Explain the genetic code.
After the cell has carried out the process of DNA transcription, we are left with a transcribed mRNA fragment. This, similar to DNA, is a chain of 4 different bases - cytosine, guanine, adenine and uracil (in this place DNA has thymine). These bases, arranged into triplets, form the basis of the genetic code. Each group of three is called a codon and in the later process of translation, codes for the addition of one specific amino acid to a growing polypeptide chain.
The genetic code has a few important characteristics. First of all, we describe it as being degenerate. What does this mean? To understand this, we first have to do some maths. Since there 4 amino acids combined in groups of 3, there will be 34 possible combinations, or 64 potential codons. However, there are only 21 naturally occurring amino acids, which the body uses for polypeptide synthesis. Hence, degenerate in the context of the genetic code means that more than one codon can code for a particular codon. We also have certain codons that code for the start of translation (AUG), which are called start codons, and some which do so for the end of translation - stop codons. The genetic code is also described as being universal, which means that it is the same in all organisms.
