FoundationalConceptsNeuroscience

It would take another few years and the combined efforts of many
investigators to work out the details of how genetic information is
encoded in DNA and how it is read out and translated into what is
needed to build and operate a cell. But Watson and Crick appreciated
the explanatory significance of their structure and were confident
that the details would be illuminated. And they were right.
The structure of DNA suggested that genetic information is
encoded in linear sequences of adenines, thymines, guanines, and
cytosines. In the decade after Watson and Crick’s publication, exactly
how sequences of As, Ts, Cs, and Gs code for sequences of amino acids
in proteins was elucidated. Each of the twenty different amino acids
used to build the structure of proteins is represented (coded for) bya
triplet (sequence of three) of nucleotides in DNA. The relationship be-
tween triplet sequences of nucleotides (called codons) and correspond-
ing amino acids is called the genetic code. Because there are 4 x 4x 4 =
64 different triplet combinations of four nucleotides (A, C, G, T) and
only twenty different amino acids needing to be represented, there is
redundancy in the code. That is, each of the amino acids is generally
represented by more than one triplet combination of nucleotides. For
example, the amino acid phenylalanine is coded for by DNA nucleo-
tide triplets AAA and AAG, and the amino acid glycine is coded for by
the triplets CCA, CCG, CCT, and CCC.
The reading out of information from the DNA takes place in the
following way (see Fig. 4.4). The DNA double helix unwinds, little bits
at a time. In the unwound portion of the helix, one of the two strands
is used as a template for the synthesis of a molecule of ribonucleic acid
(RNA), composed of a sequence of nucleotides complementary to the
sequence in the DNA. Complementary means that where there is aG
in DNA, there would be aC in RNA, where there is a C in DNA, there
would be a Gin RNA, where there is a T in DNA, there would be an Ain