Biotechnology and Bioinformatics (1+1)

1.2.7. Genetic code

Most gene s encode protein s and only a small part of the total DNA coding regions of genes act as a template for the protein. Proteins are made up of amino acids . It is the sequence of amino acids which give the protein its specific properties. DNA template is first transcribed into mRNA. The mRNA template is then translated into a chain of amino acids.

There are 20 different amino acids which are used to build up proteins. Which amino acid is signaled by which particular codes? This remained one of the great mysteries until the early 1960s.

The most outstanding work in breaking of the genetic code was done by Marshall Nirenberg and his associates in the early 1960s. They devised an elegant technique, called the triplet binding test, and discovered the first word of code dictionary.

A system was developed for synthesizing proteins in vitro ; the system included a cell extract containing ribosomes, tRNAs and other cellular components. Into this, Nirenberg added artificially synthesized mRNA molecules of known nucleotide sequences. When synthetic mRNAs consisting entirely of a single type of nucleotide were added, polypeptide s composed of only a single type of amino acid were formed. Thus, phenylalanine was formed when polyuridylic acid (poly U) was added.

Marshall Nirenberg, Severo Ochoa, Hargobind Khorana, Francis Crick and many others contributed significantly to decipher the genetic code.

• They figured out that the order in which amino acids are arranged in proteins.
• On the basis of a variety of experiments, it was found out that a particular sequence of 3 bases (triplet) would code for a particular amino acid and this triplet is referred to as codon .
• For example, if the mRNA has in its sequence a triplet code AUG, the corresponding amino acid in the protein would be methionine.
• Similarly, the sequence UUU would code for phenylalanine. Thus, the codons for all the 20 naturally occurring amino acids in proteins were figured out.
• Thus, in a mRNA molecule, the sequence of bases read in blocks of three at a time starting from a particular position in a non-overlapping fashion would automatically decide the sequence of amino acids in the poly peptide derived from that mRNA.
• Thus, 4 bases when arranged in the form of triplet code can generate 4³ or 64 codons .
• Of these, 3 codons serve as STOP (Non-Sense) codons (UAG, UAA, UGA) which simply tell the translation machinery to terminate. Not recognized by tRNAs.
• One codon AUG serve as initiating codon. Thus, many amino acids have more than one codon and codons specifying the same amino acid are said to be degenerate and differ in only the third base.