James Watson & Francis Crick (presented by Faheem) - Fp.sfasu.edu


Oct 22, 2013 (3 years and 5 months ago)



The Code of Life

The Molecular Basis
of Inheritance


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.


Although we now accept the idea that DNA

is responsible for our biological structure,

But in the early 1800s 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 is famous for challenging
this view. In 1859 he published

'The Origin of Species‘

expressing that living things

might appear to be designed,

but were actually the result

of natural selection.

Darwin showed that living

creatures evolve over several

generations through a series

of small changes.


In the 1860s Darwin's ideas were supported
when genetics was discovered by

Gregor Mendel. He found that genes

determine the characteristics a living thing
will take. The genes are passed on to later
generations, with a child taking genes from

both its parents. The great mystery was
where and how is this information stored?


The main conclusions made by Mandle were:


Inherited traits are controlled by genes, which are in pairs. When
sex cells are created one gene from each pair goes into the gamete.
When two gametes fuse at fertilization, the offspring has two copies
of each gene

one from each parent.


The genes for different traits are sorted into gametes independently

of other genes. So one inherited trait is not dependent on another.


Where there are two different forms of a gene are present in a pea
plant, the one which is dominant is the one that is observed.


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. While he 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. They were made up of 20
different building blocks (amino acids), as opposed to

the mere 4 building blocks of nucleic acids.


Search for genetic material:

In the early 1900s, Phoebus Levene, who also believed
that proteins must be the chemicals of

inheritance, studied the composition of nucleic acids.
He discovered that DNA is a chain of

nucleotides, with each nucleotide consisting of a
deoxyribose sugar, a phosphate group and a

nitrogenous base, of which there were four different
types. He proposed that the four different types of

nucleotide were repeated over and over in a specific
order. This would make DNA a relatively simple

repeat sequence

no wonder DNA wasn’t considered to
be smart enough to code for all of life!


Search for genetic material:

1928 Frederick Griffith: transforming principle

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.

Search for genetic material:


Chase Experiment



Search for genetic material:

Classic experiments for evidence

Griffith: transformation

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

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 inter
locking pair of spirals, joined by pairs of


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 indicationthat genes might have

a regular structure.


Search for genetic material:

James Watson joined the unit (its first biologist) and began by

trying to crystallize myoglobin for Kendrew. The unsuccess of this

left much time for discussion with Crick, whose office he was

sharing, and the topic of DNA structure naturally arose

particularly how to determine it. They were inclined to follow the

method of Pauling who had deduced the 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 too had a helical structure.

The publishedx
ray patterns of DNA were not very clear, and so

contact was made with King’s.

Watson attended a DNA colloquium there in November 1951, at

which Rosalind Franklin described her results.


Search for genetic material:

Watson brought back a less
accurate account

to Cambridge, but with Crick produced a

strandmodel structure only a week later.
Invited to view this,Franklin pointed out that it

was inconsistent with her results

it had
thephosphate groups on the inside

whereas her results showed they were on the

outside,and the water content was too low.

The work at Cambridge stopped abruptly for a bit.


Search for genetic material:

In July 1952, Erwin Chargaff visited the unit and told of his 1947
findings that the ratios of A/T and G/C were unity for a wide variety
of DNAs. 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 of the pure B
form showing clear helical characteristics, plus the intense 10th layer
line at 3.4A and a 20A equatorial reflection indicating the molecular

diameter. Perutz also showed them a report on the work of the King’s
group which gave the space group of the crystalline A
form as C2,
from which Crick deduced that there were two chains running in
opposite directions.


Search for genetic material:

Watson began pursuing the idea of hydrogen bonding 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 symmetry and with Chargaff’s results, and a paper was

published in April 1953 in Nature accompanied by ones from

the Wilkins and Franklin groups at King’s. 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”.

The Evidence

Search for genetic material:

James Watson and Francis Crick
used this photo with other evidence to
describe the structure of DNA.

ray diffraction photo of DNA

Image produced by Rosalind Franklin

Watson and Crick with their

DNA model

The Scientists

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.

James Watson was an American, born in

1928, so aged only 24 when the discovery

was made. He went to Chicago University

aged only 15 and had already worked on DNA.

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 recognised in the

Nobel Award.

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

model for replication


Oswald Avery


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.


Erwin Chargaff (1905

Chargaff discovered the pairing rules

of DNA letters, noticing that A

Matches to T and C to G. He later

Criticized molecular biology, the

discipline he helped invent,

as "the practice of biochemistry

without a licence",and once

described Francis Crick

as looking like "a faded racing tout".


Francis Crick (1916


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. He now studies

consciousness at California's Salk Institute.


Rosalind Franklin


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


put Watson and Crick on

the track towards the right structure.

She went on to do pioneering work on

the structures of viruses.



Linus Pauling


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



Watson went to university in Chicago

aged 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.


Maurice Wilkins



Like Crick, New Zealand
born Wilkins trained as a
physicist, and was involved with the

Manhattan project to build the nuclear

bomb. Wilkins worked on X

crystallography of DNA with Franklin

at King's College London, although

their relationship was strained. He

helped to verify Watson and Crick's

model, and shared the 1962 Nobel with them.







There are 4 different nucleotides in DNA


pairs with


pairs with



pairs with


pairs with


Does DNA fit the requirements of a hereditary material?


DNA component

Has biologically useful
information to make protein

Genetic code: 3 bases code
for 1 amino acid (protein)

Must reproduce faithfully
and transmit to offspring

Complementary bases are
faithful; found in germ cells

Must be stable within a
living organism

Backbone is strong covalent
bonds; hydrogen bonds

Must be capable of
incorporating stable changes

Bases can change through
known mechanisms

Protein Synthesis

DNA carries the instructions for the production of

proteins.A protein is composed of smaller molecules

called amino acids, and the structure and function of

the protein is determined by the sequence of its

amino acids. The sequence of amino acids, in turn,

is determined by the sequence of nucleotide bases

in the DNA. A sequence of three nucleotide bases,

called a triplet, is the genetic code word, or codon,

that specifies a particular amino acid.

Protein Synthesis

Protein synthesis begins with the separation of a DNA
molecule into two strands. In a process called
transcription, a section of the sense strand acts as a
template, or pattern, to produce a new strand called
messenger RNA (RNA). The RNA leaves the cell
nucleus and attaches to the ribosomes, specialized

cellular structures that are the sites of protein

Amino acids are carried to the ribosomes by another
type of RNA, called transfer (RNA). In a process
called translation, the amino acids are linked together
in a particular sequence, dictated by the RNA, to form
a protein.


Before replication, the parent
DNA molecule has 2
complementary strands

First the 2 strands separate

Each “old” strand serves as a
template to determine the
order of the nucleotides in the
new strand

Nucleotides are connected to
form the backbone; now have 2
identical DNA molecules.


Helicase unwinds the molecule

strand binding protein stabilized ssDNA

Primase initiates the replication with RNA

DNA polymerase extends the new DNA

Second DNA polymerase removes the RNA

DNA ligase joins all the fragments

DNA Replication is simple, but it takes a large team of
enzymes and proteins to carry out the process:

Smith & Nathans

Discovery of restriction

Hamilton Smith

• Discovered
dII in

Haemophilus influenzae

Daniel Nathans

• Used
dII to make first

restriction map of SV40



Paul Berg

Produces first recombinant DNA


Boyer, Cohen & Chang

E. coli

Recombinant plasmid



Genentech, Inc.

• Company founded by

Herbert Boyer and Robert

Swanson in 1976

• Considered the advent of

the Age of Biotechnology

First human protein (somatostatin) produced

from a transgenic bacterium.

Walter Gilbert and Allan Maxam devise a

method for sequencing DNA.


• David Botstein discovers RFLP


• U.S. Supreme Court rules that life
forms can be patented

• Kary Mullis develops PCR. Sells
patent for $300M in 1991


• First transgenic mice produced


• The USFDA approves sale of
genetically engineered human insulin


• An automated DNA sequencer is

• A screening test for Huntington’s
disease is developed using

restriction fragment length markers.


• Alec Jeffreys introduces technique
for DNA fingerprinting

to identify individuals


• Genetically engineered plants
resistant to insects, viruses,

and bacteria are field tested for the
first time

• The NIH approves guidelines for
performing experiments in

gene therapy on humans


• invention of YACs (yeast artificial
chromosomes) as

expression vectors for large


• National Center for Human
Genome Research created to

map and sequence all human DNA by


• UCSF and Stanford issued their
100th recombinant DNA

patent and earning $40 million
from the licenses by 1991.

1 discovered

• First gene therapy attempted on

a four
old girl with an
inherited immune deficiency


• U.S. Army begins "genetic dog tag"


• The Flavr Savr tomato gains FDA

• The first linkage map of the human
genome appears


• The first full gene sequence of a living
organism is

completed for
Hemophilus influenzae

• O.J. Simpson found not guilty despite
DNA evidence


• The yeast genome, containing
approximately 6,000 genes and
fourteen million nucleotides, is


• Dolly cloned from the cell of an adult

• DNA microarray technology


•The genome of the bacterium E. coli,
a classic model organism for studying
microbiological and molecular genetic
mechanisms, and a natural symbiont in
the human digestive tract, is
completely sequenced, revealing about
4,600 genes among about four and
half million nucleotides.


The genome of a nematode worm
Caenorhabditis elegans, a key model
organism for investigating genetic
regulation of development, is
sequenced, revealing approximately
18,000 genes among some 100 million
nucleotides of DNA sequence.


• 1,274 biotechnology companies in the
United States

• At least 300 biotechnology drug
products and vaccines

currently in human clinical trials

• Human Genome Project is on time and
under budget, the complete human
genome map expected in five years or


•Jesse Gelsinger, an eighteen year
with a genetic disorder affecting liver
metabolism, dies from an immune
reaction to a gene therapy treatment.
This tragic event slows gene therapy
applications and results in greater
scrutiny and caution toward the
growing number of gene therapy
research trials.


•The first complete sequence of a human
chromosome (number 22) is completed by the
public genome project and is published. This
step indicates that the genome project is
proceeding ahead of schedule, and also shows
a surprisingly small number of genes (about
300) relative to the anticipated 100,000 or
so for all twenty
four human chromosomes
two chromosomes called autosomes
shared equally by males and females, plus the
chromosome which is paired in females but
occurs in a single copy in males, plus the Y
chromosome that is unique to males).


• Celera sequences the genome of the
fruitfly (Drosophila melanogaster),
identifying approximately 13,000 genes
among 170 million nucleotides.

•First plant genome sequenced (Arabidopsis
thaliana) from the mustard family. The
Arabidopsis genome consists of about 100
million nucleotides, and approximately
20,000 genes, indicating that at the
molecular genetic level, plant and animal
genomes are about equally complex.


•"Golden rice," a genetically engineered
strain of rice manufactures its own
vitamin A. Golden rice is created by
Ingo Potrykus, plant geneticist, and his
colleagues to help alleviate severe
health problems in many areas of the
world caused by vitamin A deficiency.


In mid
February, the journal SCIENCE
publishes an analysis of the Celera Human
Genome Project, and the journal NATURE
publishes an analysis of the public Human
Genome Project.

Both revealed a surprisingly small number of
human genes, estimated jointly at about
30,000 to 35,000, barely more than a worm,
fruitfly, or plant. Both show that only about
2 percent of our DNA actually codes for
amino acid sequences of proteins, and both
identify many sequences of unknown function
and variable length present in multiple copies
making up approximately half the genome.


Each human cell has enough DNA to code for all the
traits in the human body. If the DNA in one cell was
stretched out, how long would it be? Do the math!

There are 6 X 10

base pairs/cell

Each base pair is 0.34 X 10

meters long

A human body has approximately 75 trillion cells. If the
distance to the sun is 150 X 10

meters, how many round
trips could your DNA make?

Answer: 2 meters

Answer: 500 trips


DNA from kiwi fruit