Read this article to learn about the genetic code with its features.
All the genetic information is encoded in DNA (in case of eukaryotes, in the DNA of nucleus) which produces hereditary characters in all the living beings. In fact, this genetic information lies in the specific sequence of the nucleotides in polynucleotide chains of the DNA.
The first and the most important step in the manifestation of the genetic information (gene) is the formation of specific protein in which the sequence of amino acids is determined by the sequence of specific nucleotides of the polynucleotide chain of DNA molecule.
Various nucleotides that constitute the nucleic acids are represented by code letter A, G, T, C and U.
The sequence of 3 nucleotides in polynucleotide chains of the DNA molecule is called as triplet code.
In messenger RNA molecule (m-RNA) this sequence of 3 nucleotides is complementary to the sequence of nucleotides in DNA and is called as codon. (A will be complementary to U and G to C and vice versa).
There are different codons for different amino acids. The latter are incorporated in a particular protein through specific t-RNA molecules with specific anticodons. Anticodon is complementary to codon.
Some of the codons like UAA, UAG, UGA cannot select any amino acid and are called as non-sense codons or chain termination codons.
A list of all the codons which specify amino acids constitutes the coding dictionary or genetic code (Table 9.2).
Table 9.2. Coding Dictionary or Genetic Code:
Some of the important features of the genetic code are:
(i) It is degenerate i.e., there may be more than one codons for a particular amino acid e.g., UUU, UUC=phenyl alanine.
(ii) It is non-overlapping i.e., only as many amino acids are coded as there are codons in end to end sequence e.g., UUUCCC=phenylalanine (UUU) + pro-line (CCC).
(iii) It is comma-less i.e., there are no intermediary nucleotides (or commas) between the codons.
(iv) The first and the second letters of a codon are more important than the third in specifying an amino acid (Table 9.2).
(Marshall W. Nirenberg, Robert W. Holley and Har Gobind Khorana had been instrumental in deciphering the genetic code for which they were jointly awarded Nobel Prize of 1968 in physiology or Medicine category).