Genetic Code. All 64 possible triplets of the RNA bases that code for amino acids are shown in this table. Reading off the bases in each of the three positions gives the corresponding amino acid. In the case of the bases highlighted here, Adenosine in the first position, Uracil in the second and Guanine in the third (AUG) code for the amino acid Methionine (Met). It will be seen that for some amino acids there are several base codes: Serine, for example, has 6 different codes, i.e. there are 6 different ways of specifying Serine. This is an inevitable consequence of the fact that there are 64 triplets for only 20 amino acids.

For the full names of the amino acids click here

The instructions for life are contained in the sequences of bases that make up DNA (deoxyrbonucleic acid) and its sister molecule RNA (ribonucleic acid). The DNA has the master set, and it uses different kinds of RNA as messengers to convey its instructions to the protein-making machinery.

Both DNA and RNA contain combinations of just four bases: Adenine (A), Guanine (G), Thymine (T), and Cytosine (C) in the case of DNA: in RNA the Thymine is replaced by the very similar Uracil (U).

Remembering that there are just 20 amino acids in proteins, the problem biologists faced in the 60s was: how can 4 bases code for 20 proteins: if one base = one amino acid, only four different amino acids could be produced. If the bases were read in twos, there are 16 possible combinations; still not enough. If the bases are read in threes, there are 64 possible combinations. This is too many but it doesn't matter. After much ingenious experimental work it was conclusively proved by the end of the 60s that DNA does code in triplets to make proteins. Some amino acids have no less than 6 different codes that work for them, and three of the codes are not for protein at all: they are stop signals, telling the machinery to end the chain at this point.

The whole apparatus is very similar to a computer software program. The code was established long before the actual mechanism was deduced whereby the amino acids, RNA and the cell machinery are marshalled to carry out the synthesis, and in fact this work is still going on. But the simplicity and elegance of the code means that whenever a protein structure is known the corresponding DNA and RNA structures can be inferred, and vice versa.

The Human Genome Project, which is due to be completed in 2003, will result in the complete DNA sequence for human beings; hence the structure of every protein in the human body will be known. Just 50 years after Watson and Crick's discovery of the DNA structure, the human doomsday book will exist for all time.