In this article we will discuss about the functional aspects and mechanisms of genetic code.
Functional Aspects of Genetic Codon:
All the amino acids except methionine and tryptophan are specified by more than one codon. Three amino acids—leucine, serine and orginine—are each specified by six different codons. Isoleucine has three codons, and the other amino acids each have either two or four codons. The occurrence of more than one codon per amino acid is called degeneracy of the codon. But this degeneracy of the codon is not at random, instead it is highly ordered.
The degeneracy is primarily of two types:
a) Partial degeneracy i.e. when the third base of the codon—either one of the two pyrimidines (U or C) or one of the two purines (A and G).
b) Complete degeneracy i.e. when any of the four bases may be present of the third position of the codon and it will still specify the same amino acid (please follow Codon Dictionary at the end of this chapter).
Most interesting is that the genetic codes are universal i.e. one codon specifies the same amino acids in all the organisms of the 64 codons, only three are nonfunctional i.e. these three codons can not specify any amino acid during the time of translation. These three codons are UAA, UAG and UGA and known as nonsense codon or terminator codon.
The genetic code also provides for punctuation of genetic information at the level of translation. Two codons, AUG or GUG, when any of them is present in the leader segment of the mRNA, function as a initiator codon for the beginning of the protein synthesis. Genetic codons are present on mRNA in a linear fashion.
Mechanism of Genetic Codon:
The codon is recognised during the period of translation by the anticodon site of the t-RNA which carry the amino acids. The anti-codon means the combination of three bases just complementary to the codon and present on t- RNA.
The base pairing between the third base (3′) of the codon and the 5′ base of the anticodon does not follow the normal strict base pairing rules, instead there is “wobble” at this site, permitting basepairs to form other than the usual four base pairs. Thus, the anti-codon of a single t-RNA may recognise one, two or three different codons. This wobbling is due to formation of alternate hydrogen bonds.
Following each triplet nucleotide sequence or codons refers to the nucleotide sequence in mRNA (not in DNA) that specifies the indicated amino acids (Table 5.1).