17 ge lecture presentation

BIOLOGY
Campbell • Reece • Urry • Cain • Wasserman • Minorsky • Jackson
© 2015 Pearson Education Ltd
TENTH EDITION
Global Edition
Lecture Presentation by
Nicole Tunbridge and
Kathleen Fitzpatrick
17
Expression of
Genes

The Flow of Genetic Information
a) The information content of genes is in the specific
sequences of nucleotides
b) The DNA inherited by an organism leads to
specific traits by dictating the synthesis of proteins
c) Proteins are the links between genotype and
phenotype
d)Gene expression, the process by which DNA directs
protein synthesis, includes two stages: transcription
and translation

Figure 17.1

Figure 17.1a
An albino racoon

Concept 17.1: Genes specify proteins via transcription
and translation
a) How was the fundamental relationship between
genes and proteins discovered?

Evidence from the Study of Metabolic Defects
a) In 1902, British physician Archibald Garrod first
suggested that genes dictate phenotypes through
enzymes that catalyze specific chemical reactions
b) He thought symptoms of an inherited disease reflect
an inability to synthesize a certain enzyme
c) Cells synthesize and degrade molecules in a series of
steps, a metabolic pathway

Nutritional Mutants in Neurospora: Scientific Inquiry
a) George Beadle and Edward Tatum exposed bread
mold to X-rays, creating mutants that were unable to
survive on minimal media
b) Using crosses, they and their coworkers identified
three classes of arginine-deficient mutants, each
lacking a different enzyme necessary for synthesizing
arginine
c) They developed a one gene–one enzyme hypothesis,
which states that each gene dictates production of a
specific enzyme

Figure 17.2
Precursor
Enzyme A
Enzyme B
Enzyme C
Ornithine
Citrulline
Arginine
No growth:
Mutant cells
cannot grow
and divide
Growth:
Wild-type
cells growing
and dividing
Control: Minimal medium
Results Table
Wild type
Minimal
medium
(MM)
(control)
MM +
ornithine
MM +
citrulline
MM +
arginine
(control)
Summary
of results
Can grow with
or without any
supplements
Gene
(codes for
enzyme) Wild type
Precursor
Ornithine
Gene A
Gene B
Gene C
Enzyme A
Enzyme B
Enzyme C
Enzyme A
Enzyme B
Enzyme C
Citrulline
Arginine
Precursor
Ornithine
Citrulline
Arginine
Precursor
Ornithine
Citrulline
Arginine
Precursor
Ornithine
Citrulline
Arginine
Enzyme A
Enzyme B
Enzyme C
Enzyme A
Enzyme B
Enzyme C
Class I mutants
(mutation in
gene A)
Class II mutants
(mutation in
gene B)
Class III mutants
(mutation in
gene C)
Can grow on
ornithine,
citrulline,
or arginine
Can grow only
on citrulline or
arginine
Require arginine
to grow
Class I mutants Class II mutants Class III mutants
Classes of Neurospora crassa
Condition

Figure 17.2a
Precursor
Enzyme A
Enzyme B
Enzyme C
Ornithine
Citrulline
Arginine

Figure 17.2b
No growth:
Mutant cells
cannot grow
and divide
Growth:
Wild-type
cells growing
and dividing
Control: Minimal medium

Figure 17.2c
Results Table
Wild type
Minimal
medium
(MM)
(control)
MM +
ornithine
MM +
citrulline
MM +
arginine
(control)
Summary
of results
Can grow with
or without any
supplements
Can grow on
ornithine,
citrulline,
or arginine
Can grow only
on citrulline or
arginine
Require arginine
to grow
Class I mutants Class II mutants Class III mutants
Classes of Neurospora crassa
Condition

Figure 17.2d
Gene
(codes for
enzyme) Wild type
Precursor
Ornithine
Gene A
Gene B
Gene C
Enzyme A
Enzyme B
Enzyme C
Citrulline
Arginine
Class I mutants
(mutation in
gene A)
Class II mutants
(mutation in
gene B)
Class III mutants
(mutation in
gene C)
Precursor
Ornithine
Enzyme A
Enzyme B
Enzyme C
Citrulline
Arginine
Precursor
Ornithine
Enzyme A
Enzyme B
Enzyme C
Citrulline
Arginine
Precursor
Ornithine
Enzyme A
Enzyme B
Enzyme C
Citrulline
Arginine

The Products of Gene Expression:
A Developing Story
a) Some proteins aren’t enzymes, so researchers later
revised the hypothesis: one gene–one protein
b) Many proteins are composed of several polypeptides,
each of which has its own gene
c) Therefore, Beadle and Tatum’s hypothesis is now
restated as the one gene–one polypeptide hypothesis
d) It is common to refer to gene products as proteins
rather than polypeptides

Basic Principles of Transcription and Translation
a) RNA is the bridge between genes and the proteins for
which they code
b)Transcription is the synthesis of RNA using
information in DNA
c) Transcription produces messenger RNA (mRNA)
d)Translation is the synthesis of a polypeptide, using
information in the mRNA
e) Ribosomes are the sites of translation

a) In prokaryotes, translation of mRNA can begin before
transcription has finished
b) In a eukaryotic cell, the nuclear envelope separates
transcription from translation
c) Eukaryotic RNA transcripts are modified through RNA
processing to yield the finished mRNA

Figure 17.3
Nuclear
envelope
CYTOPLASM
DNA
Pre-mRNA
mRNA
RibosomeTRANSLATION
(b) Eukaryotic cell
NUCLEUS
RNA PROCESSING
TRANSCRIPTION
(a) Bacterial cell
Polypeptide
DNA
mRNA
Ribosome
CYTOPLASM
TRANSCRIPTION
TRANSLATION
Polypeptide

Figure 17.3a-1
(a) Bacterial cell
DNA
mRNACYTOPLASM
TRANSCRIPTION

Figure 17.3a-2
(a) Bacterial cell
Polypeptide
DNA
mRNA
Ribosome
CYTOPLASM
TRANSCRIPTION
TRANSLATION

Figure 17.3b-1
Nuclear
envelope
CYTOPLASM
DNA
Pre-mRNA
(b) Eukaryotic cell
NUCLEUS
TRANSCRIPTION

Figure 17.3b-2
Nuclear
envelope
CYTOPLASM
DNA
Pre-mRNA
mRNA
(b) Eukaryotic cell
NUCLEUS
RNA PROCESSING
TRANSCRIPTION

Figure 17.3b-3
Nuclear
envelope
CYTOPLASM
DNA
Pre-mRNA
mRNA
RibosomeTRANSLATION
(b) Eukaryotic cell
NUCLEUS
RNA PROCESSING
TRANSCRIPTION
Polypeptide

a) A primary transcript is the initial RNA transcript from
any gene prior to processing
b) The central dogma is the concept that cells are
governed by a cellular chain of command:
DNA → RNA → protein

Figure 17.UN01
DNA RNA Protein

The Genetic Code
a) How are the instructions for assembling amino acids
into proteins encoded into DNA?
b) There are 20 amino acids, but there are only four
nucleotide bases in DNA
c) How many nucleotides correspond to an
amino acid?

Codons: Triplets of Nucleotides
a) The flow of information from gene to protein is based
on a triplet code: a series of nonoverlapping, three-
nucleotide words
b) The words of a gene are transcribed into
complementary nonoverlapping three-nucleotide
words of mRNA
c) These words are then translated into a chain of amino
acids, forming a polypeptide

Figure 17.4
A C C A A A C C G A G T
ACTTTT CGGGGT
U G G U U U G G C CU A
SerGlyPheTrp
Codon
TRANSLATION
TRANSCRIPTION
Protein
mRNA 5′
5′
3′
Amino acid
DNA
template
strand
5′
3′
3′

a) During transcription, one of the two DNA strands,
called the template strand, provides a template for
ordering the sequence of complementary nucleotides
in an RNA transcript
b) The template strand is always the same strand
for a given gene

a) During translation, the mRNA base triplets, called
codons, are read in the 5′ → 3′ direction
b) Each codon specifies the amino acid (one of 20)
to be placed at the corresponding position along
a polypeptide

Cracking the Code
a) All 64 codons were deciphered by the mid-1960s
b) Of the 64 triplets, 61 code for amino acids; 3 triplets
are “stop” signals to end translation
c) The genetic code is redundant (more than one codon
may specify a particular amino acid) but
not ambiguous; no codon specifies more than
one amino acid
d) Codons must be read in the correct reading frame
(correct groupings) in order for the specified
polypeptide to be produced

Figure 17.5
Second mRNA base
ThirdmRNAbase(3′endofcodon)
FirstmRNAbase(5′endofcodon)
UUU
UUC
UUA
UUG
Phe
Leu
Leu
Ile
Val
CUU
CUC
CUA
CUG
AUU
AUC
AUA
AUG
GUU
GUC
GUA
GUG
UCU
UCC
UCA
UCG
CCU
CCC
CCA
CCG
ACU
ACC
ACA
ACG
GCU
GCC
GCA
GCG GAG
GAA
GAC
GAU
AAG
AAA
AAC
AAU
CAG
CAA
CAC
CAU
Ser
Pro
Thr
Ala
Glu
Asp
Lys
Asn
Gln
His
Tyr Cys
Trp
Arg
Ser
Arg
Gly
GGG
GGA
GGC
GGU
AGG
AGA
AGC
AGU
CGG
CGA
CGC
CGU
UGG
UGA
UGC
UGUUAU
UAC
UAA
UAG Stop
Stop Stop
Met or
start
U
C
A
G
U
C
A
G
U
C
A
G
U
C
A
G
U C A G
G
A
C
U

Evolution of the Genetic Code
a) The genetic code is nearly universal, shared by the
simplest bacteria to the most complex animals
b) Genes can be transcribed and translated after being
transplanted from one species to another

Figure 17.6
Pig expressing a jellyfish
gene
(b)Tobacco plant expressing
a firefly gene
(a)

Figure 17.6a
Tobacco plant expressing
a firefly gene
(a)

Figure 17.6b
Pig expressing a jellyfish
gene
(b)

Concept 17.2: Transcription is the DNA-directed
synthesis of RNA: A closer look
a) Transcription is the first stage of gene expression

Molecular Components of Transcription
a) RNA synthesis is catalyzed by RNA polymerase,
which pries the DNA strands apart and joins together
the RNA nucleotides
b) The RNA is complementary to the DNA template
strand
c) RNA polymerase does not need any primer
d) RNA synthesis follows the same base-pairing rules as
DNA, except that uracil substitutes for thymine

Figure 17.7-1
Promoter Transcription unit
RNA polymerase
Start point
1
Template strand of DNARNA
transcript
Unwound
DNA
Initiation
5′
3′
5′
3′
5′
3′
5′
3′

Figure 17.7-2
RNA polymerase
Start point
1
transcript
Unwound
DNA
Rewound
DNA
RNA
transcript
Direction of
transcription
(“downstream”)
Initiation
Elongation2
5′
3′
5′
5′
3′
5′
3′
5′
3′
3′
5′
3′
5′
3′

Figure 17.7-3
RNA polymerase
Start point
1
transcript
Unwound
DNA
Rewound
DNA
RNA
transcript
Direction of
transcription
(“downstream”)
Completed RNA transcript
Initiation
Elongation
Termination
2
3
5′
3′
5′
5′
3′
5′
3′
5′
3′
5′
5′
3′
3′
5′
3′
3′
5′
3′
5′
3′

Animation: Transcription

a) The DNA sequence where RNA polymerase attaches
is called the promoter; in bacteria, the sequence
signaling the end of transcription is called the
terminator
b) The stretch of DNA that is transcribed is called a
transcription unit

Synthesis of an RNA Transcript
a) The three stages of transcription
a)Initiation
b)Elongation
c)Termination

Figure 17.8
Promoter
Nontemplate strand
15′
3′
5′
3′
Start point
RNA polymerase II
Template
strand
TATA box
Transcription
factors
DNA
3′
5′
3′
5′
3′
5′
2
3
Transcription factors
RNA transcript
Transcription initiation complex
3′5′
5′
3′
A eukaryotic
promoter
Several
transcription
factors bind
to DNA.
Transcription
initiation
complex
forms.
T A T AAAA
TA A T T T T

Elongation of the RNA Strand
a) As RNA polymerase moves along the DNA, it
untwists the double helix, 10 to 20 bases at a time
b) Transcription progresses at a rate of 40 nucleotides
per second in eukaryotes
c) A gene can be transcribed simultaneously by several
RNA polymerases
d) Nucleotides are added to the 3′ end of the
growing RNA molecule

Figure 17.9
Nontemplate
strand of DNA
5′
3′
3′ end
5′
3′
5′
Direction of transcription
RNA
polymerase
Template
strand of DNA
Newly made
RNA
RNA nucleotides
A
A
A
A AA
G G T T
T
C CU
U
C CT

Concept 17.3: Eukaryotic cells modify RNA after
transcription
a) Enzymes in the eukaryotic nucleus modify pre-mRNA
(RNA processing) before the genetic messages are
dispatched to the cytoplasm
b) During RNA processing, both ends of the primary
transcript are usually altered
c) Also, usually certain interior sections of the molecule
are cut out, and the remaining parts spliced together

Alteration of mRNA Ends
a) Each end of a pre-mRNA molecule is modified in a
particular way
a)The 5′ end receives a modified nucleotide 5′ cap
b)The 3′ end gets a poly-A tail
b) These modifications share several functions
a)They seem to facilitate the export of mRNA to the
cytoplasm
b)They protect mRNA from hydrolytic enzymes
c)They help ribosomes attach to the 5′ end

Figure 17.10
A modified guanine
nucleotide added to
the 5′ end
Region that includes
protein-coding segments
5′
5′ Cap 5′ UTR
Start
codon
Stop
codon
G P P P
3′ UTR
3′
AAUAAA AAA AAA
Poly-A tail
Polyadenylation
signal
50–250 adenine
nucleotides added
to the 3′ end
…

Split Genes and RNA Splicing
a) Most eukaryotic genes and their RNA transcripts
have long noncoding stretches of nucleotides that lie
between coding regions
b) These noncoding regions are called intervening
sequences, or introns
c) The other regions are called exons because they are
eventually expressed, usually translated into amino
acid sequences
d)RNA splicing removes introns and joins exons,
creating an mRNA molecule with a continuous coding
sequence

Figure 17.11
Pre-mRNA Intron Intron
Introns cut out
and exons
spliced together
Poly-A tail5′ Cap
5′ Cap Poly-A tail
1–30 31–104 105–146
1–146
3′ UTR5′ UTR
Coding
segment
mRNA

Figure 17.12
Spliceosome Small RNAs
Exon 2
Cut-out
intron
Spliceosome
components
mRNA
Exon 1 Exon 2
Pre-mRNA
Exon 1
Intron
5′
5′

Concept 17.4: Translation is the RNA-directed
synthesis of a polypeptide: A closer look
a) Genetic information flows from mRNA to protein
through the process of translation

Molecular Components of Translation
a) A cell translates an mRNA message into protein with
the help of transfer RNA (tRNA)
b) tRNAs transfer amino acids to the growing
polypeptide in a ribosome
c) Translation is a complex process in terms of its
biochemistry and mechanics

Figure 17.14
Polypeptide
Amino
acids
tRNA with
amino acid
attached
Ribosome
tRNA
Anticodon
Codons
mRNA
5′
U U U UG G G G C
A A A
3′
Gly

The Structure and Function of Transfer RNA
a) Molecules of tRNA are not identical
a)Each carries a specific amino acid on one end
b)Each has an anticodon on the other end; the
anticodon base-pairs with a complementary codon on
mRNA

a) A tRNA molecule consists of a single RNA strand that
is only about 80 nucleotides long
b) Flattened into one plane to reveal its base pairing, a
tRNA molecule looks like a cloverleaf

a) Because of hydrogen bonds, tRNA actually twists and
folds into a three-dimensional molecule
b) tRNA is roughly L-shaped

Figure 17.15
Amino acid
attachment
site
Amino acid
attachment site
5′
3′
A
C
C
A
C
G
C
U
U
A
G
G
C
G
A
U
U
U
A
A
GAA CC
CU
*
**
C
G
G U UG
C
*
*
*
*C CU
A G
G
G
G
A
GAGC
C
C
*U
* G A
G
G
U
*
*
*
A
A
A
G
C
U
G
A
A
Hydrogen
bonds
Hydrogen
bonds
Anticodon Anticodon
Symbol used
in this book
Three-dimensional
structure
(a) Two-dimensional structure (b) (c)
Anticodon
5′3′
A A G
3′
5′

Figure 17.15a
Amino acid
attachment
site 5′
3′
Hydrogen
bonds
(a) Two-dimensional structure
Anticodon
A
C
C
A
C
C
G
G
C
U
U
A
A
G
G
A
U
U
U
AA GCC
C
A
* C CU
A G *
*
G
G
G
A
GAGC
*
*
*
*
*
U
G
C
C
C
A
G
A
C
U
G
A
A
A
*
*
*
U U
U
G G
C
* G A
G
G
U

Figure 17.15b
Amino acid
attachment site
Hydrogen
bonds
Anticodon Anticodon
Symbol used
in this book
Three-dimensional
structure
(b) (c)
5′3′
3′
5′
A A G

Video: Stick and Ribbon Rendering of a tRNA

Ribosomes
a) Ribosomes facilitate specific coupling of tRNA
anticodons with mRNA codons in protein synthesis
b) The two ribosomal subunits (large and small) are
made of proteins and ribosomal RNA (rRNA)
c) Bacterial and eukaryotic ribosomes are somewhat
similar but have significant differences: some
antibiotic drugs specifically target bacterial ribosomes
without harming eukaryotic ribosomes

Figure 17.17
Exit tunnel
Large
subunit
Small
subunit
mRNA 3′
5′
E P A
tRNA
molecules
Growing
polypeptide
(a) Computer model of functioning ribosome
Growing
polypeptide Next amino
acid to be
added to
polypeptide
chain
tRNA
3′
5′
mRNA
Amino end
Codons
E
(c) Schematic model with mRNA and tRNA(b) Schematic model showing binding sites
Small
subunit
Large
subunit
Exit tunnel
A site (Aminoacyl-
tRNA binding site)
P site (Peptidyl-tRNA
binding site)
E site
(Exit site)
mRNA
binding
site
E P A

Figure 17.17a
Exit tunnel
Large
subunit
Small
subunit
mRNA 3′
5′
E P A
tRNA
molecules
Growing
polypeptide
(a) Computer model of functioning ribosome

Figure 17.17b
(b) Schematic model showing binding sites
Small
subunit
Large
subunit
Exit tunnel
A site (Aminoacyl-
tRNA binding site)
P site (Peptidyl-tRNA
binding site)
E site
(Exit site)
mRNA
binding
site
E P A

Figure 17.17c
Growing
polypeptide
Next amino
acid to be
added to
polypeptide
chain
tRNA
3′
5′
mRNA
Amino end
Codons
E
(c) Schematic model with mRNA and tRNA

Elongation of the Polypeptide Chain
a) During elongation, amino acids are added one
by one to the C-terminus of the growing chain
b) Each addition involves proteins called elongation
factors and occurs in three steps: codon recognition,
peptide bond formation, and translocation
c) Energy expenditure occurs in the first and
third steps
d) Translation proceeds along the mRNA in a
5′ → 3′ direction

Figure 17.19-1
Amino end
of polypeptide
Codon
recognition
1
3′
5′
E
P A
sitesite
E
P A
mRNA
GTP
P iGDP +

Figure 17.19-2
Amino end
of polypeptide
Codon
recognition
1
3′
5′
E
P A
sitesite
E
P A
mRNA
GTP
P i
2
GDP +
E
P A
Peptide bond
formation

Figure 17.19-3
Amino end
of polypeptide
Codon
recognition
1
3′
5′
E
P A
sitesite
E
P A
mRNA
GTP
P i
23 GTP
P i
GDP +
GDP +
Translocation
E
P A
Peptide bond
formation
E
P A
Ribosome ready for
next aminoacyl tRNA

Termination of Translation
a) Termination occurs when a stop codon in the mRNA
reaches the A site of the ribosome
b) The A site accepts a protein called a release factor
c) The release factor causes the addition of a water
molecule instead of an amino acid
d) This reaction releases the polypeptide, and the
translation assembly comes apart

Figure 17.20-1
1
Release
factor
3′
5′
Stop codon
(UAG, UAA, or UGA)
Ribosome reaches a
stop codon on mRNA.

Figure 17.20-2
1 2
Release
factor
3′
5′5′
3′
Stop codon
(UAG, UAA, or UGA)
Ribosome reaches a
stop codon on mRNA.
Release factor
promotes hydrolysis.
Free
polypeptide

Figure 17.20-3
31 2
Release
factor
3′
5′5′
3′
Stop codon
(UAG, UAA, or UGA)
Ribosome reaches a
stop codon on mRNA.
Release factor
promotes hydrolysis.
Ribosomal subunits
and other components
dissociate.
Free
polypeptide
3′
5′
2 GTP
2 GDP + 2 P i

Completing and Targeting the Functional Protein
a) Often translation is not sufficient to make a functional
protein
b) Polypeptide chains are modified after translation or
targeted to specific sites in the cell

Protein Folding and Post-Translational Modifications
a) During its synthesis, a polypeptide chain begins to
coil and fold spontaneously to form a protein with a
specific shape—a three-dimensional molecule with
secondary and tertiary structure
b) A gene determines primary structure, and primary
structure in turn determines shape
c) Post-translational modifications may be required
before the protein can begin doing its particular job in
the cell

Targeting Polypeptides to Specific Locations
a) Two populations of ribosomes are evident in cells:
free ribosomes (in the cytosol) and bound ribosomes
(attached to the ER)
b) Free ribosomes mostly synthesize proteins that
function in the cytosol
c) Bound ribosomes make proteins of the
endomembrane system and proteins that are
secreted from the cell
d) Ribosomes are identical and can switch from free to
bound

a) In eukaryotes, the nuclear envelop separates the
processes of transcription and translation
b) RNA undergoes processes before leaving
the nucleus

Figure 17.24
DNA
RNA
polymerase
RNA transcript
(pre-mRNA)
Intron
Exon
Aminoacyl-tRNA
synthetase
Amino
acid
tRNA
Aminoacyl
(charged)
tRNA
mRNA
CYTOPLASM
NUCLEUS
RNA
transcript
3′
5′
TRANSCRIPTION
RNA
PROCESSING
AMINO ACID
ACTIVATION
TRANSLATION
Ribosomal
subunits
E
P
A
E A
U U U U U GGG
A A A Anticodon
Codon
Ribosome
3′
A

Figure 17.24a
TRANSCRIPTION
DNA
RNA
polymerase
RNA transcript
(pre-mRNA)
Intron
Exon
Aminoacyl-tRNA
synthetase
Amino
acid
tRNA
AMINO ACID
ACTIVATION
Aminoacyl
(charged)
tRNA
mRNA
CYTOPLASM
NUCLEUS
RNA
PROCESSING
RNA
transcript
3′
5′

Figure 17.24b
mRNA
Growing
polypeptide
Ribosomal
subunits
Aminoacyl
(charged)
tRNA
Anticodon
TRANSLATION
AE
U U U U U GGG A
AAA
Codon
Ribosome
E P
A
3′

BioFlix: Protein Synthesis

Animation: Translation

Figure 17.UN10

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