The code that carries the hereditary characteristics of an organism is called genetic code. The code formed by the combination of three adjacent nitrogen bases in the DNA molecule is called the genetic code. It is also called mRNA code. The genetic code is the biochemical basis of heredity. Scientist Francis Crick proved that the genetic code is a triplet or 3-letter code. Nirenberg was able to synthesize artificial mRNA using poly U (polyuridylic acid) in a cell-free or in vitro system. Indian scientist Hargobind Khorana synthetically synthesized the 5-GUGUGUGUGU-3 nucleotide chain and used it to create the valine-cysteine valine-cysteine polypeptide chain. Nirenberg and Hargobind Khorana won the Nobel Prize in 1969 for this discovery.
Types of genetic code
There are 3 types of genetic codon.
1. Sense codon: All the codons that provide the signal for the creation of amino acids are called sense codons. There are 61 sense codons for 20 amino acids.
2. Start codon: All the codons that initiate protein synthesis are called start codons or initial codons. The start codon is AUG. Amino acids that initiate protein synthesis are methionine (true cells) and formyl methionine (primitive cells).
3. Stop codon: Those codons that terminate protein synthesis are called termination or stop codons. The stop codons are UAA, UAG and UGA. They do not provide any signal for the formation of amino acids. Hence they are called non sense codons.
Number of codons
If three nitrogen bases form a code, the arrangement or assembly of four nitrogen bases will be 4×4×4=64. That is, the triplet codon number is 64. 64 codes can code for 20 amino acids. The number of functional codons is 61 and termination codons are 3. In 1964, Nirenberg and Matthaei discovered 64 triplet codes for 20 amino acids.
Characteristics of the genetic code
1. Triplet Code: The genetic code is triplets. That is, the three nitrogenous bases carry the code or signal for amino acids to form proteins. In 1954, scientist George Gamow proved that the genetic code is a triplet or 3-letter code. Eg AGU, AGC, AGA, AGG etc.
2. Genetic code is continuous: The genetic code works continuously without interruption. That is, there are no extra nucleotides or gaps between two adjacent codons. Like-GUUGUCGUAGUG.
3. Genetic code is non-overlapping: The three nitrogen bases of the genetic code are never arranged in an overlapping or overlapping manner. (If the genetic code overlaps, four amino acids will be specified by 6 nitrogen bases and seven by 9 nitrogen bases. On the other hand, if the genetic code is not overlapped, two amino acids will be specified by 6 nitrogen bases and three amino acids will be specified by 9 nitrogen bases). In 1965, scientist Hargobind Khorana demonstrated that the nitrogen bases of the genetic code are non-overlapping. For example – CGUCGCCGA genetic code is universal: the genetic code of all organisms on earth is the same. That is, the genetic code never changes depending on the type or structure of the organism. For example, the AUA genetic code carries the signal for the amino acid called methionine in all organisms on earth, including viruses and bacteria.
5. Genetic Code Has Polarity: Genetic codes contain genetic information from DNA in a specific sequence. The genetic code always carries signals in the 5→3 pathway. So the polarity of the genetic code is 5→3.
6. Genetic Code Ambiguity or Degeneracy: When a genetic code carries the signal of an amino acid, it is called degeneracy. For example, a code carrying the signal for tryptophin is UGG.
7. Genetic code is degenerate: If more than one genetic code carries the signal of a single amino acid, it is called polymorphism. For example, six codes UCU, UCC, UCA, UCG, AGU and AGC carry the signal of serine.