“Genetic code is a set of rules by which information encoded in genetic material(DNA or RNA sequences) is translated into proteins by living cells.”
Genetic code is a Dictionary consists of “Genetic words” called CODONS.
“George Gamow” postulated that a 3 letter code must be employed to encode 20 standard amino acids used by living organisms to encode proteins.
First elucidation of codon was done by Marshall Nirenberg and Heinrich J.Matthaei in 1961.
They used cell free system to translate a poly-uracil sequence and as a result polypeptide synthesized consisted of only amino acid “Phenylalanine”.
Similarly another experiment confirmed that poly-adenine RNA sequence specified the amino acid “Lysine” and poly-cystosine sequence coded for amino acid “Proline”.
SALIENT FEATURES OF GENETIC CODE
Genetic code has a “sequence reading frame”. For example the string GGGAAACCC, if read from the first position contains the codons GGG,AAA,CCC, and if read from second postion the codons will be GGA,AAC.
Genetic code has “Start and Stop codons”.Startcodon is usually AUG.
Alternative start codons are GUG or UUG. Stop codons include UAG ,UGA and UAA.
Genetic code is continuous and non-overlapping.
Genetic code is Universal.
Genetic code is Degenerate.
>Degeneracy of code is not uniform. >Degeneracy results because there are more codons to encode 20 amino acids.
>This property of genetic code makes it more fault-tolerant for “Point mutation”
Codon/Anticodon recognition involves wobbling.
“5’ base of anticodon that binds to 3’ base on the mRNA was not spatially confined as the other 2 bases and could does have non-standard base pairing”.
Significance of Genetic code
>Genetic code tells how protein sequence information is stored in nucleic acids and how that information is translated into proteins.
Variations to Standard Genetic Code
>In certain proteins, non-standard aminoacids are subtituted for standard Stop codon depending on associated signal sequence in mRNA.
>For example, UGA can be code for Selenocysteine (now viewed as 21 amino acid) and UAG can code for Pyrrolysine (viewed as 22nd amino acid). >Genetic code is Universal except few mitochondrial codes and, small variants such as translation of the codon UGA as Tryptophan in the species Mycoplasma and translation of CUG as Serine rather than Leucine in the genus Candida.
>In Bacteria and Archaea, GUG and UUG are common start codons but in rare cases certain proteins may use alternative start codons not normally used by that species. Conclusion >Despite these differences, all known species codes are very similar to each other and the coding mechanism is the same for all organisms: three base codons, tRNA, ribosomes, reading the code in the same direction and translating the code 3 letters at a time into sequence of amino acids.