Codons
As we learned in Structure of Nucleic
Acids,
DNA and RNA are made up by sequences of nitrogen
bases-pairs:
adenine, thymine, guanine, and
cytosine. Scientists have long understood that these nitrogen bases somehow
contained the information that coded for specific amino acids. However, it took
some time before they figured out how the base pairs accomplished this coding.
Scientists main problem lay in the fact that while there were only 4 nitrogen
bases (nucleotides), there were 20 amino acids for which those nucleotides had
to code. If adenine, thymine, guanine, and cytosine each coded for a particular
amino acid, then the DNA/mRNA information system would only be able to code for
4 amino acids. If, however, groups of two nucleotides coded for a single amino
acid, the story is somewhat different. Given four nucleotides looked at in
groups of two, there are sixteen possible combinations (AA, AT, AG, AC, TA, TT,
TG, TC, GA, GT, GG, GC, CA, CT, CG, CC); that sixteen is still not enough to
code for twenty amino acids. But if the nucleotides code for amino acids in
groups of three then there are sixty-four possible combinations. Scientific
experiments have verified that nucleotides code for amino acids in successive
groups of threes. These groups of threes are called codons.
Degeneracy of the Genetic Code
As we know, since the genetic code is read in triplets and there are four
possible bases that can occupy each position, the number of possible codons is 4
X 4 X 4, or 64 codons. However, there are only 20 known amino acids.
Experiments have shown that three codons function also function stop codons,
acting as termination signals in translation.
Yet that brings the count up to only twenty-three necessary codons. The vast
difference between possible codon variations and needed codon variations means,
as seen in the figure below, that each amino acid is specified by more than one
codon. Because the genetic code therefore does not code to its capacity, it is
called "degenerate".
Figure %: The Entire Genetic Code Dictionary
