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
DNA
Deoxyribonucleic acid
The information necessary to
sustain and perpetuate life is
found within a molecule. This is
the genetic material that is
passed from one generation to
the next---a blue print for
building living organisms.
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.
History
Charles Darwin is famous
for challenging this view.
In 1859 he 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.
History
In the 1860s Darwin's ideas
were finally supported when
genetics was discovered by
Gregor Mendel. He found
that ‘Factors’ determine the
characteristics a living thing
will express. The genes are
passed to later generations,
with a child taking genes
from both its parents. The
great mystery was where and
how would this information
be stored?
1823-1884
Czech monk
Friedrich Miescher
Swiss physician
1844-1895
Search for genetic material:
In 1870, a German scientist named
Friedrich Miescher had isolated the
chemicals found in the nucleus. These
were proteins and nucleic acids.
(His nuclei came from pus!)
History
While Miescher found these nucleic
acids interesting, and spent a great deal of
time studying their chemical composition,
he wasn’t alone in believing that proteins
were more likely to be the chemicals
involved in inheritance, because of their
immense variability.
Proteins were made up of 20 different
building blocks (amino acids), as opposed to
the mere 4 building blocks of nucleic acids.
Discovery of a “Transforming Principle”
Frederick Griffith, in 1928
- Pneumonia (Diplococcus pneumoniae) infects mice.
- Mice develop pneumonia and die.
Two types of bacteria:
- S bacteria smooth coat – pneumonia
- R bacteria rough coat - no pneumonia
Coat type is associated with virulence.
1881 - 1941
English army medical officer
History
Search for genetic material:
1928 Frederick Griffith: transforming principle
Frederick Griffith’s 1928
Experiment
Griffith’s experiment identifying the “transforming principle”
Bacterial
colonies
+
Rough
nonvirulent
(strain R)
Smooth
virulent
(strain S)
Mouse healthy
Mouse dies
Heat-killed
smooth
virulent
(strain S)
Rough
Heat-killed
nonvirulent smooth
(strain R) virulent
(strain S)
Injection
Results
Mouse healthy
Mouse dies
Live strain S bacteria
in blood sample
from dead mouse
Transformation
• What happened in the fourth experiment?
• The harmless R cells had been transformed
by material from the dead S cells
• Descendents of the transformed cells were
also pathogenic
• The question is: what was the material???
Discovery of DNA
The extracts of heat-killed S bacteria cells
contained protein, RNA and DNA
Which of these substances were essential for
transformation?
How did they figure out which substance was
essential for transformation?
KEY PLAYERS
Oswald Avery (1877-1955)
Microbiologist Avery led the team
that showed that DNA is the unit
of Inheritance. One Nobel laureate
has called the discovery "the
historical platform of modern DNA
research", and his work inspired
Watson and Crick to seek DNA's
structure.
1600s 1800s 1850s 1900s 1950s 2000s
1877-1955
• Oswald Avery –
American bacteriologist
S
R
DNA
– 1943 – proved that DNA carries genes
S
Discovery of DNA
• They decided to use the process of elimination
• Extracts were treated with either
– Proteases (to destroy protein)
– RNase (to destroy RNA)
– DNase (to destroy DNA)
• Transformation was due exclusively to DNA
What Is the
Transforming Material?
• Avery found that cell extracts treated with
protein-digesting enzymes could still
transform bacteria
• Cell extracts treated with DNA-digesting
enzymes lost their transforming ability
• Concluded that DNA, not protein,
transforms bacteria
History
Search for genetic material:
It wasn’t until 1944 that Oswald Avery and his
colleagues, who were studying the bacteria which
causes pnuemonia, discovered by process of
elimination that bacteria contain nucleic acids, and
that DNA is the chemical which carries genes.
Despite the conclusive results of Avery’s
experiments, the theory of nucleic acids being the
genetic material was still not a popular one, but
experiments Performed with viruses also showed that
nucleic acids were the genetic material and this
confirmed Avery’s work.
1600s 1800s 1850s 1900s 1950s 2000s
• Alfred Hershey and Martha Chase 1952
–
used bacteriophage (a virus) to prove that
DNA was the hereditary material
–
the bacteriophage was the ideal organism for
settling the debate between protein and DNA.
What are viruses?
Viruses are organized
associations of
macromolecules:nucleic acid contained
within a protective shell
of protein units .
A virus is NOT alive.
A virus is NOT made out of a cell.
A bacteriophage is a virus that
infects bacteria
DNA
inside
protein
coat
Hollow
sheath
Tail
fiber
Question: what
infects the
bacterium, the
protein or the
DNA?
DNA discovery Hershey-Chase 1952
DNA discovery Hershey-Chase
1952
History
Search for genetic material:
1952 - Hershey-Chase Experiment
virus particle
labeled with 35S
DNA being
injected into
bacterium
Harvey and
Hershey
and
Chase show
protein does
not infect the
bacterium
35S
remains
outside cells
virus particle
labeled with 32P
DNA being injected
into bacterium
Harvey and
Hershey
and
Chase show
DNA infects
the bacterium
32P
remains inside cells
History
Search for genetic material:
 Classic experiments for evidence
Griffith: transformation
Hershey-Chase: DNA necessary
to produce more virus
 Other supporting evidence
DNA volume doubles before cells divide
Chargaff: ratio of nucleotides
A = T and G = C
1600s 1800s 1850s 1900s 1950s 2000s
1929-1992
• Erwin Chargaff – Austrian
American biochemist
– (1950) Discovered the basepairing regularities or
"complementarity
relationships" of nucleic acids
that provided one of the key
steps in developing a structural
model for DNA.
KEY PLAYERS
Erwin Chargaff (1905-2002)
Chargaff discovered the
pairing Rules of DNA
letters, noticing that A
Matches to T, and C to
G. He later criticized molecularbiology,
the discipline he helped invent, as "the
practice of biochemistry without a
license", and once described Francis
Crick as looking like "a faded racing
tout".
1600s 1800s 1850s 1900s 1950s 2000s
1953
• James Watson – American ornithologist
• Francis Crick – British Physicist
The Scientists
James Watson was an American, born
in 1928, was only 24 when the
discovery was made. He went to
Chicago University at the age of 15.
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.
KEY PLAYERS
James Watson (1928-
)
Watson went to university
in Chicago at age 15, and
teamed up with Crick in
Cambridge in late 1951. After solving
the double helix, he went on to work on
viruses and RNA, another genetic
information carrier. He also helped
launch the human genome project, and
is president of Cold Spring Harbor
Laboratory in New York.
KEY PLAYERS
Francis Crick (1916-2004)
Crick trained and worked
as a physicist, but
switched to biology after
the Second World War. After co
discovering the structure of DNA, he
went on to crack the genetic code that
translates DNA into protein.
At the time of his death he was
studying consciousness at California's
Salk Institute.
The Discovery
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 interlocking pair of spirals, joined by pairs
of molecules.
The Discovery
The seed that generated this was
Watson’s presence at a conference in
Naples in 1951, where an x-ray
diffraction picture from DNA was
shown by Maurice Wilkins from King’s
College in London.
This made a strong impression on
Watson – the first indication that
genes might have a regular structure.
KEY PLAYERS
Linus Pauling (1901-1994)
The titan of twentieth-century
chemistry, Pauling led the way
in working out the structure of
big biological molecules, and
Watson and Crick saw him as
their main competitor. In early
1953, working without the
benefit of X-ray pictures, he
published a paper suggesting
that DNA was a triple helix.
James Watson shared an office with Crick,
and the topic of DNA structure naturally arose
– particularly how to determine it. They were
inclined to follow the methods of Pauling who
had designed a helical structure by building a
model consistent with the x-ray patterns from
fibrous proteins.
Like proteins, DNA was built from similar
units – the bases adenine(A), thymine(T),
guanine(G) and cytosine(C), and so it seemed
likely that DNA also had a helical structure.
The published x-ray patterns of DNA were not
very clear, so contact was made with King’s.
Watson attended a DNA colloquium there in
November 1951, at which Rosalind Franklin
described her results.
1600s 1800s 1850s 1900s 1950s 2000s
1920 – 1958
Rosalind Franklin- English
Chemist
– the most beautiful X-ray
photographs of any substance ever
taken
– (1952) crucial contributions to the
solution of the structure of DNA
KEY PLAYERS
Rosalind Franklin (1920-1958)
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 - put Watson
and Crick on the track
towards the right
structure. She went on
to do pioneering work
on the structures of
viruses.
The Evidence
Search for genetic material:
James Watson and Francis Crick
used this photo with other evidence to
describe the structure of DNA.
X-ray diffraction
photo of DNA
Image produced by
Rosalind Franklin
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
helical characteristics.
OOPS!!!
Watson brought back a less-thanaccurate account to Cambridge, but
Crick produced a
three-strand
model structure only a week later.
Invited to view this,Franklin
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 & Crick
What they deduced from:
Franklin’s X-ray data
• Double helix
• Uniform width of 2 nm
• Bases stacked 0.34 nm apart
Chargoff’s “rules”
• Adenine pairs with thymine
• Cytosine pairs with guanine
Watson & Crick
What they came up with on
their own:
• Bases face inward, phosphates
and sugars outward
• Hydrogen bonding
• Hinted at semi-conservative
model for replication
History
Watson began pursuing the idea of hydrogen
bonding by using cardboard cutouts 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 oftquoted line “It has not escaped our notice
that the specific pairing we have postulated
immediately suggests a possible copying
mechanism for the genetic material”.
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.