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of DNA and to hypothesize how the genetic information was reliably replicated
(
Watson and Crick 1953
). Previous proposals had been made that suggested that
DNA “consists of three intertwined chains, with the phosphates near the fibre
axis, and the bases on the outside.” Another three-chain structure also had been
suggested in which “the phosphates are on the outside and the bases on the
inside, linked together by hydrogen bonds.”
Watson and Crick (1953)
proposed
that DNA “has two helical chains each coiled round the same axis
…
the bases on
the inside of the helix and the phosphates on the outside
…
” and indicated the
“novel feature of the structure is the manner in which the two chains are held
together by the purine and pyrimidine bases
…
They are joined together in pairs,
a single base from one chain being hydrogen-bonded to a single base from the
other chain.” Watson and Crick stated, “It has not escaped our notice that the
specific pairing we have postulated immediately suggests a possible copying
mechanism for the genetic material,” the accuracy of which is crucial to the trans-
mission of genetic information from cell to cell and from generation to genera-
tion. The next big question to be answered involved the issue of how the purine
and pyrimidine bases were able to encode the genetic information.
Crick et al. (1961)
deciphered the genetic code, by showing that three bases of
DNA code for one amino acid. As noted by
Crick et al. (1961)
, “If the coding ratio
is indeed 3 ... and if the code is the same throughout Nature, then the genetic
code may well be solved within a year.” To resolve the question,
Crick et al. (1961)
found that mutations in a bacteriophage gene caused by the insertion or dele-
tion of a single base pair resulted in a mutation that lead to a failure to pro-
duce a normal protein. The protein could be made functional again by inserting
or deleting a total of three nucleotides, indicating that the genetic code uses a
codon
of three DNA bases that correspond to an amino acid and that the code
for genes is not overlapping.
As more is learned about genomes, the concept of the gene has had to be mod-
ified. An early definition focused on protein-coding genes, but we now know that
much of the DNA in an organism is transcribed into large and small RNAs that
are not translated into proteins (
Collins and Penny 2009
). Furthermore, protein-
coding genes have a variety of regulatory elements, including enhancers and pro-
moters, whereas some genes code for RNAs that are used directly and other RNAs
regulate development, as will be described in Chapters 2 and 3.
1.3 The Central Dogma
The
Central Dogma
of molecular biology, as proposed by Francis Crick (
Crick
1958
), stated that biological information is carried in DNA, this information
