships to hereditary traits
• Molecular genetics
– uses biochemistry to study structure and function of DNA
• Genomic medicine
– treatment of genetic diseases through an understanding
of the human genome
4-3
DNA Molecular Structure
• DNA – deoxyribonucleic acid - a
long threadlike molecule with
uniform diameter, but varied length
– 46 DNA molecules in the nucleus of
most human cells
• total length of 2 meters
• average DNA molecule 2 inches long
• DNA and other nucleic acids are
polymers of nucleotides
• Each nucleotide consists of
– one sugar - deoxyribose
– one phosphate group
– one nitrogenous base
• Either pyrimidine (single carbon-nitrogen
ring) or purine (double ring) Figure 4.1a
HC
N C
N
NH2
N
H
C
C
CH
N
H
CH2OHO
O
OH
P
H
HOH
HH
O
Adenine
Phosphate Deoxyribose
(a)
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4-4
Nitrogenous Bases of DNA
• Purines - double ring
– Adenine (A)
– Guanine (G)
• Pyrimidines - single
ring
– Cytosine (C)
– Thymine (T)
• DNA bases - ATCG
Figure 4.1b
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
C
NH2 N
NH
C
CH
C
H
N
N C
Adenine (A)
Purines
C
O N
NH
C
CH
C N
HN C
NH2
Guanine (G)
H
C
NH2
C
N
H
C
HC N
O
Cytosine (C)
Uracil (U)
C
C
O
C
O
CH
HN CH
N
H
N
H
C
C
HC
CH3
NH
O
O
Thymine (T)
Pyrimidines
(b)
4-5
DNA Structure
• Molecular shape is a
double helix (resembles a
spiral staircase)
– each sidepiece is a backbone
composed of phosphate
groups alternating with the
sugar deoxyribose.
– steplike connections between
the backbones are pairs of
nitrogen bases
Figure 4.2
(a)
(b)
(c)
A T
A T
T A
A T
AT
G C
G
G
C
C
G C
Sugar–phosphate
backbone
G
A
C
T
T
G
C
Hydrogen
bond
A
Sugar–phosphate
backbone
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4-6
Complementary Base Pairing
• Nitrogenous bases united by
hydrogen bonds
– a purine on one backbone with a
pyrimidine on the other
– A – T two hydrogen bonds
– C – G three hydrogen bonds
• DNA base pairing
– A – T
– C – G
• Law of Complementary Base
Pairing
– one strand determines base
sequence of other Figure 4.2 partial
(b)
(c)
GC
Sugar–phosphate
backbone
Sugar–phosphate
backbone
G
A
C
T
A
T
A
T
G
C
Hydrogen
bond
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4-7
Discovery of the Double Helix
• By 1900: components of DNA were known
– sugar, phosphate and bases
• By 1953: x ray diffraction determined geometry of
DNA molecule
• Nobel Prize awarded in 1962 to 3 men: Watson,
Crick and Wilkins, but not to Rosalind Franklin,
who died of cancer at 37, after discovering the
x-ray data that provided the answers to the double
helix.
4-8
DNA Function
• Genes – genetic instructions for synthesis of
proteins
• Gene – segment of DNA that codes for a
specific protein
• Genome - all the genes of one person
– humans have estimated 25,000 to 35,000 genes
• 2% of total DNA
• other 98% is noncoding DNA
– plays role in chromosome structure
– regulation of gene activity
– no function at all – “junk” DNA
4-9
Chromatin and Chromosomes
• chromatin – fine filamentous DNA material
complexed with proteins
– occurs as 46 long filaments called
chromosomes
– 6 ft in first half of cell cycle
– twice that amount in preparation for cell division
– in nondividing cells, chromatin is so slender it
cannot be seen with light microscope
– granular appearance under electron microscope
– histones – disc-shaped cluster of eight proteins
– DNA molecule winds around the cluster
– repeats pattern 800,000 times
– appears to be divided into segments -
nucleosomes
– nucleosome consists of :
• core particle – histones with DNA around
them
• linker DNA – short segment of DNA
connecting core particles
– 1/3 shorter than DNA alone
Figure 4.4b
2 nm
11 nm
Nucleosome
Linker DNA
300 nm
30 nm
700 nm
700 nm
Core particle
In dividing cells only
Chromatids Centromere
1
2
3
4
5
6
30 nm fiber is
thrown into
irregular loops
to form a fiber
300 nm thick
Nucleosomes
fold accordion-
like into zigzag
fiber 30 nm in
diameter
DNA winds
around core
particles to form
nucleosomes
11 nm in
diameter
DNA double
helix
In dividing
cells, looped
chromatin coils
further into a
700 nm fiber to
form each
chromatid
Chromosome
at the midpoint
(metaphase) of
cell division
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4-10
Chromatin and Chromosomes
• chromatin – protein complex thrown into
complex, irregular loops and coils
– 1000X shorter than original molecule
• each chromosome is packed into its own
spheroidal region of the nucleus –
chromosome territory
– permeated with channels that allow regulatory
chemicals to have access to the genes
• in nondividing cell, the chromatin is not a
static structure
– changes moment to moment according to
genetic activity of cell
– genes get turned off and on
– chromosomes migrate as cells develop moving
active genes on different chromosomes closer
together
• allows genes to partner to bring about
developmental tasks in the cell Figure 4.4b
2 nm
11 nm
Nucleosome
Linker DNA
300 nm
30 nm
700 nm
700 nm
Core particle
In dividing cells only
Chromatids Centromere
1
2
3
4
5
6
30 nm fiber is
thrown into
irregular loops
to form a fiber
300 nm thick
Nucleosomes
fold accordion-
like into zigzag
fiber 30 nm in
diameter
DNA winds
around core
particles to form
nucleosomes
11 nm in
diameter
DNA double
helix
In dividing
cells, looped
chromatin coils
further into a
700 nm fiber to
form each
chromatid
Chromosome
at the midpoint
(metaphase) of
cell division
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4-11
Cells Preparing to Divide
• exact copies are made of all the
nuclear DNA
• each chromosome consists of two
parallel filaments of identical DNA -
sister chromatids
• in prophase, final coiling and
condensing
– now visible with light microscope
• final compaction enables the two
sister chromatids to be pulled apart
and carried to separate daughter
cells without damage to the DNA
– joined at centromere
– kinetochore – protein plaques on
either side of the centromere
Figure 4.5a(a)
Centromere
Kinetochore
Sister
chromatids
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4-12
RNA: Structure and Function
• RNA much smaller cousin of DNA (fewer bases)
– messenger RNA (mRNA) over 10,000 bases
– ribosomal RNA (rRNA)
– transfer RNA (tRNA) 70 - 90 bases
– DNA averages 100 million base pairs
• one nucleotide chain (not a double helix as DNA)
• ribose replaces deoxyribose as the sugar
• uracil replaces thymine as a nitrogenous base
• Essential function
– interprets code in DNA
– uses those instructions for protein synthesis
– leaves nucleus and functions in cytoplasm
4-13
What is a Gene?
• Previous definition - gene - a segment of DNA that carries
the code for a particular protein???
– Body has millions of proteins but only 35,000 genes?
– Small % of genes produce only RNA molecules
– Some segments of DNA belong to 2 different genes
• Current Definition – gene - an information-containing
segment of DNA that codes for the production of a molecule
of RNA that plays a role in synthesizing one or more proteins
• Amino acid sequence of a protein is determined by the
nucleotide sequence in the DNA
4-14
Human Genome
• Genome – all the DNA in one 23-chromosome set
– 3.1 billion nucleotide pairs in human genome
• 46 human chromosomes comes in two sets of 23
chromosomes
– one set of 23 chromosomes came form each parent
– each pair of chromosomes has same genes but different versions
(alleles) exist
• Human Genome Project (1990-2003) identified the
nitrogenous base sequences of 99% of the human genome
– genomics – the comprehensive study of the whole genome and
how its genes and noncoding DNA interact to affect the structure
and function of the whole organism.
4-15
Human Genome
• Findings of Human Genome Project
– Homo sapiens has only about 25,000 to 35,000 genes
• not the 100,000 formerly believed
– genes generate millions of different proteins
• not the old one gene one protein theory
• single gene can code for many different proteins
– genes average about 3,000 bases long
• range up to 2.4 million bases
– all humans are at least 99.99% genetically identical
• 0.01% variations that we can differ from one another in more than 3 million base
pairs
• various combinations of these single-nucleotide polymorphisms account
for all human variation
– some chromosomes are gene-rich and some gene-poor
– we now know the locations of more than 1,400 disease-producing
mutations
• opens the door for a new branch of medical diagnosis and treatment called
Genomic Medicine
• before HGP, we new locations of fewer than 100
4-16
Genetic Code
• body can make millions of different proteins, all from the
same 20 amino acids, and encoded by genes made of just
4 nucleotides (A,T,C,G)
• Genetic code – a system that enables these 4 nucleotides
to code for the amino acid sequence of all proteins
• minimum code to symbolize 20 amino acids is 3 nucleotides
per amino acid
• Base triplet – a sequence of 3 DNA nucleotides that stands
for one amino acid
– codon - the 3 base sequence in mRNA
– 64 possible codons available to represent the 20 amino acids
• 61 code for amino acids
• Stop Codons – UAG, UGA, and UAA – signal the ‘end of the message’,
like a period at the end of a sentence
• Start Codon – AUG codes for methionine , and begins the amino acid
sequence of the protein
4-17
Overview of Protein Synthesis
• all body cells, except sex cells and some immune cells,
contain identical genes.
• different genes are activated in different cells
• any given cell uses 1/3 to 2/3rds of its genes
– rest remain dormant and may be functional in other types of cells
• activated gene
– messenger RNA (mRNA) – a mirror-image copy of the gene is made
• migrates from the nucleus to cytoplasm
• its code is read by the ribosomes
– ribosomes – cytoplasmic granules composed of ribosomal RNA
(rRNA) and enzymes
– transfer RNA (tRNA) – delivers amino acids to the ribosome
– ribosomes assemble amino acids in the order directed by the codons
of mRNA
4-18
Summary of Protein Synthesis
• process of protein synthesis
– DNA mRNA protein
• transcription – step from DNA to mRNA
– occurs in the nucleus where DNA is located
• translation – step from mRNA to protein
– most occurs in cytoplasm
– 15-20% of proteins are synthesized in the nucleus
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4-21
Transcription
• DNA too large to leave nucleus and participate directly in cytoplasmic
protein synthesis
– necessary to make a small mRNA copy that can migrate through a nuclear
pore into the cytoplasm
• Transcription – copying genetic instructions from DNA to RNA
• RNA Polymerase – enzyme that binds to the DNA and assembles the
mRNA
– base sequences TATATA or TATAAA inform the polymerase where to begin
– RNA polymerase opens up the DNA helix about 17 base pairs at a time
– reads base from one strand of DNA
– makes corresponding mRNA
• where it finds C on the DNA, it adds G to the mRNA
• where it finds A on the DNA, it adds U to the mRNA
– RNA polymerase rewinds the DNA helix behind it
– gene can be transcribed by several polymerase molecules at once
– terminator – base sequence at the end of a gene which signals polymerase
to stop
4-22
Transcription
• pre-mRNA – immature RNA produced by transcription
• exons – “sense” portions of the immature RNA
– will be translated to protein
• introns – “nonsense” portions of the immature RNA
– must be removed before translation
• alternative splicing – removing the introns by enzymes and splicing
the exons together into a functional RNA molecule
– one gene can code for more than one protein
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4-24
Alternative Splicing of mRNA
• One gene can code for more than one protein
• Exons can be spliced together into a variety of
different mRNAs.
Figure 4.6
Gene (DNA)
Pre-mRNA Intron Exon
mRNA 2mRNA 1 mRNA 3
Protein 2Protein 1 Protein 3
Transcription1
Translation3
Splicing2
A B C D E F
A C D A E FB D E
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4-25
Translation
• translation – the process that converts the language of
nucleotides into the language of amino acids
• ribosomes - translate sequence of nucleotides into the
sequence of amino acids
– occur mainly in cytosol, on surface of rough ER, and
nuclear envelope
– consists of two granular subunits, large and small
• each made of several rRNA and enzyme molecules
4-26
Translation of mRNA
• mRNA molecule begins with leader sequence
– acts as binding site for small ribosomal subunit
– large subunit attaches to small subunit
– ribosome pulls mRNA molecule through it like a ribbon, reading
the bases as it goes
– when start codon (AUG) is reached, protein synthesis begins
– all proteins begin with methionine when first synthesized
Figure 4.7
Free amino acids
Cytosol
Free tRNA
NucleusDNA
G U A C G U
C A U G C A
Ribosomal subunits
rejoin to repeat the
process with the same
or another mRNA.
Protein
1
4
6
7
8
mRNA
tRNA
+ PiATP ADP
3
5
2
tRNA is released from
the ribosome and is
available to pick up a
new amino acid and
repeat the process.
After translating the
entire mRNA, the
ribosome dissociates
into its two subunits.
The preceding tRNA hands off
the growing protein to the new
tRNA, and the ribosome links the
new amino acid to the protein.
mRNA leaves
the nucleus.
tRNA binds an
amino acid; binding
consumes 1 ATP.
Ribosome
binds mRNA.
tRNA anticodon binds
to complementary
mRNA codon.
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4-27
Translation
• requries the participation of transfer RNA (tRNA)
– small RNA molecule
– coils on itself to form an angular L shape
– one end of the L includes three nucleotides called an anticodon
– other end has binding site specific for one amino acid
– each tRNA picks up specific amino acid from pool of free amino
acids in cytosol
• one ATP molecule is used to bind amino acid to site
• provides energy for peptide bond formation
– some imprecision in codon-anticodon pairing
• takes only 48 different tRNAs to pair with 61 different codons
– ribosome binds and holds tRNA with its specific amino acid
– large ribosomal subunit contains an enzyme that forms peptide bond
that links amino acids together
– first tRNA released from ribosome
– second tRNA temporarily anchors growing peptide chain
– ribosome shifts and third tRNA brings its amino acid to the site
– one ribosome can assemble a protein of 400 amino acids in about 20
seconds
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4-29
Transfer RNA (tRNA)
Figure 4.8
Amino acid –
accepting end
Loop 1
Loop 3
Loop 2
C
C
U U A
Anticodon
A
Amino acid –
accepting end
Loop 1
Loop 3
Loop 2
UU A
A
C
C
(a) (b)
Anticodon
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4-30
Polyribosomes
• polyribosome - one mRNA holding multiple ribosomes
– One ribosome can assemble protein of 400 amino acids in 20
seconds
– 300,000 identical mRNA molecules may be undergoing
simultaneous translation
• cell can produce 150,000 protein molecules per second
Figure 4.9
60 nm
mRNA Ribosomes Protein
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© E.V. Kiseleva, February 5 Letter 257:251-253, 1989
4-31
Review of Peptide Formation
Figure 4.10
1
2
3
4
5
6
DNA double helix
Seven base triplets on the
template strand of DNA
The corresponding codons of
mRNA transcribed from the
DNA triplets
The anticodons of tRNA that
bind to the mRNA codons
The amino acids carried by
those six tRNA molecules
The amino acids linked into a
peptide chain
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4-32
Protein Processing and Secretion
• protein synthesis is not finished when the amino
acid sequence (primary structure) has been
assembled.
• to be functional it must coil or fold into precise
secondary and tertiary structure
• Chaperone proteins
– older proteins that pick up new proteins and guides the
new protein in folding into the proper shapes
– helps to prevent improper association between different
proteins
– also called stress proteins or heat-shock proteins
• chaperones produced in response to heat or stress
• help damaged protein fold back into correct functional shapes
4-33
Protein Packaging and Secretion
Figure 4.11
Nucleus
Rough ER
Golgi
complex
Lysosome
Ribosomes
Protein packaged into transport
vesicle, which buds from ER.
Transport vesicles fuse into clusters that
unload protein into Golgi complex.
Golgi complex modifies
protein structure.
Golgi vesicle containing
finished protein is formed.
Secretory vesicles
release protein by
exocytosis.
1
2
3
4
5
6
Clathrin-coated
transport vesicle
Protein formed by
ribosomes on rough ER.
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4-34
Protein Processing and Secretion
• proteins to be used in the cytosol are likely to be made on
free ribosomes in cytosol
• proteins destined for packaging into lysosomes or secretion
from the cell are assembled on rough ER and sent to golgi
complex for packaging
– entire polyribosome migrated to the rough ER and docks on its
surface
– assembled amino acid chain completed on rough ER
– sent to Golgi for final modification
4-35
Protein Processing and Secretion
• proteins assembled on ER surface
• threads itself through a pore in the ER membrane into cisterna
• modifies protein by posttranslational modification
– removing some amino acid segments
– folding the protein
– stabilizing protein with disulfide bridges
– adding carbohydrates
• when rough ER finished with protein
– pinches off bubblelike transport vesicle coated with clathrin
– clathrin helps select the proteins to be transported in vesicles and helps mold forming
vesicle
– vesicles detach from ER and carry protein to the nearest cisterna of Golgi complex
• vesicles fuse and unloads proteins into Golgi cisterna
• Golgi complex further modifies the protein
• passes from cisterna closest to ER to cisterna farthest from ER
• buds off new coated Golgi vesicles containing finished protein
• some Golgi vesicles become lysosomes
• others become secretory vesicles migrate to plasma membrane, fuse to it, and
release their cell product by exocytosis
4-36
Mechanism of Gene Activation
Figure 4.12
mRNA
for casein
Casein
Exocytosis
Regulatory
protein
(transcription
activator)
Casein
gene
RNA
polymerase
Rough
Endoplasmic
reticulum
Golgi
complex
Secretory
vesicles
Prolactin
Prolactin
receptors
2
1
3
4
5
6
7
ATP
Pi
ADP
+
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Gene Regulation
• genes are turned on and off from day to
day
• their products are need or not
• many genes are permanently turned off in
any given cell
4-37
Gene Regulation
• example of several ways to turn genes on or off
• mother giving birth to first baby
– hormone prolactin stimulates cells of the mammary glands to begin
synthesizing components of breast milk
– including protein casein – something never secreted before
1. prolactin binds to receptors
- pair of proteins in plasma membrane of mammary cell
1. receptors trigger the activation of a regulatory protein (transcription
activator) in cytoplasm
2. regulatory protein moves into the nucleus and binds to the DNA near
the casein gene
3. the binding enables RNA polymerase to bind to the gene and
transcribe it, producing the mRNA for casein
4-38
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4-40
Synthesizing Compounds Other
Than Proteins
• cells synthesize glycogen, fat, steroids, phospholipids,
pigments, and other compounds
– no genes for these
– synthesis under indirect genetic control
– produced by enzymatic reactions
– enzymes are proteins encoded by genes
• example – testosterone production
– a steroid
– a cell of the testes takes in cholesterol
– enzymatically converts it to testosterone
– only occurs when genes for enzyme are activate
• genes may greatly affect such complex outcomes as
behavior, aggression, and sex drive
4-41
DNA Replication and Cell Cycle
• before cells divide, it must duplicate its DNA so
it can give a complete copy of all its genes to
each daughter cell.
• since DNA controls all cellular function, this
replication process must be very exact
• Law of Complementary Base Pairing – we can
predict the base sequence of one DNA strand if
we know the sequence of the other
– enables a cell to reproduce one strand based on the
information in another
4-42
DNA Replication
Figure 4.14
DNA polymerase
Incoming
nucleotides
Key
A T
C G
Parental DNA
Gap in
replication
Daughter DNA
Old strand
New strand
DNA ligase
Replication
fork
(a)
(b)
(c) (d)
(e)
DNA helicase
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4-43
Steps of DNA Replication
1. Double helix unwinds from histones
2. enzyme DNA helicase opens one short segment of helix at a time
-exposing its nitrogen bases
1. replication fork – the point where the DNA is opened up (like two
separated halves of a zipper)
2. DNA polymerase molecules move along each strand
-read the exposed bases
-matches complementary free nucleotides
1. the two separated strands of DNA are copied by separate
polymerase molecules proceeding in opposite directions
-the polymerase molecule moving toward the replication fork makes a
long, continuous, new strand of DNA
-the polymerase molecule moving away from the replication fork
makes short segments of DNA at a time…DNA ligase joins them
together
4-44
Steps of DNA Replication
6. from the old parental DNA molecule, two new daughter DNA
molecules are made
7. semiconservative replication - each daughter DNA consists of one
new helix synthesized from free nucleotides and one old helix
conserved from the parental DNA
8. new histones are synthesized in cytoplasm
-millions of histones are transported into the nucleus within a few
minutes after DNA replication
-each new DNA helix wraps around them to make a new
nucleosome
9. each DNA polymerase works at a rate of 100 base pairs per second
-would take weeks for one polymerase to replicate one
chromosome
-thousands of polymerase molecules work simultaneously on each
DNA molecule
10. all 46 chromosomes are replicated in 6 - 8 hours
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4-46
Errors and Mutations
• DNA polymerase does make mistakes
– multiple modes for correction of replication errors
– double checks the new base pair and tend to replace
incorrect, biochemically unstable pairs with more stable
correct pairs
– result is only 1 error per 1 billion bases replicated
• mutations - changes in DNA structure due to
replication errors or environmental factors
(radiation, viruses, chemicals)
– some mutations cause no ill effects. others kill the cell,
turn it cancerous or cause genetic defects in future
generations.
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4-49
Cell Cycle
• Cell Cycle – the cell’s life cycle that
extends from one division to the next
• G1 phase, the first gap phase
– interval between cell division and DNA
replication
– accumulates materials needed
to replicate DNA
• S phase, synthesis phase
– duplicates centrioles
– DNA replication occurs
• G2 phase, second gap phase
– interval between DNA replication andcell division
– finishes centriole duplication
– synthesizes enzymes that control cell division
– repairs DNA replication errors
• M phase, mitotic phase
– cell replicates its nucleus
– pinches in two to form new daughter cells
• Interphase – collection of G1, S, and G2 phases
• G0 (G zero) phase - cells that have left the cycle for a “rest”
– muscle and nerve cells
Figure 4.15
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
G2
Second gap phase
Growth and preparation
for mitosis
G1
First gap phase
Growth and normal
metabolic roles
S
Synthesis phase
DNA
replication
Interphase
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4-51
Mitosis
• cell division in all body cells except the eggs and
sperm
• Functions of mitosis
– development of the individual from one fertilized egg to
some 40 trillion cells
– growth of all tissues and organs after birth
– replacement of cells that die
– repair of damaged tissues
• 4 phases of mitosis
– prophase, metaphase, anaphase, telophase
4-52
Mitosis
Figure 4.16
Aster
2
3
1
4
Chromatids
Centriole
Spindle fibers
Kinetochore
Nucleolus
Chromatin
Cleavage furrow
Metaphase
Chromosomes lie along midline
of cell. Some spindle fibers
attach to kinetochores.
Fibers of aster attach
to plasma membrane.
Anaphase
Centromeres divide in two.
Spindle fibers pull sister
chromatids to opposite poles
of cell. Each pole (future
daughter cell) now has an
identical set of genes.
Telophase
Chromosomes gather
at each pole of cell.
Chromatin decondenses.
New nuclear envelope
appears at each pole.
New nucleoli appear
in each nucleus.
Mitotic spindle
vanishes.
Prophase
Chromosomes condense
and nuclear envelope
breaks down. Spindle
fibers grow from centrioles.
Centrioles migrate to
opposite poles of cell.
Nuclear envelope
re-forming
Daughter
cells in
interphase
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4-53
Mitosis: Prophase
• chromosomes shorten and thicken coiling into compact rods
– easier to distribute to daughter cells than chromatin
• 46 chromosomes
– two chromatids per chromosome
– one molecule of DNA in each chromatid
• nuclear envelope disintegrates and releases chromosomes into the
cytosol
• centrioles sprout elongated microtubules – spindle fibers
– push centrioles apart as they grow
– pair of centrioles lie at each pole of the cell
• some spindle fibers grow toward chromosomes and attach to the
kinetochore on each side of the centromere
• spindle fibers then tug the chromosomes back and forth until they line
up along the midline of the cell
4-54
Mitosis: Prophase
Figure 4.16 (1)
1
1 Prophase
Chromosomes condense
and nuclear envelope
breaks down. Spindle
fibers grow from centrioles.
Centrioles migrate to
opposite poles of cell.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© Ed Reschke
4-55
Mitosis: Metaphase
• Chromosomes are aligned on cell equator
– oscillating slightly and awaiting signal that stimulates each of them
to split
• mitotic spindle – lemon-shaped array of spindle fibers
– long spindle fibers (microtubules) attach to chromosomes
– shorter microtubules (aster) anchor centrioles to plasma
membrane at each end of cell
Figure 4.16 (2)
Aster
2
2
Metaphase
Chromosomes lie along
midlineof cell. Some spindle fibers
attach to kinetochores.
Fibers of aster attach
to plasma membrane.
Spindle fibers
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© Ed Reschke
4-56
Mitosis: Anaphase
• activation of an enzyme that cleaves two sister chromatids apart at
centromere
• daughter chromosomes migrate towards each pole of the cell with
centromere leading the way
– motor proteins in kinetochore crawling along the spindle fiber as the fiber
itself is ‘chewed up’ and disassembled at the chromosomal end
– daughter cells of mitosis are genetically identical
Figure 4.16 (3)
3
3
Anaphase
Centromeres divide in two.
Spindle fibers pull sister
chromatids to opposite poles
of cell. Each pole (future
daughter cell) now has an
identical set of genes.
Chromatids
Kinetochore
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
© Ed Reschke
4-57
Mitosis: Telophase
• chromatids cluster on each side of the cell
• rough ER produces new nuclear envelope
around each cluster
• chromatids begin to uncoil and form chromatin
• mitotic spindle breaks up and vanishes
• each nucleus forms nucleoli
– indicating it has already begun making RNA and
preparing for protein synthesis
4-58
Cytokinesis
• cytokinesis – the division of cytoplasm into two
cells
– telophase is the end of nuclear division but overlaps
cytokinesis
– early traces of cytokinesis visible in anaphase
• achieved by motor protein myosin pulling on
microfilaments of actin in the terminal web of
cytoskeleton
• creates the cleavage furrow around the equator
of cell
• cell eventually pinches in two
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operating systems, some animations
will not appear until the presentation is
viewed in Presentation Mode (Slide
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in the “Normal” or “Slide Sorter” views.
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in Presentation Mode and playing each
animation. Most animations will require
the latest version of the Flash Player,
which is available at
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4-60
Timing of Cell Division
Cells divide when:
• they have enough cytoplasm for two daughter cells
• they have replicated their DNA
• adequate supply of nutrients
• are stimulated by growth factor
– chemical signals secreted by blood platelets, kidney cells, and other
sources
• neighboring cells die, opening up space in a tissue to be
occupied by new cells
Cells stop dividing when:
• snugly contact neighboring cells
• when nutrients or growth factors are withdrawn
• contact inhibition – the cessation of cell division in
response to contact with other cells
4-61
Chromosomes and Heredity
• heredity - transmission of genetic characteristics from parent to
offspring
• karyotype - chart of 46 chromosomes laid out in order by size and other
physical features
• 23 pairs – the two members of each pair are called homologous
chromosomes
– 1 chromosome from each pair inherited from each parent
• 22 pairs called autosomes
– look alike and carry the same genes
• one pair of sex chromosomes (X and Y)
– normal female has homologous pair of X chromosomes
– normal male has one X and one much smaller Y chromosome
• diploid – any cell with 23 pairs of chromosomes (somatic cells)
• haploid – contain half as many chromosomes as somatic cells – sperm
and egg cells (germ cells)
• fertilization restores diploid number to the fertilized egg and the
somatic cells arise from it.
4-62
Karyotype of Normal Male
Figure 4.17
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Peter Arnold, Inc.
4-63
Genes and Alleles
• locus – the location of a particular gene on a chromosome
• alleles - different forms of gene at same locus on two
homologous chromosomes
• dominant allele (represented by capital letter)
– corresponding trait is usually detectable in the individual
– masks the effect of any recessive allele that may be present
– produces protein responsible for visible trait
• recessive allele (represented by lower case letter)
– expressed only when present on both of the homologous
chromosomes
– no dominant alleles at that locus
4-64
Genetics of Cleft chin
Figure 4.18 bAllele for cleft chin is dominant.
(b)
C c
c
C
Cc
Heterozygous
cleft chin
Cc
Heterozygous
cleft chin
cc
Homozygous
uncleft chin
CC
Homozygous,
cleft chin
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4-65
Genes and Alleles
• genotype – the alleles that an individual possesses for a
particular trait
– homozygous alleles – two identical alleles for a trait
– heterozygous alleles – different alleles for that gene
• phenotype – an observable trait
– an allele is expressed if it shows in the phenotype of an individual
• genetic counselors – perform genetic testing or refer
clients for tests, advise couples on the probability of
transmitting genetic diseases, and assist people on coping
with genetic disease
• Punnett square shows how 2 heterozygous parents with
cleft chins can have child with uncleft chin
– heterozygous carriers of hereditary diseases such as cystic fibrosis
– both parents healthy
4-66
Multiple Alleles, Codominance,
and Incomplete Dominance
• gene pool - collective genetic makeup of population as a
whole
• multiple alleles - more than two allelic forms for a trait
– 100 alleles are responsible for cystic fibrosis
– 3 alleles for ABO blood types
• IA
, IB
, i alleles for ABO blood types
• codominant - both alleles equally dominant
– IA
IB
= type AB blood
– both are phenotypically expressed
• incomplete dominance
– phenotype intermediate between traits each allele would
have produced alone
4-67
Polygenic Inheritance
• genes at two or more loci, or even different
chromosomes, contribute to a single phenotypic
trait (skin and eye color, alcoholism, mental illness,
cancer, and heart disease)
Figure 4.19
Gene 1
Gene 2
Phenotype
(eye color)
Gene 3
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(top): Comstock/Getty Images; (middle & bottom): Photodisc Red/Getty Images
4-68
Pleiotropy
• One gene produces multiple phenotypic effects
– alkaptonuria - mutation on chromosome 3 that blocks
the breakdown of tyrosine
– sickle-cell disease is another example of pleiotrophy
Figure 4.20
Gene
Phenotype 3
(urine darkens
upon standing)
Phenotype 1
(darkened skin)
Phenotype 2
(darkened sclera
of eye)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
(1&3): From G. Pierrard, A. Nikkels. April 5, 2001, A Medical Mystery. New England Journal of Medicine, 344: p. 1057. © 2001
Massachusetts Medical Society. All rights reserved; (2): British Journal of Ophthalmology 1999; 83:680 © by BMJ Publishing Group
Ltd/http://www.bjophthalmol.com
4-69
Sex Linkage
• Sex-linked traits – carried on the X and Y chromosomes,
and therefore tend to be inherited by one sex more than the
other
• Recessive color blindness allele on X, no gene locus for trait
on Y, so red-green color blindness more common in men
(mother is carrier)
Figure 4.21
X X X Y
C c c
Female (XX)
Genotype Cc
Normal vision
Male (XY)
Genotype c–
Color blindness
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
4-70
Penetrance and Environmental
Effects• Penetrance – the
percentage of a population with
a given genotype that actually
exhibits the predicted
phenotype
– polydacyly
• Role of environment
– no gene can produce a
phenotypic effect without
nutritional and other
environmental input
– need both the genetic
recipe and the ingredients
– brown eye color requires
phenylalanine from diet to
produce melanin pigment Figure 4.22
Phenotype
(brown eyes)
From diet
(environment)
Phenvlalanine
(amino acid)
Melanin
(pigment)
Genes
(DNA)
Tyrosine
Enzyme 1
mRNA 1
Enzyme 2
mRNA 2
4-71
Dominant and Recessive Alleles
at the Population Level
• common misconception that dominant
alleles must be more common in the gene
pool than recessive alleles
• some recessive alleles, blood type O, are
the most common
• some dominant alleles, polydactyly and
blood type AB, are rare in the population
4-72
Cancer
• benign tumor
– slow growth
– contained in fibrous capsule
– will not metastasize
– usually easy to treat
• malignant tumor – called cancer
– fast growing
– metastasize – give off cells that seed the growth of
multiple tumors elsewhere
• Oncology – medical specialty that deals with both
benign and malignant tumors
• tumor angiogenesis – ingrowth of blood vessels
stimulated by energy-hungry tumors
4-73
Cancer
• Cancers are named for the tissue of origin
– carcinomas – originate in epithelial tissue
– lymphomas – originate in lymph nodes
– melanomas – originate in pigment cells of
epidermis (melanocytes)
– leukemias – in blood forming tissues
– sarcomas – in bone, other connective tissue,
or muscle
• carcinogen – environmental cancer-causing
agents
4-74
Causes of Cancer
• Carcinogens – environmental cancer- causing
agents
– radiation – ultraviolet rays, X-rays
– chemical - cigarette tar, food preservatives, industrial
chemicals
– viruses – human papilloma virus, hepatitis C, and type 2
herpes simplex
• 5 -10% of cancers are hereditary
• carcinogens trigger gene mutations
4-75
Malignant Tumor Genes
• Oncogenes
– causes cell division to accelerate out of control
• excessive production of growth factors that stimulate
mitosis
• the production for excessive growth factor receptors
• Tumor suppressor genes
– inhibit development of cancer
• oppose action of oncogenes
• codes for DNA repairing enzymes
4-76
Lethal Effects of Cancer
• replace functional tissue in vital organs
• steal nutrients from the rest of the body
– cachexia – severe wasting away of depleted
tissues
• weaken one’s immunity
• open the door for opportunistic infections
• often invade blood vessels, lung tissue, or
brain tissue

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Genetics and Cellular Function

  • 1. 4-1 Genetics and Cellular Function • DNA and RNA – the nucleic acids • Genes and their action • DNA replication and the cell cycle • Chromosomes and heredity
  • 2. 4-2 Modern Genetics • Mendelian genetics – Gregor Mendel – investigates family patterns of inheritance • Cytogenetics – uses techniques of cytology and microscopy to study chromosomes and their relationships to hereditary traits • Molecular genetics – uses biochemistry to study structure and function of DNA • Genomic medicine – treatment of genetic diseases through an understanding of the human genome
  • 3. 4-3 DNA Molecular Structure • DNA – deoxyribonucleic acid - a long threadlike molecule with uniform diameter, but varied length – 46 DNA molecules in the nucleus of most human cells • total length of 2 meters • average DNA molecule 2 inches long • DNA and other nucleic acids are polymers of nucleotides • Each nucleotide consists of – one sugar - deoxyribose – one phosphate group – one nitrogenous base • Either pyrimidine (single carbon-nitrogen ring) or purine (double ring) Figure 4.1a HC N C N NH2 N H C C CH N H CH2OHO O OH P H HOH HH O Adenine Phosphate Deoxyribose (a) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 4. 4-4 Nitrogenous Bases of DNA • Purines - double ring – Adenine (A) – Guanine (G) • Pyrimidines - single ring – Cytosine (C) – Thymine (T) • DNA bases - ATCG Figure 4.1b Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. C NH2 N NH C CH C H N N C Adenine (A) Purines C O N NH C CH C N HN C NH2 Guanine (G) H C NH2 C N H C HC N O Cytosine (C) Uracil (U) C C O C O CH HN CH N H N H C C HC CH3 NH O O Thymine (T) Pyrimidines (b)
  • 5. 4-5 DNA Structure • Molecular shape is a double helix (resembles a spiral staircase) – each sidepiece is a backbone composed of phosphate groups alternating with the sugar deoxyribose. – steplike connections between the backbones are pairs of nitrogen bases Figure 4.2 (a) (b) (c) A T A T T A A T AT G C G G C C G C Sugar–phosphate backbone G A C T T G C Hydrogen bond A Sugar–phosphate backbone Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 6. 4-6 Complementary Base Pairing • Nitrogenous bases united by hydrogen bonds – a purine on one backbone with a pyrimidine on the other – A – T two hydrogen bonds – C – G three hydrogen bonds • DNA base pairing – A – T – C – G • Law of Complementary Base Pairing – one strand determines base sequence of other Figure 4.2 partial (b) (c) GC Sugar–phosphate backbone Sugar–phosphate backbone G A C T A T A T G C Hydrogen bond Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 7. 4-7 Discovery of the Double Helix • By 1900: components of DNA were known – sugar, phosphate and bases • By 1953: x ray diffraction determined geometry of DNA molecule • Nobel Prize awarded in 1962 to 3 men: Watson, Crick and Wilkins, but not to Rosalind Franklin, who died of cancer at 37, after discovering the x-ray data that provided the answers to the double helix.
  • 8. 4-8 DNA Function • Genes – genetic instructions for synthesis of proteins • Gene – segment of DNA that codes for a specific protein • Genome - all the genes of one person – humans have estimated 25,000 to 35,000 genes • 2% of total DNA • other 98% is noncoding DNA – plays role in chromosome structure – regulation of gene activity – no function at all – “junk” DNA
  • 9. 4-9 Chromatin and Chromosomes • chromatin – fine filamentous DNA material complexed with proteins – occurs as 46 long filaments called chromosomes – 6 ft in first half of cell cycle – twice that amount in preparation for cell division – in nondividing cells, chromatin is so slender it cannot be seen with light microscope – granular appearance under electron microscope – histones – disc-shaped cluster of eight proteins – DNA molecule winds around the cluster – repeats pattern 800,000 times – appears to be divided into segments - nucleosomes – nucleosome consists of : • core particle – histones with DNA around them • linker DNA – short segment of DNA connecting core particles – 1/3 shorter than DNA alone Figure 4.4b 2 nm 11 nm Nucleosome Linker DNA 300 nm 30 nm 700 nm 700 nm Core particle In dividing cells only Chromatids Centromere 1 2 3 4 5 6 30 nm fiber is thrown into irregular loops to form a fiber 300 nm thick Nucleosomes fold accordion- like into zigzag fiber 30 nm in diameter DNA winds around core particles to form nucleosomes 11 nm in diameter DNA double helix In dividing cells, looped chromatin coils further into a 700 nm fiber to form each chromatid Chromosome at the midpoint (metaphase) of cell division Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 10. 4-10 Chromatin and Chromosomes • chromatin – protein complex thrown into complex, irregular loops and coils – 1000X shorter than original molecule • each chromosome is packed into its own spheroidal region of the nucleus – chromosome territory – permeated with channels that allow regulatory chemicals to have access to the genes • in nondividing cell, the chromatin is not a static structure – changes moment to moment according to genetic activity of cell – genes get turned off and on – chromosomes migrate as cells develop moving active genes on different chromosomes closer together • allows genes to partner to bring about developmental tasks in the cell Figure 4.4b 2 nm 11 nm Nucleosome Linker DNA 300 nm 30 nm 700 nm 700 nm Core particle In dividing cells only Chromatids Centromere 1 2 3 4 5 6 30 nm fiber is thrown into irregular loops to form a fiber 300 nm thick Nucleosomes fold accordion- like into zigzag fiber 30 nm in diameter DNA winds around core particles to form nucleosomes 11 nm in diameter DNA double helix In dividing cells, looped chromatin coils further into a 700 nm fiber to form each chromatid Chromosome at the midpoint (metaphase) of cell division Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 11. 4-11 Cells Preparing to Divide • exact copies are made of all the nuclear DNA • each chromosome consists of two parallel filaments of identical DNA - sister chromatids • in prophase, final coiling and condensing – now visible with light microscope • final compaction enables the two sister chromatids to be pulled apart and carried to separate daughter cells without damage to the DNA – joined at centromere – kinetochore – protein plaques on either side of the centromere Figure 4.5a(a) Centromere Kinetochore Sister chromatids Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 12. 4-12 RNA: Structure and Function • RNA much smaller cousin of DNA (fewer bases) – messenger RNA (mRNA) over 10,000 bases – ribosomal RNA (rRNA) – transfer RNA (tRNA) 70 - 90 bases – DNA averages 100 million base pairs • one nucleotide chain (not a double helix as DNA) • ribose replaces deoxyribose as the sugar • uracil replaces thymine as a nitrogenous base • Essential function – interprets code in DNA – uses those instructions for protein synthesis – leaves nucleus and functions in cytoplasm
  • 13. 4-13 What is a Gene? • Previous definition - gene - a segment of DNA that carries the code for a particular protein??? – Body has millions of proteins but only 35,000 genes? – Small % of genes produce only RNA molecules – Some segments of DNA belong to 2 different genes • Current Definition – gene - an information-containing segment of DNA that codes for the production of a molecule of RNA that plays a role in synthesizing one or more proteins • Amino acid sequence of a protein is determined by the nucleotide sequence in the DNA
  • 14. 4-14 Human Genome • Genome – all the DNA in one 23-chromosome set – 3.1 billion nucleotide pairs in human genome • 46 human chromosomes comes in two sets of 23 chromosomes – one set of 23 chromosomes came form each parent – each pair of chromosomes has same genes but different versions (alleles) exist • Human Genome Project (1990-2003) identified the nitrogenous base sequences of 99% of the human genome – genomics – the comprehensive study of the whole genome and how its genes and noncoding DNA interact to affect the structure and function of the whole organism.
  • 15. 4-15 Human Genome • Findings of Human Genome Project – Homo sapiens has only about 25,000 to 35,000 genes • not the 100,000 formerly believed – genes generate millions of different proteins • not the old one gene one protein theory • single gene can code for many different proteins – genes average about 3,000 bases long • range up to 2.4 million bases – all humans are at least 99.99% genetically identical • 0.01% variations that we can differ from one another in more than 3 million base pairs • various combinations of these single-nucleotide polymorphisms account for all human variation – some chromosomes are gene-rich and some gene-poor – we now know the locations of more than 1,400 disease-producing mutations • opens the door for a new branch of medical diagnosis and treatment called Genomic Medicine • before HGP, we new locations of fewer than 100
  • 16. 4-16 Genetic Code • body can make millions of different proteins, all from the same 20 amino acids, and encoded by genes made of just 4 nucleotides (A,T,C,G) • Genetic code – a system that enables these 4 nucleotides to code for the amino acid sequence of all proteins • minimum code to symbolize 20 amino acids is 3 nucleotides per amino acid • Base triplet – a sequence of 3 DNA nucleotides that stands for one amino acid – codon - the 3 base sequence in mRNA – 64 possible codons available to represent the 20 amino acids • 61 code for amino acids • Stop Codons – UAG, UGA, and UAA – signal the ‘end of the message’, like a period at the end of a sentence • Start Codon – AUG codes for methionine , and begins the amino acid sequence of the protein
  • 17. 4-17 Overview of Protein Synthesis • all body cells, except sex cells and some immune cells, contain identical genes. • different genes are activated in different cells • any given cell uses 1/3 to 2/3rds of its genes – rest remain dormant and may be functional in other types of cells • activated gene – messenger RNA (mRNA) – a mirror-image copy of the gene is made • migrates from the nucleus to cytoplasm • its code is read by the ribosomes – ribosomes – cytoplasmic granules composed of ribosomal RNA (rRNA) and enzymes – transfer RNA (tRNA) – delivers amino acids to the ribosome – ribosomes assemble amino acids in the order directed by the codons of mRNA
  • 18. 4-18 Summary of Protein Synthesis • process of protein synthesis – DNA mRNA protein • transcription – step from DNA to mRNA – occurs in the nucleus where DNA is located • translation – step from mRNA to protein – most occurs in cytoplasm – 15-20% of proteins are synthesized in the nucleus
  • 19. Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
  • 20. Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
  • 21. 4-21 Transcription • DNA too large to leave nucleus and participate directly in cytoplasmic protein synthesis – necessary to make a small mRNA copy that can migrate through a nuclear pore into the cytoplasm • Transcription – copying genetic instructions from DNA to RNA • RNA Polymerase – enzyme that binds to the DNA and assembles the mRNA – base sequences TATATA or TATAAA inform the polymerase where to begin – RNA polymerase opens up the DNA helix about 17 base pairs at a time – reads base from one strand of DNA – makes corresponding mRNA • where it finds C on the DNA, it adds G to the mRNA • where it finds A on the DNA, it adds U to the mRNA – RNA polymerase rewinds the DNA helix behind it – gene can be transcribed by several polymerase molecules at once – terminator – base sequence at the end of a gene which signals polymerase to stop
  • 22. 4-22 Transcription • pre-mRNA – immature RNA produced by transcription • exons – “sense” portions of the immature RNA – will be translated to protein • introns – “nonsense” portions of the immature RNA – must be removed before translation • alternative splicing – removing the introns by enzymes and splicing the exons together into a functional RNA molecule – one gene can code for more than one protein
  • 23. Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
  • 24. 4-24 Alternative Splicing of mRNA • One gene can code for more than one protein • Exons can be spliced together into a variety of different mRNAs. Figure 4.6 Gene (DNA) Pre-mRNA Intron Exon mRNA 2mRNA 1 mRNA 3 Protein 2Protein 1 Protein 3 Transcription1 Translation3 Splicing2 A B C D E F A C D A E FB D E Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 25. 4-25 Translation • translation – the process that converts the language of nucleotides into the language of amino acids • ribosomes - translate sequence of nucleotides into the sequence of amino acids – occur mainly in cytosol, on surface of rough ER, and nuclear envelope – consists of two granular subunits, large and small • each made of several rRNA and enzyme molecules
  • 26. 4-26 Translation of mRNA • mRNA molecule begins with leader sequence – acts as binding site for small ribosomal subunit – large subunit attaches to small subunit – ribosome pulls mRNA molecule through it like a ribbon, reading the bases as it goes – when start codon (AUG) is reached, protein synthesis begins – all proteins begin with methionine when first synthesized Figure 4.7 Free amino acids Cytosol Free tRNA NucleusDNA G U A C G U C A U G C A Ribosomal subunits rejoin to repeat the process with the same or another mRNA. Protein 1 4 6 7 8 mRNA tRNA + PiATP ADP 3 5 2 tRNA is released from the ribosome and is available to pick up a new amino acid and repeat the process. After translating the entire mRNA, the ribosome dissociates into its two subunits. The preceding tRNA hands off the growing protein to the new tRNA, and the ribosome links the new amino acid to the protein. mRNA leaves the nucleus. tRNA binds an amino acid; binding consumes 1 ATP. Ribosome binds mRNA. tRNA anticodon binds to complementary mRNA codon. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 27. 4-27 Translation • requries the participation of transfer RNA (tRNA) – small RNA molecule – coils on itself to form an angular L shape – one end of the L includes three nucleotides called an anticodon – other end has binding site specific for one amino acid – each tRNA picks up specific amino acid from pool of free amino acids in cytosol • one ATP molecule is used to bind amino acid to site • provides energy for peptide bond formation – some imprecision in codon-anticodon pairing • takes only 48 different tRNAs to pair with 61 different codons – ribosome binds and holds tRNA with its specific amino acid – large ribosomal subunit contains an enzyme that forms peptide bond that links amino acids together – first tRNA released from ribosome – second tRNA temporarily anchors growing peptide chain – ribosome shifts and third tRNA brings its amino acid to the site – one ribosome can assemble a protein of 400 amino acids in about 20 seconds
  • 28. Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
  • 29. 4-29 Transfer RNA (tRNA) Figure 4.8 Amino acid – accepting end Loop 1 Loop 3 Loop 2 C C U U A Anticodon A Amino acid – accepting end Loop 1 Loop 3 Loop 2 UU A A C C (a) (b) Anticodon Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 30. 4-30 Polyribosomes • polyribosome - one mRNA holding multiple ribosomes – One ribosome can assemble protein of 400 amino acids in 20 seconds – 300,000 identical mRNA molecules may be undergoing simultaneous translation • cell can produce 150,000 protein molecules per second Figure 4.9 60 nm mRNA Ribosomes Protein Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © E.V. Kiseleva, February 5 Letter 257:251-253, 1989
  • 31. 4-31 Review of Peptide Formation Figure 4.10 1 2 3 4 5 6 DNA double helix Seven base triplets on the template strand of DNA The corresponding codons of mRNA transcribed from the DNA triplets The anticodons of tRNA that bind to the mRNA codons The amino acids carried by those six tRNA molecules The amino acids linked into a peptide chain Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 32. 4-32 Protein Processing and Secretion • protein synthesis is not finished when the amino acid sequence (primary structure) has been assembled. • to be functional it must coil or fold into precise secondary and tertiary structure • Chaperone proteins – older proteins that pick up new proteins and guides the new protein in folding into the proper shapes – helps to prevent improper association between different proteins – also called stress proteins or heat-shock proteins • chaperones produced in response to heat or stress • help damaged protein fold back into correct functional shapes
  • 33. 4-33 Protein Packaging and Secretion Figure 4.11 Nucleus Rough ER Golgi complex Lysosome Ribosomes Protein packaged into transport vesicle, which buds from ER. Transport vesicles fuse into clusters that unload protein into Golgi complex. Golgi complex modifies protein structure. Golgi vesicle containing finished protein is formed. Secretory vesicles release protein by exocytosis. 1 2 3 4 5 6 Clathrin-coated transport vesicle Protein formed by ribosomes on rough ER. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 34. 4-34 Protein Processing and Secretion • proteins to be used in the cytosol are likely to be made on free ribosomes in cytosol • proteins destined for packaging into lysosomes or secretion from the cell are assembled on rough ER and sent to golgi complex for packaging – entire polyribosome migrated to the rough ER and docks on its surface – assembled amino acid chain completed on rough ER – sent to Golgi for final modification
  • 35. 4-35 Protein Processing and Secretion • proteins assembled on ER surface • threads itself through a pore in the ER membrane into cisterna • modifies protein by posttranslational modification – removing some amino acid segments – folding the protein – stabilizing protein with disulfide bridges – adding carbohydrates • when rough ER finished with protein – pinches off bubblelike transport vesicle coated with clathrin – clathrin helps select the proteins to be transported in vesicles and helps mold forming vesicle – vesicles detach from ER and carry protein to the nearest cisterna of Golgi complex • vesicles fuse and unloads proteins into Golgi cisterna • Golgi complex further modifies the protein • passes from cisterna closest to ER to cisterna farthest from ER • buds off new coated Golgi vesicles containing finished protein • some Golgi vesicles become lysosomes • others become secretory vesicles migrate to plasma membrane, fuse to it, and release their cell product by exocytosis
  • 36. 4-36 Mechanism of Gene Activation Figure 4.12 mRNA for casein Casein Exocytosis Regulatory protein (transcription activator) Casein gene RNA polymerase Rough Endoplasmic reticulum Golgi complex Secretory vesicles Prolactin Prolactin receptors 2 1 3 4 5 6 7 ATP Pi ADP + Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 37. Gene Regulation • genes are turned on and off from day to day • their products are need or not • many genes are permanently turned off in any given cell 4-37
  • 38. Gene Regulation • example of several ways to turn genes on or off • mother giving birth to first baby – hormone prolactin stimulates cells of the mammary glands to begin synthesizing components of breast milk – including protein casein – something never secreted before 1. prolactin binds to receptors - pair of proteins in plasma membrane of mammary cell 1. receptors trigger the activation of a regulatory protein (transcription activator) in cytoplasm 2. regulatory protein moves into the nucleus and binds to the DNA near the casein gene 3. the binding enables RNA polymerase to bind to the gene and transcribe it, producing the mRNA for casein 4-38
  • 39. Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
  • 40. 4-40 Synthesizing Compounds Other Than Proteins • cells synthesize glycogen, fat, steroids, phospholipids, pigments, and other compounds – no genes for these – synthesis under indirect genetic control – produced by enzymatic reactions – enzymes are proteins encoded by genes • example – testosterone production – a steroid – a cell of the testes takes in cholesterol – enzymatically converts it to testosterone – only occurs when genes for enzyme are activate • genes may greatly affect such complex outcomes as behavior, aggression, and sex drive
  • 41. 4-41 DNA Replication and Cell Cycle • before cells divide, it must duplicate its DNA so it can give a complete copy of all its genes to each daughter cell. • since DNA controls all cellular function, this replication process must be very exact • Law of Complementary Base Pairing – we can predict the base sequence of one DNA strand if we know the sequence of the other – enables a cell to reproduce one strand based on the information in another
  • 42. 4-42 DNA Replication Figure 4.14 DNA polymerase Incoming nucleotides Key A T C G Parental DNA Gap in replication Daughter DNA Old strand New strand DNA ligase Replication fork (a) (b) (c) (d) (e) DNA helicase Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 43. 4-43 Steps of DNA Replication 1. Double helix unwinds from histones 2. enzyme DNA helicase opens one short segment of helix at a time -exposing its nitrogen bases 1. replication fork – the point where the DNA is opened up (like two separated halves of a zipper) 2. DNA polymerase molecules move along each strand -read the exposed bases -matches complementary free nucleotides 1. the two separated strands of DNA are copied by separate polymerase molecules proceeding in opposite directions -the polymerase molecule moving toward the replication fork makes a long, continuous, new strand of DNA -the polymerase molecule moving away from the replication fork makes short segments of DNA at a time…DNA ligase joins them together
  • 44. 4-44 Steps of DNA Replication 6. from the old parental DNA molecule, two new daughter DNA molecules are made 7. semiconservative replication - each daughter DNA consists of one new helix synthesized from free nucleotides and one old helix conserved from the parental DNA 8. new histones are synthesized in cytoplasm -millions of histones are transported into the nucleus within a few minutes after DNA replication -each new DNA helix wraps around them to make a new nucleosome 9. each DNA polymerase works at a rate of 100 base pairs per second -would take weeks for one polymerase to replicate one chromosome -thousands of polymerase molecules work simultaneously on each DNA molecule 10. all 46 chromosomes are replicated in 6 - 8 hours
  • 45. Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
  • 46. 4-46 Errors and Mutations • DNA polymerase does make mistakes – multiple modes for correction of replication errors – double checks the new base pair and tend to replace incorrect, biochemically unstable pairs with more stable correct pairs – result is only 1 error per 1 billion bases replicated • mutations - changes in DNA structure due to replication errors or environmental factors (radiation, viruses, chemicals) – some mutations cause no ill effects. others kill the cell, turn it cancerous or cause genetic defects in future generations.
  • 47. Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
  • 48. Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
  • 49. 4-49 Cell Cycle • Cell Cycle – the cell’s life cycle that extends from one division to the next • G1 phase, the first gap phase – interval between cell division and DNA replication – accumulates materials needed to replicate DNA • S phase, synthesis phase – duplicates centrioles – DNA replication occurs • G2 phase, second gap phase – interval between DNA replication andcell division – finishes centriole duplication – synthesizes enzymes that control cell division – repairs DNA replication errors • M phase, mitotic phase – cell replicates its nucleus – pinches in two to form new daughter cells • Interphase – collection of G1, S, and G2 phases • G0 (G zero) phase - cells that have left the cycle for a “rest” – muscle and nerve cells Figure 4.15 Prophase Metaphase Anaphase Telophase Cytokinesis G2 Second gap phase Growth and preparation for mitosis G1 First gap phase Growth and normal metabolic roles S Synthesis phase DNA replication Interphase Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 50. Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
  • 51. 4-51 Mitosis • cell division in all body cells except the eggs and sperm • Functions of mitosis – development of the individual from one fertilized egg to some 40 trillion cells – growth of all tissues and organs after birth – replacement of cells that die – repair of damaged tissues • 4 phases of mitosis – prophase, metaphase, anaphase, telophase
  • 52. 4-52 Mitosis Figure 4.16 Aster 2 3 1 4 Chromatids Centriole Spindle fibers Kinetochore Nucleolus Chromatin Cleavage furrow Metaphase Chromosomes lie along midline of cell. Some spindle fibers attach to kinetochores. Fibers of aster attach to plasma membrane. Anaphase Centromeres divide in two. Spindle fibers pull sister chromatids to opposite poles of cell. Each pole (future daughter cell) now has an identical set of genes. Telophase Chromosomes gather at each pole of cell. Chromatin decondenses. New nuclear envelope appears at each pole. New nucleoli appear in each nucleus. Mitotic spindle vanishes. Prophase Chromosomes condense and nuclear envelope breaks down. Spindle fibers grow from centrioles. Centrioles migrate to opposite poles of cell. Nuclear envelope re-forming Daughter cells in interphase Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 53. 4-53 Mitosis: Prophase • chromosomes shorten and thicken coiling into compact rods – easier to distribute to daughter cells than chromatin • 46 chromosomes – two chromatids per chromosome – one molecule of DNA in each chromatid • nuclear envelope disintegrates and releases chromosomes into the cytosol • centrioles sprout elongated microtubules – spindle fibers – push centrioles apart as they grow – pair of centrioles lie at each pole of the cell • some spindle fibers grow toward chromosomes and attach to the kinetochore on each side of the centromere • spindle fibers then tug the chromosomes back and forth until they line up along the midline of the cell
  • 54. 4-54 Mitosis: Prophase Figure 4.16 (1) 1 1 Prophase Chromosomes condense and nuclear envelope breaks down. Spindle fibers grow from centrioles. Centrioles migrate to opposite poles of cell. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © Ed Reschke
  • 55. 4-55 Mitosis: Metaphase • Chromosomes are aligned on cell equator – oscillating slightly and awaiting signal that stimulates each of them to split • mitotic spindle – lemon-shaped array of spindle fibers – long spindle fibers (microtubules) attach to chromosomes – shorter microtubules (aster) anchor centrioles to plasma membrane at each end of cell Figure 4.16 (2) Aster 2 2 Metaphase Chromosomes lie along midlineof cell. Some spindle fibers attach to kinetochores. Fibers of aster attach to plasma membrane. Spindle fibers Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © Ed Reschke
  • 56. 4-56 Mitosis: Anaphase • activation of an enzyme that cleaves two sister chromatids apart at centromere • daughter chromosomes migrate towards each pole of the cell with centromere leading the way – motor proteins in kinetochore crawling along the spindle fiber as the fiber itself is ‘chewed up’ and disassembled at the chromosomal end – daughter cells of mitosis are genetically identical Figure 4.16 (3) 3 3 Anaphase Centromeres divide in two. Spindle fibers pull sister chromatids to opposite poles of cell. Each pole (future daughter cell) now has an identical set of genes. Chromatids Kinetochore Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. © Ed Reschke
  • 57. 4-57 Mitosis: Telophase • chromatids cluster on each side of the cell • rough ER produces new nuclear envelope around each cluster • chromatids begin to uncoil and form chromatin • mitotic spindle breaks up and vanishes • each nucleus forms nucleoli – indicating it has already begun making RNA and preparing for protein synthesis
  • 58. 4-58 Cytokinesis • cytokinesis – the division of cytoplasm into two cells – telophase is the end of nuclear division but overlaps cytokinesis – early traces of cytokinesis visible in anaphase • achieved by motor protein myosin pulling on microfilaments of actin in the terminal web of cytoskeleton • creates the cleavage furrow around the equator of cell • cell eventually pinches in two
  • 59. Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer.
  • 60. 4-60 Timing of Cell Division Cells divide when: • they have enough cytoplasm for two daughter cells • they have replicated their DNA • adequate supply of nutrients • are stimulated by growth factor – chemical signals secreted by blood platelets, kidney cells, and other sources • neighboring cells die, opening up space in a tissue to be occupied by new cells Cells stop dividing when: • snugly contact neighboring cells • when nutrients or growth factors are withdrawn • contact inhibition – the cessation of cell division in response to contact with other cells
  • 61. 4-61 Chromosomes and Heredity • heredity - transmission of genetic characteristics from parent to offspring • karyotype - chart of 46 chromosomes laid out in order by size and other physical features • 23 pairs – the two members of each pair are called homologous chromosomes – 1 chromosome from each pair inherited from each parent • 22 pairs called autosomes – look alike and carry the same genes • one pair of sex chromosomes (X and Y) – normal female has homologous pair of X chromosomes – normal male has one X and one much smaller Y chromosome • diploid – any cell with 23 pairs of chromosomes (somatic cells) • haploid – contain half as many chromosomes as somatic cells – sperm and egg cells (germ cells) • fertilization restores diploid number to the fertilized egg and the somatic cells arise from it.
  • 62. 4-62 Karyotype of Normal Male Figure 4.17 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Peter Arnold, Inc.
  • 63. 4-63 Genes and Alleles • locus – the location of a particular gene on a chromosome • alleles - different forms of gene at same locus on two homologous chromosomes • dominant allele (represented by capital letter) – corresponding trait is usually detectable in the individual – masks the effect of any recessive allele that may be present – produces protein responsible for visible trait • recessive allele (represented by lower case letter) – expressed only when present on both of the homologous chromosomes – no dominant alleles at that locus
  • 64. 4-64 Genetics of Cleft chin Figure 4.18 bAllele for cleft chin is dominant. (b) C c c C Cc Heterozygous cleft chin Cc Heterozygous cleft chin cc Homozygous uncleft chin CC Homozygous, cleft chin Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 65. 4-65 Genes and Alleles • genotype – the alleles that an individual possesses for a particular trait – homozygous alleles – two identical alleles for a trait – heterozygous alleles – different alleles for that gene • phenotype – an observable trait – an allele is expressed if it shows in the phenotype of an individual • genetic counselors – perform genetic testing or refer clients for tests, advise couples on the probability of transmitting genetic diseases, and assist people on coping with genetic disease • Punnett square shows how 2 heterozygous parents with cleft chins can have child with uncleft chin – heterozygous carriers of hereditary diseases such as cystic fibrosis – both parents healthy
  • 66. 4-66 Multiple Alleles, Codominance, and Incomplete Dominance • gene pool - collective genetic makeup of population as a whole • multiple alleles - more than two allelic forms for a trait – 100 alleles are responsible for cystic fibrosis – 3 alleles for ABO blood types • IA , IB , i alleles for ABO blood types • codominant - both alleles equally dominant – IA IB = type AB blood – both are phenotypically expressed • incomplete dominance – phenotype intermediate between traits each allele would have produced alone
  • 67. 4-67 Polygenic Inheritance • genes at two or more loci, or even different chromosomes, contribute to a single phenotypic trait (skin and eye color, alcoholism, mental illness, cancer, and heart disease) Figure 4.19 Gene 1 Gene 2 Phenotype (eye color) Gene 3 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (top): Comstock/Getty Images; (middle & bottom): Photodisc Red/Getty Images
  • 68. 4-68 Pleiotropy • One gene produces multiple phenotypic effects – alkaptonuria - mutation on chromosome 3 that blocks the breakdown of tyrosine – sickle-cell disease is another example of pleiotrophy Figure 4.20 Gene Phenotype 3 (urine darkens upon standing) Phenotype 1 (darkened skin) Phenotype 2 (darkened sclera of eye) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. (1&3): From G. Pierrard, A. Nikkels. April 5, 2001, A Medical Mystery. New England Journal of Medicine, 344: p. 1057. © 2001 Massachusetts Medical Society. All rights reserved; (2): British Journal of Ophthalmology 1999; 83:680 © by BMJ Publishing Group Ltd/http://www.bjophthalmol.com
  • 69. 4-69 Sex Linkage • Sex-linked traits – carried on the X and Y chromosomes, and therefore tend to be inherited by one sex more than the other • Recessive color blindness allele on X, no gene locus for trait on Y, so red-green color blindness more common in men (mother is carrier) Figure 4.21 X X X Y C c c Female (XX) Genotype Cc Normal vision Male (XY) Genotype c– Color blindness Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
  • 70. 4-70 Penetrance and Environmental Effects• Penetrance – the percentage of a population with a given genotype that actually exhibits the predicted phenotype – polydacyly • Role of environment – no gene can produce a phenotypic effect without nutritional and other environmental input – need both the genetic recipe and the ingredients – brown eye color requires phenylalanine from diet to produce melanin pigment Figure 4.22 Phenotype (brown eyes) From diet (environment) Phenvlalanine (amino acid) Melanin (pigment) Genes (DNA) Tyrosine Enzyme 1 mRNA 1 Enzyme 2 mRNA 2
  • 71. 4-71 Dominant and Recessive Alleles at the Population Level • common misconception that dominant alleles must be more common in the gene pool than recessive alleles • some recessive alleles, blood type O, are the most common • some dominant alleles, polydactyly and blood type AB, are rare in the population
  • 72. 4-72 Cancer • benign tumor – slow growth – contained in fibrous capsule – will not metastasize – usually easy to treat • malignant tumor – called cancer – fast growing – metastasize – give off cells that seed the growth of multiple tumors elsewhere • Oncology – medical specialty that deals with both benign and malignant tumors • tumor angiogenesis – ingrowth of blood vessels stimulated by energy-hungry tumors
  • 73. 4-73 Cancer • Cancers are named for the tissue of origin – carcinomas – originate in epithelial tissue – lymphomas – originate in lymph nodes – melanomas – originate in pigment cells of epidermis (melanocytes) – leukemias – in blood forming tissues – sarcomas – in bone, other connective tissue, or muscle • carcinogen – environmental cancer-causing agents
  • 74. 4-74 Causes of Cancer • Carcinogens – environmental cancer- causing agents – radiation – ultraviolet rays, X-rays – chemical - cigarette tar, food preservatives, industrial chemicals – viruses – human papilloma virus, hepatitis C, and type 2 herpes simplex • 5 -10% of cancers are hereditary • carcinogens trigger gene mutations
  • 75. 4-75 Malignant Tumor Genes • Oncogenes – causes cell division to accelerate out of control • excessive production of growth factors that stimulate mitosis • the production for excessive growth factor receptors • Tumor suppressor genes – inhibit development of cancer • oppose action of oncogenes • codes for DNA repairing enzymes
  • 76. 4-76 Lethal Effects of Cancer • replace functional tissue in vital organs • steal nutrients from the rest of the body – cachexia – severe wasting away of depleted tissues • weaken one’s immunity • open the door for opportunistic infections • often invade blood vessels, lung tissue, or brain tissue

Editor's Notes

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