In this article we will discuss about the discovery and special features of genetic code.
Once the role of DNA as a hereditary material was established, it was clear that DNA contained the information for the formation of protein molecules or polypeptides. The sequence of purines and pyrimidines bases along the DNA molecule determines the sequence of amino acids in protein molecules. But the question was how DNA instructs the sequence of amino acids. Only four bases of DNA in some way must be determining 20 types of amino acids that form the proteins.
It was known that the information for the sequence of amino acids of a protein was contained in the sequence of bases on mRNA, which in turn was governed by the sequence of nucleotide bases in the DNA. The genetic code is a sequence of nitrogenous bases along a sugar phosphate strand of a DNA molecule.
The discovery of genetic code became possible through the significant contributions of Francis H.C. Crick, Severo Ochoa, Marshall Nirenberg, Hargobind Khorana, and J.H. Matthei in early 1960s. For this work, Hargobind Khorana shared the Nobel Prize in 1968 with Nirenberg and Holley.
The set of nucleotides that specify one amino acid is a codon. The simplest possible code is a singlet code in which one nucleotide codes for one amino acid (Table 5). Such a code is inadequate, for only four amino acids could be specified. A doublet code is also inadequate because it could specify only 16 amino acids, whereas a triplet code could specify 64 amino acids.
The first experimental evidence in support to the concept of triplet codes is provided by Crick and co-workers in 1961. Crick observed that deletion or addition of one or two base pairs in DNA of T4 bacteriophage disturbed normal DNA functioning i.e. normal protein synthesis could not take place.
The mutation produces a ‘frame shift’ in the reading frame. However, when three base pairs were added or deleted, the disturbance caused was minimum. Crick’s experiment also suggested that the code is degenerate i.e. many of the amino acids are specified by more than one triplet.
The genetic code has following special, features:
1. Triplet Codon:
A codon is triplet in nature and is dependent on the sequence of nitrogenous bases in the DNA molecule. A codon codes for a particular amino acid (see Table 6).
The genetic code is commaless. There are no ‘punctuation marks’ (gaps) between the coding triplets. Reading of the code begins at a fixed point and continues three nucleotides at a time, without a pause till the terminator codon, which marks the end of the message is reached. There is no punctuation (comma) between the adjacent codons.
3. Chain Initiation:
The codon present in the beginning of the cistron is known as initiation codon. It marks the beginning of the message for a polypeptide chain. The initiation codon is AUG in majority of cases and it codes for amino acid methionine.
4. Termination Codon:
Similarly, the last codon of a cistron helps in reading the termination of polypeptide chain. This is known as termination codon. There are three termination codons- UAA, UGA and UAG. These were called nonsense codons, because these do not code for any of the 20 amino acids.
5. Degeneracy of Genetic Code:
More than one codon can code for a particular amino acid. This multiple system of coding is known as degenerate system or degenerate code. The major degeneracy occurs at the third position (3′ end of the triplet codon). When first two bases are specified, the same amino acid may be coded for whether the third base is U, C, A or G. The third base is known as Wobbly base.
Note the genetic codes for the following amino acids:
i. Serine – UCU, UCC, UCA, UCG, AGU and AGC.
ii. Arginine – CGU, CGC, CGA, CGG, AGA and AGG.
6. Code is Non-Overlapping:
The genetic code is non-overlapping. A base is a part of only one codon. The sequence CCGCAC is read only as CCG and CAC and not as CCG, CGC, GCA, CAC.
7. The code is usually non-ambiguous since a particular codon always codes for the same amino acid throughout the living world. But in the presence of streptomycin, UUU which normally codes for phenylalanine may also code for isoleucine, leucine or serine.
Genetic code represents sequence of codons in mRNA and the corresponding amino acid residues of a polypeptide chain are arranged in the same linear sequence. mRNA is linear with DNA and with amino acids in polypeptide chain.