Genetic Code: The Genetic Code

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".