The discovery in 1953 of the double helix, the twisted-ladder
structure of deoxyribonucleic acid (DNA),
by James Watson and Francis Crick marked a milestone in the history of science and gave rise
to modern molecular biology, which is largely concerned with understanding how
genes control the chemical processes.
DNA was first isolated by
the Swiss physician Friedrich Miescher who, in 1869,
discovered a microscopic substance in the pus of discarded surgical bandages.
As it resided in the nuclei of cells, he called it "nuclein". In
1878, Albrecht Kosselisolated the non-protein
component of "nuclein", nucleic acid, and later isolated its five
primary nucleobases.
In 1919, Phoebus Levene
identified the base, sugar, and phosphate nucleotide unit. Levene
suggested that DNA consisted of a string of nucleotide units linked together
through the phosphate groups. Levene thought the chain was short and the bases
repeated in a fixed order.
In 1937, William Astbury produced the first X-ray diffraction patterns that showed
that DNA had a regular structure. In
1927, Nikolai Koltsov proposed that inherited traits would be inherited via a
"giant hereditary molecule" made up of "two mirror strands that
would replicate in a semi-conservative fashion using each strand as a
template".
In 1928, Frederick Griffith in his experiment discovered
that traits of the
"smooth" form of Pneumococcus could be transferred
to the "rough" form of the same bacteria by mixing killed
"smooth" bacteria with the live "rough" form.
This system provided the
first clear suggestion that DNA carries genetic information—the Avery–MacLeod–McCarty
experiment—when Oswald Avery, along with coworkers Colin MacLeod and Maclyn McCarty,
identified DNA as the transforming principle in 1943.
DNA's role in heredity was confirmed in 1952 when Alfred Hershey and Martha Chase in the Hershey–Chase
experiment showed
that DNA is the genetic material of the T2 phage.
Late in 1951, Francis Crick started working with James Watson at the Cavendish Laboratory within the University of Cambridge. In 1953, Watson and
Crick suggested what is now accepted as the first correct double-helix model
of DNA structure in the journal Nature.
Their double-helix,
molecular model of DNA was then based on one X-ray diffraction image (labeled as "Photo 51") taken by Rosalind Franklin and Raymond Gosling in May 1952, and the information that the DNA bases are
paired. On 28 February 1953 Crick interrupted patrons' lunchtime at The Eagle pub in Cambridge to
announce that he and Watson had "discovered the secret of life".
Experimental evidence
supporting the Watson and Crick model was published in a series of five
articles in the same issue of Nature. Of these, Franklin and
Gosling's paper was the first publication of their own X-ray diffraction data and
original analysis method that partly supported the Watson and Crick
model; this issue also contained an article on DNA structure by Maurice Wilkins and two of his colleagues, whose analysis and in
vivo B-DNA X-ray patterns also supported the presence in vivo of
the double-helical DNA configurations as proposed by Crick and Watson for their
double-helix molecular model of DNA in the prior two pages of Nature.
In 1962, after Franklin's
death, Watson, Crick, and Wilkins jointly received the Nobel Prize in
Physiology or Medicine. Nobel Prizes are awarded only to living recipients. A
debate continues about who should receive credit for the discovery.
In an influential
presentation in 1957, Crick laid out the central dogma of
molecular biology, which foretold the relationship between DNA, RNA, and
proteins, and articulated the "adaptor hypothesis".
Final confirmation of the
replication mechanism that was implied by the double-helical structure followed
in 1958 through the Meselson–Stahl experiment. Further work by Crick and
coworkers showed that the genetic code was based on non-overlapping triplets of
bases, called codons, allowing Har Gobind Khorana, Robert W. Holley, and Marshall Warren
Nirenberg to
decipher the genetic code. These findings represent the birth of molecular
biology.
The Human Genome Project was an international scientific research project with the goal of determining the sequence of
nucleotide base pairs that
make up human DNA, and of identifying and mapping all of
the genes of the human
genome from both a physical
and a functional standpoint
Key
findings of the draft (2001) and complete (2004) genome sequences include:
There are approximately
22,300 protein-coding genes in human beings, the same range as in other
mammals.
The human genome has significantly more segmental duplications (nearly
identical, repeated sections of DNA) than had been previously suspected.
At the time when the draft sequence was
published fewer than 7% of protein families appeared
to be vertebrate specific.
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