Genetic Code

The gene is represented by the sequences of bases in the DNA molecule, which can, in a sense, be thought of as a "storage molecule" for genetic information. DNA is extremely stable, a property critical to the maintenance of the integrity of the gene. This stability is evidenced by the fact that DNA has been extracted from Egyptian mummies and extinct animals such as the woolly mammoth. It can be extracted from dried blood or from a single hair at a crime scene.

Each cell contains a complete set of genes, but only certain of these genes are active or "expressed" at any one time. When a gene is active, a "disposable" copy is transcribed from the gene into codons contained in a messenger RNA (mRNA) molecule. Unlike the DNA molecule, the mRNA molecule is relatively unstable and short-lived. This is so that when a gene is turned off, the mRNA does not remain in the cell forever, running off more proteins on the ribosomes that are no longer needed by the cell.

Another RNA molecule, called transfer RNA (tRNA), contains a specific region called the anticodon. The tRNA anticodon can base pair with the codon region of the mRNA during protein synthesis, using the base pairing rules of A-U, U-A, C-G, and G-C. Each tRNA carries a specific amino acid. Thus the tRNA carrying methionine has a UAC anticodon that pairs with the AUG codon of the mRNA bound to the ribosome. Similarly the tRNA for proline has a GGA anticodon.

In examining the table of codons (Table 1) you will see that there is more than one codon for each amino acid, except for methionine (AUG) and tryptophan (UGG). Different codons that code for the same amino acid are said to be "synonyms," and the code is said to be "degenerate" in the sense that there is not a single, unique codon for each of the twenty amino acids.

Table 1. Genetic code.