Genetic Code,Codons,Termination Codons,Degeneracy,Pyrimidines,Genetic code is Degenerate,amino Acids,Purine,Pyrimidine,Positions

www.molecular-plant-biotechnology.info

Back to Home

Home >> Genetic Translation >>Genetic Code

Back to Home

	Genetic Code - Genetic information is the sequence of bases in the DNA. Genetic information present in DNA is copied or expressed into RNA sequence. Genetic information that is expressed as an mRNA sequence must be translated from the four letter language of nucleotides into the 21-letter language of proteins. The letters A, G, T and C correspond to the nucleotides found in DNA. They are organized into three letter code words called codons. A collection of these codons is called as genetic code. Genetic code is concerned with the processes involved in translating or decoding the information contained in the primary structure of DNA.
The genetic code provides a foundation for explaining the way in which protein defects may cause genetic disease and for the diagnosis and perhaps treatment of these disorders. Some of the important properties of a genetic code are listed below. Genetic code is degenerate The occurrence of more than one codon per amino acid is called degeneracy. All amino acids except methionine and tryptophan have more than one codon, so that all the possible triplets have a meaning, despite there being 64 triplets and only 20 amino acids. Leucine, serine and arginine have six different codons.
Isoleucine has three codons. The degeneracy in the genetic code is not at random: instead, it is highly ordered. Usually multiple codons specifying an amino acid differ by only one base, the third or 3 base of codon. The degeneracy can be either partial or complete degeneracy. A complete degeneracy is a condition in which the third base position is of less significance. Any base present in this position will lead to same amino acid. But if the third position is occupied by one type of purine/pyrimidine, then they code for one type of amino acid and if the position is occupied by another type of purine/ pyrimidines, without any change in the first two positions, they code for another amino acid.
Such a condition is called as partial degeneracy, e.g. CAU or CAC code for His whereas CAA and CAG code for Gly.Because of the degeneracy of the genetic code there must either be several different tRNAs that recognize the different codons specifying a given amino acid or the anticodon of a given tRNA must be able to base-pair with several different codons. Actually both of these occur. Several tRNAs exist for certain amino acids and some tRNAs recognize more than one codon. The hydrogen bonding between the bases in the anticodon of tRNA and the codon of mRNA appears to follow strict base pairing rules only for the first two bases of the codon and is apparently less stringent, allowing what Crick has called wobble at this site.
	Codon is triplet Twenty different amino acids are incorporated during translation. Thus, at least 20 different codons must be formed using the four symbols (bases) available in the mRNA. Two bases per codon would yield only 42 or 16 possible codons which is clearly not enough. Three bases per codon yield 43 or 64 possible co dons apparently excess. It is now known that each codon consists of a sequence of three nucleotides, i.e., it is a triplet code. The deciphering of the genetic code depended heavily on the chemical synthesis of nucleotide polymer, particularly triplets in repeated sequence.

Code contains punctuation codons Three codons do not code for specific amino acids. These codons are called as nonsense codons. These nonsense codons cause termination of protein synthesis.

Thus they are called as termination codons. Similarly, AUG codon codes for starting of the gene and position where translation should begin. AUG codes for the initiation codon and because it codes for methionine, almost all newly synthesized polypeptides have this amino acid at the start.

Codon is commaless There are no commas or some specific nucleotide sequences to separate the codons, i.e., CCCAAAUUUGGG has four code words and upon translation we have a tetrapeptide chain of pro-lys-phen-gly. So all the letters are used to code for one or other amino acid.