Ch2 chromosome structure (In brief)

SBB 1054 Genetics
1
Prepared by Pratheep Sandrasaigaran
Lecturer at Manipal International University

IMPORTANT DATES..!
1. Test 1- Week 6 (20th – 24th MAC 2017)
2. Test 2- Week 11 ( 24th – 28th APR 2017)
3. Assignment 2- 3rd May 2017 (Week 12)
4. Study Break- Week 15 (22nd – 26th MAY
2017)
5. Exam- Week 16 & 17 (29th MAY – 9th JUN
2017)
Figure taken from Internet
2

Lecturer at Manipal International University
2.0 Chromosome
structure
3

Figure taken from Internet
• Define the basis of
“Central Dogma”
• Chromosome Structure
and DNA packaging
• Understanding genetics
Code
• Compare the nuclear DNA
with
- Mitochondrial DNA
- Chloroplast DNA
By the end of this
chapter you should
be able to:
4

2.1 The Central Dogma
5

The “Central Dogma”
• Central dogma of:
- Molecular Biology
- Genetics
• Primarily to direct the production of proteins.
• Each DNA molecule can carry thousands of genes which
in return plan for building a particular protein, or part
of a particular protein.
6

The “Central Dogma”
• The type of protein produced
decides everything about a cell and
eventually the human body as a
whole:
- The color of the hair and skin
- Propensity to certain diseases
- Unique ability
- Fat, thin, tall, short, bold, hairy….
7
Diagram adopted from: Human Genetics: Concepts
and Applications, 9th Edition, Lewis

8
a. Transcription
• Transcription occurs in three stages:
- Initiation
- Elongation
- Termination.

a. Transcription
• Initiation is the control point that determines which genes are
transcribed.
9

a. Transcription
• RNA nucleotides are added during elongation
10

a. Transcription
• A terminator sequence in the gene signals the end of
transcription.
11

a. Transcription
12
• Many identical copies of RNA are transcribed simultaneously.
• Usually 100 or more DNA bases lie between RNA polymerases.

13
• The double helix become detached from
each other and exposes the genes
(sequences of bases)
• Just one of the strands serves as a
template to produce the RNA strand.
• RNA polymerase binds to the DNA and
moves along it, attaching free nucleotides
making strand of mRNA.
• What is the sequence of mRNA that you
expect?
a. Transcription
Diagram adopted from
The Facts On File Illustrated Guide to the Human Body: Cells and Genetics

14
• The mRNA consists of a single chain of
nucleotides with that the base uracil
(U) occurs instead of thymine (T).
• mRNA carries the genetic code, in the
form of base triplets, or codons.
• What happen to the other strand of
DNA?
• They are not transcribed
a. Transcription

i. Transcription- mRNA Processing
15

i. Transcription- Alternative mRNA
Splicing
16

ii. Ribosomal RNA
17

iii. Transfer RNA
18

Types of RNA
19

TEST YOUR KNOWLEDGE 1
20

21
1. DNA is the genetic code.
2. Transcription is needed to only synthesis mRNA.
3. Both strand of DNA involve in the transciption
process.
4. Guanine and Thymine are the purine component
of the nitrogenous bases that are found in DNA.
5. Instead of Adenine, Uracil is replaced in the RNA
molecule.
6. RNA is a single strand molecule and information
is read in triplet code of codon.

b. Translation
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• Initiation
• Elongation
• Termination
Diagram adopted from internet

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Diagram adopted from internet
Termination

24

25

2.2 Chromosome Structure and
DNA packaging
26

Introduction
• A gene provides each cell in the body with instructions
for making (synthesizing) a particular protein (or part of
a protein).
• Each protein, either directly or indirectly, determines a
particular measurable characteristic or cell function.
• Proteins are formed either as enzymes or structural
proteins.
27

Where and when genes are active?
• Except for sex cells, all human cells contain a set
of approximately 25,000 genes.
• Which genes manufacture proteins depends on
where in the body a cell is situated.
• Where is the gene is located in the body and
how does such vast information can be
coordinated?
28

Basic structure of a chromosome
• Genes are organized into structures called
chromosomes, which serve as vehicles for
transmitting genetic information.
• In eukaryotes, nuclear chromosomes are
packaged by proteins into a condensed structure
called chromatin.
• Two types of chromatin can be seen with
electron microscopy.
- Heterochromatin
- Euchromatin
29

a. Heterochromatin
• Is an electron dense and distributed around the
periphery of the nucleus and in discrete masses
within the nucleus.
• The DNA is in close association with
nucleoproteins, and it is not active in RNA
synthesis.
• Serves as structural purpose during the
chromosomal stages.
30

b. Euchromatin
• Is an electron lucent and represents DNA that is
actually or potentially active in RNA synthesis.
• Example, the protein coding gene region
31

Basic structure of a chromosome
• Chromosomes are usually diffuse, threadlike
structures, not easily distinguishable from each
other within the nucleus.
• Just before and during cell division the
chromosomes condense (become shorter and
fatter), so that their different shapes become
visible under a microscope.
• The chromosome also copies itself, making two
identical chromatids that meet at a narrow point
called the centromere.
32
2 type of
Chromosome
• Sex
• Autosomal

a. Sex chromosomes
• These chromosomes determine gender.
• Human cells contain two sex chromosomes.
• If you’re female, you have two X chromosomes,
and if you’re male, you have an X and a Y
chromosome.
33

b. Autosomal chromosomes
• Autosomal simply refers to non-sex
chromosomes.
• So, sticking with the human example, do the
math, and you can see that humans have 44
autosomal chromosomes.
• In humans, chromosomes come in pairs.
• How many pairs of chromosome that you will
find in your cells?
34

The 46 Human chromosomes
35
Diagram adopted from Genetics for Dummies, Tara. R. R

Packing DNA into Small Spaces
• Human cells contain 6 billion base pairs of DNA, which
would measure 1.8 meters stretched end to end.
• In the nucleus of a normal human cell, there are 46
chromosomes each containing 48–240 million bases of
DNA
• Smallest human chromosome, the DNA would stretch
14,000 times the length of the nucleus.
• Virtually all of the genomic DNA is distributed among
the 23 chromosomes that reside in the cellular nucleus
36

Packing DNA into Small Spaces
• However, the average nucleus has a diameter of
approximately 5 μm.
• Tightly packaging is required to fit DNA into such small
spaces.
• How it is possible?
• Complexed with histone protein and tightly packed
within the nucleus of the cell
37

Histone protein
• Positively charged proteins of five
major types: H1, H2A, H2B, H3, and
H4.
• The positive charges attract the
negative charges on the
phosphates of DNA and holds the
DNA in contact with the histones
38
Diagram adopted from Genetics for Genetics- Conceptual
Approach, Benjamin A. Pierce

39
Chromatosome
consists of a
nucleosome plus the
H1 histone
Diagram adopted from Genetics for Genetics- Conceptual
Approach, Benjamin A. Pierce

40
Diagram adopted from Concepts of Genetics, Klug.W.S., 10th ED
Chromatin fibers Metaphase chromosomes
Metaphase chromosomes

41

42
1. Heterochromatin are the dense region of
chromosome that involve in the RNA
transcriptions.
2. Human autosomal chromosome accounts 46 all
together.
3. Chromosome has both DNA and Protein.
4. H1, H2, H3 and H4 are the protein parts of
histone protein.

43
Chromatid
Centromere
Telomeres
Heterochromatin
Euchromatin
Diagram adopted from Internet

Centromeres
• Each metaphase chromosome is composed of two
identical sister chromatids.
• Chromatids are connected at a central region called the
centromere
• Centromeres consist of hundreds of kilobases of
repetitive DNA and are responsible for the movement of
chromosomes at cell division.
• Each centromere divides the chromosome into short (p)
and long (q) arms.
44

Anatomy of a chromosome showing
the three shapes
45
Diagram adopted from Crash course: Cell Biology and Genetics, 4th ED

46
p arm
Centromere
Diagram adopted from Internet
q arm
A
B
C

Diagram adopted from Concepts of Genetics, Klug.W.S., 10th ED
Centromere locations and
designations of chromosomes
47

Telomeres
• The ends of chromosomes are
protected by DNA structures
called telomeres.
• Telomeres are tandem repeats
of the hexameric sequence
‘TTAGGG’ and loops back on
itself to form the T-loop.
48
Crash course: Cell Biology and Genetics, 4th ED

Telomeres
• Telomeres have several functions in preserving
chromosome stability:
• Preventing abnormal end-to-end fusion of
chromosomes
• Protecting the ends of chromosomes from
degradation
• Ensuring complete DNA replication
• Having a role in chromosome pairing during meiosis.
49

50

51
Chromosome structure- Summary
Diagram adopted from Genetics for Dummies, Tara. R. R

52

53

Karyotype
Analysis
54
1. Look at the following Karyotype; is
this organism male or female?
2. How many chromosomes are found in
the somatic cells of this organism?
3. How many chromosomes are found in
sperm cells from this organism?
4. How many pairs of chromosomes
does this organism’s karyotype
contain?
5. How many chromosomes would be
found in this organism’s skin cells?
6. How many chromosomes would be
found in this organism’s egg cells?
Figure adopted from Internet

Define these terms
• Gene
- A discrete unit of hereditary information consisting
of a specific nucleotide sequence in DNA (or RNA, in
some viruses)
• Locus
- A specific place along the length of a chromosome
where a given gene is located
• Gamete
- A haploid reproductive cell, such as an egg or
sperm. Gametes unite during sexual reproduction
to produce a diploid zygote.
55

Define these terms
• Male gamete
- Sperm
• Female gamete
- Eggs
• Asexual reproduction
- The generation of offspring from a single parent that
occurs without the fusion of gametes (by budding,
division of a single cell, or division of the entire
organism into two or more parts).
- In most cases, the offspring are genetically identical
to the parent.
56

1. What is a somatic cell? Give examples of two human
somatic cell types
- A somatic cell is any cell in a multicellular organism
except a sperm or egg or their precursors. Examples
may vary but could include bone cells, skin cells,
blood cells, etc.
2. How does a somatic cell compare to a gamete in
terms of chromosome number?
- Unlike somatic cells, gametes contain a single set of
chromosomes.
- Such cells are called haploid cells, and each has a
haploid number of chromosomes (n).
- For humans, the haploid number is 23.
57

3. Can you distinguish allele, gene and locus?
- Homologous chromosomes can have different
alleles on them.
- Alleles are variants of the same gene that occur on
the same place on a chromosome. (Through a
mutation, they are different).
- A locus refers to the location on the chromosome
where the gene is found.
58

59
• Human genome encodes app. 100,000 proteins.
• Coding portions of a gene app. 2000
• Coding sequences are interrupted by from 1 to 50 noncoding sequences

2.3 Understanding genetics Code
60
- Identify the chemical components of DNA.
- Assemble the double helix: The structure of
DNA.
- Genetic code

61
- Identify the chemical components of DNA.

• Nucleic acids are produced from
nucleotide polymerization.
• During synthesis a series of nucleic
acid condensation reactions occur
between phosphate and sugar
groups.
• A-T-C-G (DNA) A-U-C-G (RNA)
62
Nucleic Acid

63
Nucleic Acid
• Pentose sugar
• Nitrogenous
base
• Phosphates
• Pentose sugar
• Nitrogenous
base
Nucleotides
Nucleosides
Phosphates Nucleosides+ Nucleotides=

64
Nucleic Acid
N-glycosidic
bond
Phosphodiester
bonds

65
DNA vs RNA

66
DNA vs RNA- Sugar
Sugar:
• RNA – ribose (OH)
• DNA – deoxyribose (H)

67
DNA vs RNA- Sugar
A B
• The nucleotide bases in nucleic acids
contain nitrogen derived from either
purines or pyrimidines.
• Purines (Double ring)
• Adenine
• Guanine
• Pyrimidines (Single ring)
• Cytosine
• Thymine*
• Uracil*

68
DNA vs RNA
PURINES PYRIMIDINES

• Discovered by Watson and Crick in 1953
• Base composition analysis of hydrolyzed samples of
DNA
• X-ray diffraction studies of DNA.
• Adenine and thymine pair via two hydrogen bonds
between opposing strands.
• Guanine and cytosine pair via three hydrogen bonds.
• Base pairing results in two complementary
polynucleotides, which run antiparallel to each other.
69
DNA is double helix

70
DNA is double helix

71
The Road to the Double Helix

• Messenger RNA
• Carries genetic information from the nucleus into the
cytoplasm.
• In eukaryotes, it is derived by splicing the initial RNA
transcript (heteronuclear RNA that holds introns)
72
RNA is Single strand- mRNA

73
RNA is Single strand- mRNA
A A A A A A A A

• Transfer RNA
• Linear molecule with an average of 76 nucleotides
• Exhibits extensive intramolecular base pairing, giving it
a ‘clover-leaf’-shaped secondary structure
• Carries specific amino acids to the site of protein
synthesis.
74
RNA is Single strand- tRNA

75
RNA is Single strand- tRNA
• Terminal CCA group
can accept a specific
amino acid.
• anticodon arm,
recognizes the
corresponding mRNA
codon.
• Specific base pairing
within the five arms
helps to maintain the
secondary structure.

• Ribosomal RNA (ribosomal component)
• In a eukaryotic cell each ribosome consists of two
unequal subunits, made up of proteins and RNA, called
the S (small) and L (large) subunits held together by
magnesium ions.
• The RNA molecules undergo extensive intramolecular
base pairing, which determines the ribosomal
structure.
76
RNA is Single strand- rRNA

77

78
1. What are nucleic acids?
• DNA and RNA, the nucleic acids, are the molecules responsible for the
hereditary information that commands the protein synthesis in living
beings.
2. What are the units nucleic acids constituted? What
are the chemical entities that compose that unit?
• Nucleic acids are formed by sequences of nucleotides
• Nucleotides are constituted by one molecule of sugar (deoxyribose in
DNA and ribose in RNA) bound to one molecule of phosphate and to one
nitrogen-containing base
3. Which two groups can the nitrogen-containing
bases that form DNA and RNA be classified?
• The nitrogen-containing bases that form DNA and RNA are classified as
pyrimidine and purine bases

4. What is the criterion used in purine-pyrimidine
classification?
• Cytosine, thymine and uracil, have only one nitrogenized carbon ring.
The others, adenine and guanine, have two nitrogenized associated
carbon rings
5. Which type of chemical bond maintains the
pairing of each chain in the DNA molecule?
• To form the DNA molecule, purine bases bind to pyrimidine bases by
intermolecular bonds called hydrogen bonds. Hydrogen bonds occur
when there is hydrogen near one of these electronegative elements:
fluorine, oxygen or nitrogen
79

6. What is the completing sequence of nitrogen-
containing bases for a AGCCGTTAAC fragment of
a DNA chain?
7. What are the three main types of RNA?
• Messenger RNA, or mRNA, transfer RNA, or tRNA, and ribosomal RNA,
or rRNA, are the three main types of RNA.
• TCGGCAATTG
8. What is meant by heterogeneous RNA?
• The newly formed RNA molecule, a precursor of mRNA, is called
heterogeneous RNA (hnRNA). The heterogenous RNA bears portions
called introns and portions called exons. The hnRNA is processed in
many chemical steps, introns are removed and mRNA is created formed
only of exons, the biologically active nucleotide sequences
80

9. Concerning their biological function what is the
difference between DNA and RNA?
10. Is there any situation in which DNA is made
based on a RNA template?
• The process in which DNA is synthesized having as template a RNA chain
is called reverse transcription. In cells infected by retroviruses (RNA
viruses, like the AIDS or SARS viruses) reverse transcription occurs and
DNA is made from information contained in the viral RNA. Viral RNA
within the host cell produces DNA with the help of an enzyme called
reverse transcriptase.
• DNA is the source of information for RNA production (transcription) and
thus for protein synthesis. DNA is still the basis of heredity due to its
replication capability
81

82
- Assemble the double helix: The structure of
DNA.

• Between 1949 and 1953, Erwin Chargaff and his
colleagues used chromatographic methods to separate
the four bases in DNA samples from various organisms.
83
Evidence 1: Base-Composition Studies

• The amount of adenine residues is proportional to the
amount of thymine residues in DNA.
• The amount of guanine residues is proportional to the
amount of cytosine residues.
• The sum of the purines (A + G) equals the sum of the
pyrimidines (C + T).
• The percentage of (G + C) does not necessarily equal
the percentage of (A + T).
84
What can be derived from Erwin
Chargaff?

• When fibers of a DNA molecule are subjected to X-ray
bombardment, the X rays scatter (diffract) in a pattern
that depends on the molecule’s atomic
• William Astbury (1938) detected a periodicity 3.4
angstroms (3.4-Å)
• Bases were stacked like coins on top of one another.
85
Evidence 2: X-Ray Diffraction Analysis

86
Evidence 2: X-Ray Diffraction Analysis
• Between 1950 and 1953, Rosalind Franklin suggested
that the structure of DNA was some sort of helix.
• X-ray diffraction photograph by
Rosalind Franklin using the purified
DNA fibers.
Diagram adopted from William S and Klug. Concept of Genetics, 10th ED

87
The Watson–Crick Model
• Two long polynucleotide chains are coiled
around a central axis, forming a right-handed
double helix.
• The two chains are antiparallel; that is, their
C-5’-to-C-3’orientations run in opposite
directions.
• The bases of both chains are flat structures
lying perpendicular to the axis; they are
“stacked” on one another, 3.4 Å (0.34 nm)
apart, on the inside of the double helix.
Diagram adopted from Crash course: Cell Biology and
Genetics, 4th ED

88
• The nitrogenous bases of opposite chains are
paired as the result of the formation of
hydrogen bonds
• In DNA, only A -T and G-C pairs occur.
• Each complete turn of the helix is 34 Å (3.4
nm) long; thus, each turn of the helix is the
length of a series of 10 base pairs.
Genetics, 4th ED

89
• A larger major groove alternating with a
smaller minor groove winds along the length
of the molecule.
• The double helix has a diameter of 20 Å (2.0
nm).
Genetics, 4th ED

90
DNA and
Inheritance
• The structure of DNA is a linear
sequence of deoxyribonucleotides.
• There are regions within the DNA
that contain protein-coding genes.
• How is the information within DNA
decoded for translation of proteins?
• Central dogma – flow of information
from?

91

92
A
B
C
D
E

93
1. Who were James Watson, Francis Crick and
Maurice Wilkins
• Watson (North American), Crick (British) and Wilkins (New Zealander)
were the discoverers of the molecular structure of DNA, the double helix
made of two polynucleotide chains paired by their nitrogen-containing
bases. They won the Nobel prize in Medicine in 1962 for the discovery.
2. What can you conclude from the table below?

94
- Genetic code

95
Deciphering the Code
• Marshall Nirenberg and Heinrich Matthaei
at the National Institutes of Health used a
precise and logical series of experiments to
“crack the code”.
• They were among the first to characterize
specific coding sequences.
• Made possible by advancements that:
• Allowed protein synthesis in vitro
• Synthesizing RNA strands in vitro
Diagram adopted from Human Genetics
concepts and Application 9th ed

96
Deciphering the Code
https://profiles.nlm.nih.gov/ps/access/JJBCBS.pdf
https://profiles.nlm.nih.gov/ps/access/JJBCBR.pdf

8 Characteristics of the Genetic Code
1. Written in linear form of ribonucleotide bases
(mRNA).
2. Each word consists of 3 ribonucleotide letters
which (triplet code- codon) specifies one amino
acid.
3. The code is non-overlapping
4. The code is degenerate, given amino acid can be
specified by more than one triplet codon.
97

The Genetic code
Diagram adopted from Internet source
98

5. The code is commaless; Once translation of
mRNA begins, the codons are read one after the
other with no breaks between them (until a stop
signal is reached).
6. The code contains 1 start and 3 stop codons
7. The code is (nearly) universal. With only minor
exceptions, a single coding dictionary is used by
almost all viruses, prokaryotes, archaea, and
eukaryotes.
8. The code is unambiguous- each triplet specifies
only a single amino acid.
8 Characteristics of the Genetic Code
99

The Genetic Code
Uses Ribonucleotide
Bases as “Letters”
• Even though genetic information is
stored in DNA, the code that is
translated into proteins resides in RNA.
• How only four nucleotides could
specify 20 the amino acids?
100

• List down the characteristics of DNA that you
know?
• List down the characteristics of RNA that you
know?
• Explain how information is passed from DNA
to the genetic code?
• Explain how Gene is translated?
101

2. Evidence for the Triplet Code
• How Many RNA Bases Specify One amino acid
code; 20 amino acids code?
• If a codon consisted of only one mRNA base?
• Two base, for example, provides only 16 unique
code words (42). Not enough..!
• A triplet code yields 64 words (43) and therefore is
sufficient for the 20 amino acids.
• A four-letter code (44), which would specify 256
words.
Theory
102

• Experimental work of Francis Crick, Leslie Barnett,
Brenner and R. J. Watts-Tobin provided the solid
evidence for a triplet code.
• The insertion of a single nucleotide shifts all
subsequent triplets out of the reading frame
(frameshift mutations).
• They exposed the chemicals that add or remove
one, two, or three bases at the rII locus of
bacteriophage T4 and examine their reproduction
on E. coli K12 (viral gene is well known).
103

• Alteration in DNA sequence happen due
to disruption of the reading frame.
• However, continues experiment shows
no huge variation in the protein
production and the reproduction ability
of T4 phage… WHY?
• The result was a protein with a stretch of
the wrong amino acids, like a sentence
with a few words in the middle that are
misspelled
Diagram adopted from Human Genetics concepts
and Application 9th ed
104

https://profiles.nlm.nih.gov/ps/access/JJBCBT.pdf
105

3. Evidence for a Non-Overlapping
Code
• If code was overlapping:
• Amino acid sequences would be restricted-
Sydney Brenner
• Translation would be too complex to be
efficient- Francis Crick
106

Code
• Consider a hypothetical mRNA sequence:
AUGCCCAAG
• If the genetic code is triplet and a DNA
sequence is “read” in a nonoverlapping
manner, then this sequence has only three
codons and specifies three amino acids.
• Do you agree?
• However, if the DNA sequence is
overlapping, the sequence specifies seven
codons.
107

Code
• Even though the genetic code is
non-overlapping, it is possible to
read any DNA or RNA sequence in
three different reading frames,
depending upon the “start” base.
Diagram adopted from Human Genetics concepts
108

4 & 5. Evidence for a Commaless and
Degenerate Code
• Crick hypothesized, on the basis of genetic evidence, that
the code would be commaless.
• Only 20 of the 64 possible codons would specify an amino
acid and that the remaining 44 would carry no coding
assignment. What do you think?
• More than one codon specifies the same amino acid.
• Crick’s frameshift studies suggest that wild-type function
is restored when there is, (+) with (-); (++) with (- -); and
(+++) with (- - -); the original frame of reading is restored.
109

6. The code contains 1 start and 3 stop
codons
• Chemical analysis eventually showed that the
genetic code includes directions for starting and
stopping translation.
• The codon AUG signals “start,” and the codons
UGA, UAA, and UAG signify “stop.”
AUG
UGA
UAA
UAG
110

7. The code is (nearly) universal
• All life evolved from a common ancestor, hence
all species use the same mRNA codons to specify
the same amino acids.
• Do you think there is exceptions to the
universality of the genetic code?
• Mitochondria
• Certain single-celled eukaryotes (ciliated
protozoa)
• These deviations may be tolerated because they
do not affect the major repositories of DNA.
111

8. The code is unambiguous
• Which codons specify which amino acids?
• Marshall Nirenberg and Heinrich Matthaei experiments.
Human Genetics concepts
112

Marshall Nirenberg and
Heinrich Matthaei experiments
• Synthesized simple mRNA molecules were added
to test tubes that contained all the chemicals and
structures needed for translation, extracted from
E. coli.
• The 1st synthetic mRNA they made had the
sequence UUUUUU…. Result: Peptides of
phenylalanine.
• Conclusion: The codon UUU specifies the amino
acid phenylalanine
113

• The 2nd experiments AAAAAA….. Result: Peptide
of lysine
• The 3rd experiments CCCCCC…. Result: Peptide
for Proline.
• GGGGGG…. was unstable, so this part of the
experiment could not be done.
• Next to prove other codon-amino acid pairs
hence researchers synthesized chains of
alternating bases
114

• Synthetic mRNA of sequence AUAUAU . . .
introduced codons AUA and UAU alternating
Isoleucines and Tyrosines, but which one is which?
• Another experiment with a more complex sequence
answered the question.
• The mRNA UUUAUAUUUAUA, UUU codes for
phenylalanine, AUA code for isoleucine.
• Hence if AUA codes for isoleucine, then UAU must
code for tyrosine… right?
115

8. The code is unambiguous
• Sixty of the possible 64 codons specify
particular amino acids
• Three indicate “stop,” and one encodes both
the amino acid methionine and “start.”
• This means that some amino acids are
specified by more than one codon.
• For example, both UUU and UUC encode
phenylalanine.
116

The Genetic code
Diagram adopted from Internet source
117

The Genetic code
• Almost all amino acids are specified by two,
three, or four different codons.
• Three amino acids (arginine, serine, and leucine)
are specified by six codons.
• Methionine and tryptophan are encoded by
single codons.
• In many codons specifying the same amino acid
(synonymous codons), the first two positions are
the same but the third position differs (Proline);
degenerate
118

Wobble hypothesis
• Crick postulated the wobble hypothesis.
• He predicted that the initial two ribonucleotides of
triplet codes are often more critical than the third
member in attracting the correct tRNA.
• He hypothesized that hydrogen bonding at the
third position of the codon-anticodon interaction
would be less spatially constrained.
• In addition, it may also need not to adhere strictly
to the base-pairing rules.
119

Wobble hypothesis
• 61 different tRNAs could theoretically exist, one for
each codon that specifies an amino acid, however
only 49 different genes encode tRNAs.
• This is because the same type of tRNA can detect
synonymous codons that differ only in whether the
wobble (third) position is U or C.
• The same type of tRNA, for example, binds to both
UUU and UUC codons, which specify the amino acid
phenylalanine.
• Synonymous codons ending in A or G use different
tRNAs
120

121

• The genetic code is linear
• The genetic code is triplet code- codon
• The code is non-overlapping
• The code is degenerate
• The code is commaless
• The code contains 1 start and 3 stop codons
• The code is (nearly) universal.
• The code is unambiguous
Summary- The Genetic Code
122

1. In the film Jurassic Park, which is about cloned dinosaurs, a
cartoon character named Mr. DNA talks about the billions of
genetic codes in DNA. Why is this statement incorrect?
2. Titin is a muscle protein named for its size. its gene has the largest known
coding sequence of 80,781 DNA bases. How many amino acids long is it?
3. An extraterrestrial life form has a triplet genetic code with five different
bases. How many different amino acids can this code specify, assuming
no degeneracy?
123

4. Explain the work of Francis Crick at al. on how did they prove a codon is
triplet.
5. What is Marshall Nirenberg and Heinrich Matthaei experiments is about?
Explain in details.
6. What is a Wobble hypothesis?
124

2.4 Compare the nuclear DNA
With
The Mitochondrial DNA
125

Mitochondrial DNA and the
genetic code
• Mitochondria contain their own unique DNA,
which in humans consists of 16 kb of circular
dsDNA
• This makes 37 essential genes
126

Mitochondrial DNA and the
genetic code
• Mitochondrial DNA code for:
• 22 mitochondrial (mt) tRNAs
• Two mt rRNAs
• 13 proteins synthesized by the
mitochondrion’s machinery; subunits of the
oxidative phosphorylation pathway.
127

• Codon/anticodon pairings show more ‘wobble’ pairings than in the
process originating in the nucleus due to unusual mt tRNA sequences
Nuclear DNA vs Mitochondrial DNA
128

Mitochondrial inheritance
• Mitochondrial DNA (mtDNA) is only
maternally inherited
• Sperm mtDNA (mid-piece) normally will be
degraded after fertilization.
• Hence, affected males cannot transmit the
disease to their offspring, single ancestral
lineage
• Used to track back the evolutionary history
of human
Crash course: Cell Biology and Genetics, 4th ED
129

Mitochondrial inherited disease
• Leber hereditary optic neuropathy- an
inherited form of vision loss
• Mitochondrial encephalomyopathy, lactic
acidosis and stroke-like syndrome (MELAS)
• Myoclonus with epilepsy and with ragged
red fibres (MERRF).
130

2.4 Compare the nuclear DNA
WIth
The Chloroplast DNA
131

Chloroplast DNA and the genetic
code
• Chloroplast DNA (cpDNA) has between 100 and 225
kb in length.
• It is circular and double stranded.
• The size of cpDNA is much larger than that of mtDNA,
hence account for a larger number of genes.
• Most cpDNA are non coding and duplications of same
DNA sequences
132

Chloroplast DNA and the genetic
code
• Numerous gene products encoded by chloroplast
DNA function during translation within the
organelle.
• Genes specific to the photosynthetic function have
also been identified.
• Mutations in these genes may inactivate
photosynthesis.
133

Eg. Ribulose-1-5-bisphosphate
carboxylase
• Also known as Rubisco
• This enzyme has its small subunit
encoded by a nuclear gene,
whereas the large subunit is
encoded by cpDNA.
134

135

1. Codon/ anticodon pairing in mitochondria are
very wobble.
2. mt DNA is important to make 37 essential genes.
3. Most of the cp DNA are the junk DNA.
4. Mutation in Rubisco gene will cease the
photosynthesis in plant.
136

Ch2 chromosome structure (In brief)

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