DNA and genetic code

DNA

The genetic material of all cellular life forms is made up of two long strands of deoxyribonucleic acid - DNA - which are coiled around one another in a double helix. Four DNA bases - adenine, cytosine, guanine and thymine - form the "rungs" in this "ladder" and contain all the information needed for this double helix to perform its functions. These bases can be compared with the letters forming words in a manuscript: each sequence of three bases constitutes a "word".

Genetic code

The sequence of "letters" (DNA sequence) in the DNA molecule represents a code governing the production of specific proteins in cells. Known as a TRIPLET CODE, it determines which of the 20 amino acids found in the organism are to be incorporated at a given point in a protein.

This system of producing proteins from a chain of amino acids, the position of which is determined by a genetic code made up of three bases, applies equally to all living organisms on earth. The universality of the genetic code makes it possible to transfer genes across species barriers. If this were not the case, genetic engineering could not have developed.

The illustration shows the possible combinations of triplets in a genetic code chart. Reading from the inside outwards, each triplet represents the code for one of 20 different proteins, with several different possible combinations for the majority of proteins. In addition, some triplets also code for the beginning and end of a chain.

The genetic code
The triplet code is read from the inside outwards, with each triplet coding for one of 20 amino acids