Within each cell is a separate enclosure, the nucleus. Inside this nucleus is a truly amazing substance, commonly known as DNA. Think of a ladder with its two side rails joined by the rungs. Then imagine that some giant twisted the ladder along its length, until the side rails looked like two huge corkscrews cross-connected by the rungs. Reduce this in size to a minute fraction of a millimetre and you have, in essence, the structure of a DNA molecule. The diagrams show the idea. The simple diagram shows the twisted ladder arrangement and the more complicated one the actual structure of just a short length of DNA. A complete DNA molecule would be very much longer, having many thousands of twists in its spiral rather than the few you see here. In fact if the total DNA in just one human cell could be stretched out, it would be about 2 metres long!

The wonderful thing about DNA is that along its length it contains the instructions for making all the different types of enzymes the cell needs. As the enzymes are responsible for making the chemical reactions in the cell work, you can see that DNA therefore controls the whole cell. The information about the correct sequence of amino acids in each enzyme is contained in coded form on the ‘rungs’ of the DNA ladder. There are only four different kinds of ‘rungs’, each composed of chemicals paired together (T, A, G, C in the diagram) and it needs three ‘rungs’ to code for one amino acid.

If we call the four types of rungs A B C D, then ABC might be the code for amino acid 1, BCD for amino acid 2, BCB for amino acid 3, DBA for amino acid 4 and so on. Continuing until all the 20 amino acids are coded, using only four ‘rungs’. So, in our example above, if the sequence of ‘rungs’ on the DNA molecule were BCDBCBABCDBA it would mean that the sequence of amino acids would be 2,3,1,4. In this way, a ladder of 600 ‘rungs’ could code for an enzyme of 200 amino acids in its chain. If the code sequence on the DNA was correct, then every enzyme produced from that section of its length would have its amino acids in the right order too and would therefore be able to do its job.

This only explains the principle of the code's operation. In practice, the transfer of the coded information to the site of enzyme production is very complicated and involves other very special substances. It is estimated that to make one protein molecule, about another hundred different proteins are required as enzymes to effect the production. 4

The total amount of information along that microscopic chain of DNA is mind-boggling. In a simple cell, like a bacterium, there are several million coded symbols and in those of man there are between two and four billion. The total DNA code in a cell is styled the genome; and it is a measure of scientific progress that in recent years the whole of the human genome has been elucidated.



4 M Denton: Evolution: A theory in Crisis, page 265 (Adler and Adler 1986).