Genetic Material

• Ritesh Ranjan Pal, PhD

Assistant Professor,
School Of Biological Sciences (SBS),
Indian Association For The Cultivation Of Science (IACS),
Jadavpur, Kolkata-700032, INDIA
ROLE OF THE GENETIC MATERIAL

“A genetic material must carry out

two jobs: duplicate itself and control
the development of the rest of the
cell in a specific way.”
-Francis Crick
 HISTORY
• Although we now accept the idea that DNA is
responsible for our biological structure, before
the mid-1800’s it was unthinkable for the leading
Scientists and Philosophers that a chemical
molecule could hold enough information to build
a human. They believed that plants and animals
had been specifically designed by a creator.
 Charles Darwin
• Charles Robert Darwin, was an English
naturalist, geologist and biologist, best
known for his contributions to the
science of evolution. Darwin began
detailed investigations and in 1838
conceived his theory of natural selection.
• Charles Darwin is famous for challenging
this view. In 1859 Charles Darwin
published ‘The Origin of Species‘ which
expressed that living things appear to be
designed, but may actually be the result
of natural selection. Darwin showed that
living creatures evolve over several
generations through a series of small
changes.
 Gregor Mendel
In the 1860s Darwin's ideas were finally
supported when genetics was discovered
by Gregor Mendel. He discover the basic
rules of Heredity of garden pea. His
experiment involved growing thousands
of pea plants for 8 years and observed 7
characters.
Conclusion Of Mendel Experiment
▪An individual organism has two
alternative heredity units for a given trait
(Dominant trait vs. Recessive trait)
▪The genes are passed to later
generations from both its parents.
▪He gave three laws
1. Law of Dominance
2. Law of Segregation
1823-1884 3. Law of Independent Assortment
 Chromosomes uniformly segregate in daughter cells

Drawing of mitosis in salamander embryo by Walther Flemming was

published in Zellsubstanz, Kern und Zelltheilung (1882).
Walter Sutton (left) and Theodor Boveri (right) independently
developed the chromosome theory of inheritance in 1902.

Chromosomal Theory of Inheritance, which states that

chromosomes carry the unit of heredity (genes)
 Johannes Friedrich
Miescher Swiss physician
In 1869, a German scientist named
Friedrich Miescher had isolated various
phosphate rich chemicals found in the
nucleus called nuclein from the nuclei of
white blood cells. These were proteins and
nucleic acids.
Friedrich Miescher raised the idea
that the nucleic acids could be involved in
heredity.
Later he isolated a pure sample of
the material now known as DNA from the
sperm of salmon, and in 1899 his
pupil, Richard Altmann named it "nucleic
acid". The extract is rich in Phosphorus and
Nitrogen, but not in sulfur. This substance
was found to exist only in the
1844-1895 chromosomes.
– 1881 Edward Zacharias showed chromosomes are
composed of nuclein.

– 1899 Richard Altmann renamed nuclein to nucleic

acid.

– By 1900, chemical structures of all 20 amino acids

had been identified
 Thomas Hunt Morgan

In 1908, Morgan began to study the

genetic characteristics of the fruit
fly Drosophila melanogaster. In his
famous Fly Room at Columbia
University, Morgan demonstrated
that genes are carried
on Chromosomes and are the basis
of heredity. These discoveries
formed the basis of the modern
science of genetics.
Is the genetic material protein or DNA
Topic of Discussion till 1944 was which chromosomal
component DNA or Protein carries hereditary
information or is the genetic material.

Until 1940 Proteins were considered as genetic

material as Proteins are polymer of 20 protein amino
acids and present in larger quantity, encode more and
variety of information.

DNA is polymer of only 4 different

deoxyribonucleotides (ATP, CTP, GTP & TTP) and is
present in smaller quantity

Most geneticists focused on “transmission genetics”

and passively accepted proteins as the genetic
material
 Frederick Griffith

He was working on a project in

1928 that formed the basis that DNA
was the molecule of
inheritance. Griffith's experiment
involved mice and two types of
pneumonia. One was virulent and the
other non-virulent.
-S bacteria (smooth coat)- pneumonia
-R bacteria (rough coat) - no
1881 - 1941 pneumonia
 Griffith’s experiment identifying the “transforming principle”

Bacterial
colonies +
Rough Smooth Heat-killed Rough Heat-killed
nonvirulent virulent smooth nonvirulent smooth
(strain R) (strain S) virulent (strain R) virulent
(strain S) (strain S)

Injection

Results
Mouse healthy Mouse dies Mouse healthy Mouse dies

Live strain S bacteria

Found in blood sample
from dead mouse

• Discovery: something in heat-killed virulent bacteria could be

transferred to live harmless bacteria and make them virulent.
 Phoebus Levene
In 1929 Phoebus Levene at the
Rockefeller Institute identified the
components that make up a DNA
Molecule. Those components are:
1.The four bases
1.Adenine (A) 3. Thymine (T)
2.Guanine (G) 4. Cytosine (C)
2.Sugar
3.Phosphate
• He showed that the
components of DNA were linked
in the order phosphate-sugar-N
base.
1869-1940
• He said that each of these units is a nucleotide and
suggested the DNA molecule consisted of a string of
nucleotide units linked together through the
phosphate groups. He suggested that these form a
'backbone' of the molecule.
• However, Levene thought the chain was short and
that the bases repeated in the same fixed order. It
was Torbjorn Caspersson and Einar Hammerstein
who showed that DNA was a polymer.
 Oswald Avery ,
McCarty & MacLeod
Avery continued working with Griffith’s
findings in hope of discovering what
factor in bacteria carried the trait of
virulence and showed that DNA is the
unit of Inheritance in 1944. And his work
inspired Watson and Crick to seek DNA's
Structure.
• They decided to use the process of
elimination
• Extracts were treated with either
1. Proteases (to destroy protein)
2. RNase (to destroy RNA)
3. DNase (to destroy DNA)
• Transformation was due exclusively
(1877-1955) to DNA
 Erwin Chargaff and
Chargaff’s rule
• To understand the DNA molecule
better, scientists were trying to make a
model to understand how it works and
what it does. In the 1947 Erwin
Chargaff found the pattern in the
amounts of the four bases: adenine,
guanine, cytosine, and thymine.
• He took samples of DNA of different
cells and found that the amount of
adenine was almost equal to the
amount of thymine, and that the
amount of guanine was almost equal
to the amount of cytosine.
(1905-2002)
• The first rule was that in DNA the number of guanine
units is equal to the number of cytosine units, and the
number of adenine units is equal to the number of
thymine units. This hinted at the base pair makeup of
DNA.
• Thus you could say: A=T and G=C
• The second rule was that the relative amounts of
guanine, cytosine, adenine and thymine bases vary
from one species to another. This hinted that DNA
rather than protein could be the genetic material.
• Bases present in characteristic ratio
humans:
A = 30.9% T = 29.4%
G = 19.9% C = 19.8%
“This discovery later became Chargaff’s Rule”
 Alfred Hershey and
Martha Chase
• In 1952, Alfred Hershey and
Martha Chase used
bacteriophage (a viruses that
infect the Bacteria) to prove
that DNA was the hereditary
material.
• grew phage viruses in 2 media,
radioactively labeled with
either
35S in their proteins
32P in their DNA
 Blender experiment
Protein coat labeled DNA labeled with 32P
with 35S
T2 bacteriophages
are labeled with
radioactive isotopes
Sulphur vs Phosphorus

bacteriophages infect
bacterial cells

bacterial cells are agitated in blender

to remove viral protein coats

35S 32P radioactivity found

radioactivity
found in the medium in the bacterial cells

Confirmed DNA is the genetic material.

 Rosalind Franklin (1920-1958)
[THE DARK LADY OF DNA]

• Franklin, trained as a chemist,

was expert in deducing the
structure of molecules by firing X-
rays through them. Her images of
DNA disclosed without her
knowledge & Watson and Crick
on the track towards the right
structure. She went on to do
pioneering work on the
structures of viruses.
Rosalind Franklin
 The Evidence
James Watson and Francis Crick used this photo as an evidence to
describe the structure of DNA

X-ray diffraction photo of DNA

Image produced by Rosalind
Franklin gave in 1952

Photo-51
James Watson – American
ornithologist
Francis Crick – British Physicist

• James Watson was an

American, born in 1928, was
only 24 when the discovery
was made.
• Francis Crick was born in 1916.
He went to London University
and trained as a physicist. After
the war he changed the
direction of his research to
molecular biology.
• The DNA molecule was discovered in 1951 by Francis
Crick, James Watson and Maurice Wilkins using X-ray
Diffraction.
• In Spring 1953, Francis Crick and James Watson, two
scientists working at the Cavendish Laboratory in
Cambridge, discovered the structure of the DNA a
double helix, or inter-locking pair of spirals, joined by
pairs of molecules.
• In July 1952, Erwin Chargaff visited Watson and
Crick and told of his 1947 findings that the ratios of
A/T and G/C were statistically equal for a wide
variety of DNA’s. Crick became convinced that base
pairing was the key to the structure.
• Prompted by receiving a flawed manuscript on
DNA structure from Pauling, Watson again visited
King’s and Wilkins showed him a DNA x-ray pattern
taken by Franklin showing clear double helical
characteristics.
• Along with Francis Crick, Watson used
characteristics and features of Photo 51 to develop
the chemical model of the DNA molecule
• Watson brought back to Cambridge, but Crick produced a
three-strand model structure only a week later.
• Invited to view this, Franklind pointed out that it was
inconsistent with her results – it had the phosphate groups
on the inside whereas her results showed they were on the
outside, and the water content was too low.
• Watson began pursuing the idea of hydrogen bonding of
the four bases. He found that (A+T) and (G+C) could be
bonded together to form pairs with very similar shapes.
• On this basis, a model was built consistent with the
Franklin’s symmetry and Chargaff’s results, and Watson &
Crick published in April 1953 in Nature accompanied by
ones from the Wilkins and Franklin groups.
• Watson and Crick’s paper ends with the oft-
quoted line “It has not escaped our notice that
the specific pairing we have postulated
immediately suggests a possible copying
mechanism for the genetic material”.
 What they deduced from
1. Franklin’s X-ray data 3. What they came up with
• Double helix on their own:
• Uniform width of 2 nm • Bases face inward,
• Bases stacked 0.34 nm apart phosphates and sugars
outward
2. Chargaff's rules • Hydrogen bonding
• Adenine pairs with thymine • Hinted at semi-conservative
• Cytosine pairs with guanine model for replication
Watson and Crick with their DNA model
 The Nobel Prize

Crick, Watson and Wilkins won the Nobel Prize for medicine in
1962. Maurice Wilkins was at King's College, London and was
an expert in X-ray photography. His colleague, Rosalind
Franklin, did brilliant work developing the technique to
photograph a single strand of DNA. She received little
recognition for this at the time and died tragically of cancer in
1958, so could not be recognized in the Nobel Award.
 Meselson–Stahl

The Meselson–Stahl experiment is an experiment

by Matthew Meselson and Franklin Stahl in 1958
which supported Watson and Crick's hypothesis
that DNA replication was semiconservative. In
semiconservative replication, when the double
stranded DNA helix is replicated, each of the two new
double-stranded DNA helices consisted of one strand
from the original helix and one newly synthesized. It
has been called "the most beautiful experiment in
biology”
 Models of DNA Replication
conservative semiconservative dispersive
 Frederick Sanger
• In 1977, Frederick Sanger
developed the classical “DNA
sequencing” technique, now
known as the Sanger method, to
determine the order of bases in a
strand of DNA.
• Special enzymes are used to
synthesize short pieces of DNA,
which end when a selected
“terminating” base is added to the
stretch of DNA being synthesized.
• Typically, each of these
terminating bases is tagged with a
radioactive marker, so it can be
identified.
• Then the DNA fragments, of varying lengths,
are separated by how rapidly they move
through a gel matrix when an electric field is
applied – a technique called electrophoresis.
• Frederick Sanger shared the 1980 Nobel Prize
in Chemistry for his contributions to DNA-
sequencing methods, which he shared with
Walter Gilbert and Paul Berg.
 Scientific History
T.H. Morgan (1908) :: genes are on chromosomes
Frederick Griffith (1928) :: a transforming factor can change
phenotype
Phoebus Levene (1929) :: identified the components that
make up a DNA Molecule
Avery, McCarty & MacLeod (1944) :: transforming factor is
DNA
Erwin Chargaff (1947) :: Chargaff rules: A = T, C = G
Hershey & Chase (1952) :: confirmation that DNA is
genetic material
Rosalind Franklin (1952) :: Gave Photo-51 which showed X-
ray diffraction photo of DNA
Watson & Crick (1953) :: determined double helix
structure of DNA
Meselson & Stahl (1958) :: supported DNA replication is
semi-conservative replication

Frederick Sanger :: DNA sequencing