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1.2.4 Translation and Posttranslational Modifications
The process of protein synthesis in the cell involves a very complex machin-
ery, with hundreds of molecules. The main players of translation consist of:
(1) the mRNA molecules that work as template for protein synthesis; (2) the
ribosomes where the actual decoding takes place; and (3) a special class of
RNAs (transfer RNA) that carries the appropriate amino acid to the complex
mRNA/ribosome.
Translation
The logistics of the synthesis of both DNA and RNA molecules, based on
the complementarity of nucleotides, is fairly simple when compared with
the synthesis of proteins. Chemically, there is no simple way of pairing
directly the triplet codons with the appropriate amino acids. Indeed, most
of the sophisticated machinery of translation evolved to solve exactly this
problem. The amino acid must be correctly attached to special molecules
which are then coupled to the correct codons in the mRNA molecule. This
special class of molecules are also RNA molecules (transfer RNAs), but
they are structurally and functionally very different from mRNA. As we
have seen, tRNAs have tertiary structure and can therefore have varied
functionalities. Although the identification of the correct amino acids is
not made by the tRNAs themselves, their unique three-dimensional struc-
ture is fundamental to their correct identification by particular enzymes.
Each such enzyme recognizes both a particular amino acid and the appro-
priate tRNA, further attaching the amino acid to the tRNA. Furthermore,
each tRNA contains also a nucleotide sequence (the anticodon) that is com-
plementary to the appropriate codon, through which the coupling of the
correct amino acid carrier to the mRNA is made.
The set of rules to translate the triplet codons of mRNA into amino acids
is the genetic code. Figure 1.6 shows the 64 codons and the amino acids or
instructions each codes for. The amino acids are represented by a three-letter
abbreviation and by a one-letter abbreviation often used in describing the
primary structure in proteins. (I'll stick to this one-letter abbreviation to rep-
resent protein chains in all the figures of this chapter.)
The message in the mRNA molecule is read one codon at a time, each
codon being correctly paired with the appropriate tRNA through the antico-
don, and the transported amino acids are linked one after another, forming a
long, linear protein chain whose sequence exactly reflects the sequence of
