The Genetic Code
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