The translation of genetic information from the 4-letter alphabet of
polynucleotides into the 20-letter alphabet of proteins is a complex process.
The rules of this translation seem in some respects neat and rational, in other
respects strangely arbitrary, given that they are (with minor exceptions)
identical in all living things. These arbitrary features, it is thought, reflect
frozen accidents in the early history of life chance properties of the earliest
organisms that were passed on by heredity and have become so deeply
embedded in the constitution of all living cells that they cannot be changed
without wrecking cell organization.
The information in the sequence of a messenger RNA molecule is read out in
groups of three nucleotides at a time: each triplet of nucleotides, or
specifies (codes for) a single amino acid in a corresponding protein. Since
there are 64 (= 4 × 4 × 4) possible codons, but only 20 amino acids, there are
necessarily many cases in which several codons correspond to the same amino
acid. The code is read out by a special class of small RNA molecules, the
transfer RNAs (
specific amino acid, and displays at its other end a specific sequence of three
nucleotides an
a particular codon or subset of codons in mRNA (Figure 1-9).
For synthesis of protein, a succession of tRNA molecules charged with theirappropriate amino acids have to be brought together with an mRNA molecule
and matched up by base-pairing through their anticodons with each of its
successive codons. The amino acids then have to be linked together to extend
the growing protein chain, and the tRNAs, relieved of their burdens, have to be
released. This whole complex of processes is carried out by a giant
multimolecular machine, the ribosome, formed of two main chains of RNA,
called ribosomal RNAs (
evolutionarily ancient molecular juggernaut latches onto the end of an mRNA
molecule and then trundles along it, capturing loaded tRNA molecules and
stitching together the amino acids they carry to form a new protein chain
(Figure 1-rRNAs), and more than 50 different proteins. Thiscodon,tRNAs). Each type of tRNA becomes attached at one end to aanticodon that enables it to recognize, through basepairing,
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