††††The two uprights are linked together by "rungs" consisting of four important chemicals arranged in pairs. These four chemicals are the bases adenine, thymine, guanine and cytosine, known as A, T, D and C for short. Watson and Crick discovered that A always pairs with T, and C with G.
So DNA consists of a series of pairs of these bases along the length of the molecule. Now the order in which the pairs of bases are arranged is variable, and this is how the genes exert their effects on a living thing. For example, the order of pairs of bases for producing, say, brown eyes, will be different from the order that produces blue eyes, and so on. In other words, DNA contains a set of coded instructions which tell the living organism how to develop.
These instructions form the genetic code. Forensic scientists use many tests to help solve crimes and a new test, known as DNA profiling can be used to identify the origins of a trace of blood or skin cells.
