Genetic Code
Genetic code is a combination of three nucleotides, which specify a perticular Amino-acid.
DNA is the basic genetic and hereditory material.It caries information in the form of genetic code.There are three nucleotides in a codon.There are following reasons of forming triplet:
Two Nucleotides:
Two nucleotide codon does not form enough combinations to code for different 20 amino-acids.
Four Nucleotides:
Four nucleotides forms only 42 or 16 different combinations of nucleotides which was also not satisfactory.
Three Nucleotides:
Three nucleotides can form 64 different combinations.It is more than enough to code for the 20 amino-acids.
Discovery:
Genetic code was tested & discovered by Cricks, Nirenberg, Leader & Khorana.They made artificial mRNAs for synthesizing protein Aminoacyl-tRNA complexes with the help of artificial mRNA.They tested all 64 codons for specifying different amino-acids.
Characteristics:
Genetic code have following major characteristics:
Triplet Code:
Each genetic code consists of a triplet of nucleotides or nitrogen bases, in a perticular sequence, which can specify a perticular amino-acid.
Initiating Codon:
Out of 64 codons AUG is always present at the start of mRNA that codes for Methionine and it acts as a Initiating codon.
Stop Codon:
Three codons UAA, UAG, UGA do not code for any amino-acid and they act as Stop Codon.They are present at the end of mRNA, so called as stop codon.
Code is Ambiguous:
The genetic code is ambiguous, that is, same codon may specifies more than one amino-acid, e.g., UUU codon usually code for Phenylalanine but in presence of Streptomycin, also code for Serin.
Code is Degenerative:
Genetic code is degenerative, which means that more than one codon (means many codons) can specify the same amino-acid, e.g., CCU, CCC, CCA & CCG specify the same amino-acid Prolin.One interesting thing is observed in these codons is that the first two bases of the triplet are constant, whereas the third base can vary, e.g., all codon starting with CC specify Prolin.
None-overlapping:
The genetic code shows non-overlapping, meaning that no single base can take part in the formation of more than one codon at a time.
Commaless:
The genetic code is commaless, which means that there is no punctuation between three nucleotide units.
Universal:
Genetic code is universal.It means that they are same in almost all the organisms, e.g., AGA specifies Arginine in Bacteria, Human & all other organisms.Thus the genes can be transformed from one organism to another due to their universality.They can be successfully transcribed and translated in their new Host.The study of genetic code of Mitochondrial DNA showed that Genetic codes are not so universal, e.g., UGA codon is normally a stop codon but in Mitochondria it codes for Tryptophan.Thus it appears that Genetic codes are not quite universal.
