It is genetic information present in the nucleotide sequences of DNA with three consecutive nucleotides functioning as a codon in a non-overlapping fashion. The genetic vocabulary consists of 64 codons. Each codon has a message for construction of polypeptide chain. Genetic code is therefore, also defined as a coRrespondence between the sequence of amino acids in polypeptides and the nucleotide sequence in DNA.
Context
- Genetic code information
- Characteristics Of Genetic Code
Genetic Code information
- Gamow (1955) theoretically proposed a triplet genetic code. If one nucleotide corresponds to one amino acid, only four amino acids can be specified. A four letter code would specify 4X4 = 16 aming acids. The number of amino acids is 20. A triplet code consisting of three nucleotide bases can produce 4x4x4 64 codons which are sufficient for all the twenty amino acids and their incorporation or termination in polypeptide.
- The genetic code was deciphered by the efforts of a number of workers like Crick, Ochoa Nirenberg. Khorana and Matthei. Severo Ochoa discovered polynucleotide polymerase (Ochoa enzyme for template independent polymerisation of nucleotides. Khorana synthesised RNAs with defined combination of nucleotides (homopolymers, copolymers). Nirenberg could synthesise protein in cell free systems.
- Nirenberg et al (1961) synthesised poly-U RNA (polyuridylic acid). It was added in twenty lot of bacterial extracts each having all the 20 amino acids but with only one of them being labelled. The observed that poly-U caused synthesis of a polypeptide made of repeated units of phenylalanine Therefore, they deduced that the codon UUU corresponds to amino acid phenylalanine. The scientist also found that codon CCC corresponds to amino acid proline while AAA signifies amino acid lysine.
- Khorana (1964) synthesised a chain of alternate nucleotide GUGUGUGUGU. He found that stimulated synthesis of a peptide having alternate valine-cysteine-valine-cysteine.
- Ochoa et al (1965,67) prepared a number of nucleotide co-polymers and deciphered several codons,
- Brenner (1965) had discovered the nonsense codon UAG which has been named 'amber' by him.
Characteristics Of Genetic Code
1. Triplet Nature. Genetic code is triplet code where three adjacent nitrogen bases specify one amino cid, e.g., UGG for tryptophan, AUG for methionine.
2. Non-overlapping. Three successive nucleotides or nitrogen bases code for only one amino acid.
None of these nitrogen bases become part of any other codon.
3. Commaless. Genetic code does not possess punctuation. Reading of the genetic code, when once started, is continuous till the end.
4. Specificity or Unambiguity. A codon has a code for only one amino acid. Therefore, genetic code is specific as well as unambiguous.
5. Universality. Same codon specifies the same amino acid from bacteria to human beings. Exceptions, however, occur in some mitochondrial and viral genes.
6. Number. The total codons are 64 in number. Most of them specify amino acids with some functioning as start and stop signals.
7. Initiation Codons. They function as start signals for the synthesis of polypeptides. Their number is two, AUG or methionine codon and GUG or valine codon.
8. Termination Codons. They are stop signals which when encountered cause termination of polypeptide synthesis. Termination codons are also called nonsense codons because they do not specify any amino acid.
The various termination codons are UAA (ochre), UAG (amber) and UGA (opal).
9. Polarity. The code is read in 5'-3' direction. Usually the code present on mRNA is recognised as genetic code. Its complementary code present on template DNA is not considered. If DNA code is to be mentioned, the same is the one which is present on the sense strand (complementary to template strand).
Because of it the sense strand is also called coding strand. Here the code is similar to that of mRNA with replacement of U with T.
10. Degeneracy. Out of 64 codons only 3 signify stop signals. Only two codons specify a single amino acids.
Those were AUG for methionine and UGG for tryptophan. Three amino acids have 6 codons each, 5 amino acids 4 codons, one amino acid with 3 codons and 9 amino acids are specified by two codons each.
11. Colinearity. Both DNA (or mRNA) as well as polypeptide has linear arrangements of components. The sequence of amino acids present in polypeptide corresponds to sequence of nitrogen bases of DNA with three successive nitrogen bases forming a single codon. A change in any codon due to addition, deletion or replacement of nucleotides results in change in the reading of genetic code as well as incorporation of amino Acid.
12. Related Codons. Amino acids having similar properties are also specified by related codons, e.g., aromatic amino acids tryptophan-UGG, tyrosine-UAU, UAC, phenylalanine-UUU, UUC.
13. Cistron-polypeptide parity. A cistron or gene takes part in synthesis of single polypeptide. The number of cistrons must, therefore, be equal to the number of polypeptides.
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