THE GENETIC CODE

If genes are segments of DNA and if DNA is just a string of nucleotide pairs. then how does the sequence of nucleotide pairs dictate the sequence of amino acids in  proteins? The analogy to a code springs to mind at once. By the mid-1960's the genetic code was largely solved. The most important properties of the genetic code are:-

1. The genetic code is a triplet code.
2. The code is non-overlapping
3. The code is degenerate.
4. The code is comma-free.
5. The code is ordered.
6. The code contains start and stop codons 
7. The code is universal.

1. Triplet Code:-

• Twenty different amino acids are incorporated into polypeptides during translation. (20 amino acids commonly found in cellular proteins)
• Thus, atleast 20 different codons must be formed with the four bases available in mRNA.
• If the words are two letters long, then 4X4=16 codons are possible. for example AU, CU OR CC. This is not large enough,
• If  the words are three letters  long then (4cube)4X4X4=64 words are possible. for example AUU, GCG OR UGC. This code provides more than enough words to describe the amino acids. Thus by experimental evidence it was proved that the genetic code is a triplet code. However, if all words are "triplets", then we have a considerable excess to possible words over the 20 needed to name the common amino acids.
• Three bases code for an amino acid. These triplets are termed codons.
• The code is read from a fixed starting point and continues to the end of the coding sequence.
• The reading frame of mRNA is the series of nucleotide triplets that are (used to) read during translation.

             For example, the reading frame of the sequence AAAGGGCCCTTT is (AAA) (GGG) (CCC) (TTT).

2. Non-overlapping:- 

                                 By 1961, it was already clear that the genetic code was non-overlapping. The analysis of mutationally altered proteins. In particularly, the nitrous acid-generated mutants of tobacco mosaic virus showed that only a single amino acid change at one time in one region of the protein. This is predicted by a non-overlapping chains.

3. Degenerate Code:- 

                                  The occurance of more than one codon per amino acid is called degeneracy.

Example:- Leucine, Serine, Arginine are coded each by six different codons. 

Isoleucine has 3 codons.

All the amino-acids except methionine and tryptophan are coded by more than one codon.

The degeneracy is of two types:-

I). Partial degeneracy and

II). Complete degeneracy.