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Unit VI
CHEMICAL BASIS OF INHERITANCE
MLT, FAMS, TU
Dr. Nabil MTIRAOUI, M.Sc, Ph.D
Molecular Genetics
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The branch of genetics that deals with hereditary
transmission and variation on the molecular level.
Deals with the expression of genes by studying the DNA
sequences of chromosomes
The study of the molecular structure of genes, involving
DNA and RNA.
Lecture 11
DNA Structure & Properties
I. DNA’s Discovery & Structure
What is a Genetic “Factor”?

From Mendel:
 we
now accepted that there was genetic transmission
of traits.

Traits are transmitted by “factors”
 Organisms

carry 2 copies of each “factor”
The question now was: what is the factor that carries the
genetic information?
Requirements of Genetic Material


Must be able to replicate, so it is reproduced in each cell of
a growing organism.
Must be able to control expression of traits
Traits are determined by the proteins that act within us
 Proteins are determined by their sequences

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Therefore, the genetic material must be able to encode the
sequence of proteins
It must be able to change in a controlled way, to allow
variation, adaptation, thus survival in a changing environment.
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

Chromosomes – The First Clue
First ability to visualize chromosomes in the nucleus came at
the turn of the century
 construction
of increasingly powerful microscopes
 the discovery of dyes that selectively colored various components
of the cell
Scientists examined cellular nuclei and observed nuclear
structures, which they called chromosomes
Observation of these structures suggested their role in
genetic transmission
Implications

Chromosomes behaved like Mendel’s “factors”
 Mendel's
hereditary factors were either located on the chrs or
were the chromosomes themselves.


Proof chromosomes were hereditary factors – 1905:
The first physical trait was linked to the presence of specific
chromosomal material
 conversely,
the absence of that chromosome meant the absence
of the particular physical trait.

Discovery of the sex chromosomes
 "X"
and "Y."
 distinguished from other chromosomes and from each other
What Carries the Genetic Information?

Chromosomes are about 40% DNA & 60% protein.


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Protein molecules are composed of 20 different subunits
DNA molecules are composed of only four
Therefore protein molecules could encode more information,
and a greater variety of information


Protein is the larger component
protein had the possibility for much more diversity than in DNA
Therefore, scientists believed that the protein in chromosomes
must carry the genetic information
1. A History of DNA
F. Griffiths (1928)
Tried to determine what
genetic material was made of.
The Transforming Principle


Fredrick Griffith - 1928
Discovered that different strains of the bacterium
Strepotococcus pneumonae had different effects on mice
One strain could kill an injected mouse (virulent)
 Another strain had no effect (avirulent)
 When the virulent strain was heat-killed and injected into mice, there
was no effect.
 But when a heat-killed virulent strain was co-injected with the avirulent
strain, the mice died.



Concluded that some factor in the heat killed bacteria was
transforming the living avirulent to virulent?
What was the transforming principle and was this the genetic
material?
Griffiths’ Experiment
Griffiths’ Experiment
Pneumococcus bacteria on mice
2 STRAINS
S-type
Smooth colonies
Virulent
R-type
Rough colonies
Avirulent
Innoculate into mice
Innoculate into mice
Dead from pneumonia
Not killed
Avery, MacCleod & McCarthy (1944)
Tried purifying the transforming
principle to change R-type
Pneumococcus to S-type
The Transforming Principle is DNA

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
Avery, Macleod, & McCarty – 1943
Attempted to identify Griffith’s “transforming principle”
Separated the dead virulent cells into fractions
The protein fraction
 DNA fraction

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Co-injected them with the avirulent strain.
When co-injected with protein fraction, the mice lived
 with the DNA fraction, the mice died


Result was IGNORED
Most scientists believed protein was the genetic material.
 They discounted this result and said that there must have been some
protein in the fraction that conferred virulence.

The Hershey-Chase Experiment
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Hershey & Chase – 1952
Performed the definitive
experiment that showed that
DNA was the genetic
material.
Bacteriphages = viruses that
infect bacteria
Bacteriphage is composed
only of protein & DNA
Inject their genetic material
into the host
The Hershey-Chase Experiment
The Experiment


Prepared 2 cultures of bacteriophages
Radiolabeled sulphur in one culture
there is sulphur in proteins, in the amino acids methionine and cysteine
 there is no sulphur in DNA


Radiolabeled phosphorous in the second culture
there is phosphorous in the phosphate backbone of DNA
 none in any of the amino acids.


So this one culture in which only the phage protein was
labeled, and one culture in which only the phage DNA was
labeled.
Experiment Summary
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Performed side by side experiments with separate phage
cultures in which either the protein capsule was labeled
with radioactive sulfur or the DNA core was labeled with
radioactive phosphorus.
The radioactively labeled phages were allowed to infect
bacteria.
Agitation in a blender dislodged phage particles from
bacterial cells.
Centrifugation pelleted cells, separating them from the
phage particles left in the supernatant.

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Results Summary
Radioactive sulfur was found predominantly in the supernatant
Radioactive phosphorus was found predominantly in the cell
fraction, from which a new generation of infective phage was
generated.
Thus, it was shown that the genetic material that encoded the
growth of a new generation of phage was in the phosphorouscontaining DNA.
Chargaff’s Rule
Chargaff’s rule is a rule about DNA,
Chargaff’s Rule


Once DNA was recognized as the genetic material, scientists
began investigating its mechanism and structure.
Erwin Chargaff – 1950


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
discovered the % content of the 4 nucleotides was the same in all tissues
of the same species
percentages could vary from species to species.
He also found that in all animals (Chargaff’s rule):
%G = %C
%A = %T
This suggested that the structure of the DNA was specific and
conserved in each organism.
The significance of these results was initially overlooked
The Double Helix: Watson & Crick
Watson and Crick shared the 1962 Nobel Prize for Physiology and
Medicine with Maurice Wilkins. Rosalind Franklin died before this date.
The Double Helix: Watson & Crick

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James Watson and Francis Crick – 1953
Presented a model of the structure of DNA.
It was already known from chemical studies that DNA was
a polymer of nucleotide (sugar, base and phosphate) units.
X-ray crystallographic data obtained by Rosalind Franklin,
combined with the previous results from Chargaff and
others, were fitted together by Watson and Crick into the
double helix model.
2. Chemical Bases in DNA

Two types of nucleic acid can be recognized:
deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

DNA is mostly found in the nucleus where it forms the
principal substance of the chromosomal material, the
chromatin. In addition to DNA, chromatin contains proteins,
mainly histones, and little RNA.
2. Chemical Bases in DNA

In prokaryotes, DNA is present in a single chromosome in the
nucleoid.

Little DNA is also found in mitochondria and in chloroplasts.

Many viruses are made up of DNA, mostly double stranded,
but some are single stranded.
3. Primary Structure: Nucleotide & Nucleoside
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The addition of a pentose sugar to a base produces a nucleoside .
If the sugar is ribose, a ribonucleoside is produced; if the sugar is 2deoxyribose, a deoxyribonucleoside is produced
Addition of phosphate group to nucleoside produces nucleoside
mono-phosphate (NMP) like AMP or CMP or a nucleotide
3. Primary Structure: Mononucleotide
3. Primary Structure: Nitrogenous Bases

PURINES
1. Adenine (A)
2. Guanine (G)

PYRIMIDINES
3. Thymine (T)
4. Cytosine (C)
T or C
3. Primary Structure: Dinucleotide
3. Primary Structure: Polynucleotide
3. Primary Structure: Polynucleotide
O
A
5’-End
H
H3C
N
Thymine
O
H
5’
I
O=P-O-CH2 O
I
O4’
H
1’
H
N
3’
O
N
2’
N
Adenine
N
H
O
5’
I
O=P-O-CH2 O
I
O4’
N
N
3’
H
1’
2’
NH2
H
N
H
O
5’
I
O=P-O-CH2 O
I
O4’
O
N
O
1’
N
2’
HN
3’
Cytosine
H
N
O
5’
I
O=P-O-CH2 O
I
O4’
3’
N
3’ 5’ Phosphodiester bond
NH2
1’
2’
OH
3’-End
Guanine
4. Secondary structure: double helical structure
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
The 2 strands are twisted about each other,
coiled around a common axis, forming a righthanded double helix.
The hydrophilic sugar- phosphate backbone of
each chain lies on the outside of the molecule.
The hydrophobic nitrogenous bases project
inwards from the outer sugar-phosphate
framework, perpendicular to the long axis of
the helix and are stacked one above the other.
The stacking of bases is held by hydrophobic
bonds .This helps in holding the helical structure.
4. Secondary structure: double helical structure

The nitrogenous bases of the 2 strands meet each other near the
central axis of the helix where they become connected by hydrogen
bonds between the amino, or imino, hydrogen and the ketonic
oxygen atoms. The hydrogen bonding between the bases helps to
hold the 2 strands of the DNA together.
Thymine
Adenine
Cytosine
Guanine
4. Secondary Structure: Chargaff’s Rule
A nitrogen-containing ring structure called a
base. The base is attached to the 1' carbon
atom of the pentose. In DNA, four different
bases are found:
two purines, called adenine (A) and guanine
(G)
two pyrimidines, called thymine (T) and
cytosine (C)
*A always pairs with T : two hydrogen bonds
*C always pairs with G : three hydrogen bonds
4. Secondary Structure: Direction of Strands

The 2 strands of the
double helical molecule
are antiparallel, i.e.,
they run in opposite
direction; one runs in
the 5’ to 3’ direction,
while the other runs in
the 3’ to 5’ direction.
5. DNA Conformations
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B conformation (B-DNA):

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A-DNA:
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The most common form of DNA.
The minor groove and major groove, are of
different widths on the outside of DNA.
Forms under conditions of low salt and low humidity.
There can be transient shifts from B to A form.
Z-DNA:
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Consists of alternating purines and pyrimidines
Found infrequently.
Z-DNA is:



long and thin
Left-handed,
Phosphate backbone has a zig-zag appearance.
6. Key Features of a DNA molecule
7. Key Features of a RNA molecule
8. Biochemistry of DNA

DNA is the carrier of
genetic information,
which is stored in the
Replication
DNA
form of a nucleotide
Transcription
sequence. DNA has 2
important functions:
“replication” and
RNA
Translation
“transcription”.
Protein
9. What is Gene ?

The gene, the basic units
of inheritance; it is a
segment within a very
long strand of DNA
with specific instruction
for the production of
one specific protein.
Genes located on
chromosome on it's
place or locus.
9. What is Gene ?
A gene in relation to the double helix structure of DNA and to a
chromosome (right). Introns are regions often found in eukaryote
genes that are removed in the splicing process (after the DNA is
transcribed into RNA): only the exons encode the protein. This
diagram labels a region of only 40 or so bases as a gene. In reality
most genes are hundreds of times larger.