Animal Biotechnology

THE GENETIC CODE

• The genetic code is the correspondence between the sequence of four bases in nucleic acids and the sequence of the 20 amino acids in protein. The code is a triplet code where three nucleotides encode one aminoacid. Since there are only 20 aminoacids to be specified and potentially 64 triplets, most aminoacids are specified by more than one triplet and the genetic code is said to be degenerate or to have redundancy.
• The genetic code is degenerate ,this is because 18 out of 20 aminoacids have more than one codon to specify them, called synonymous codons. Only methionine and tryptophan have single codons.
• Among the 64 triplet codons, only the 61 codons represent aminoacids and 3 codons cause termination. The three codons UAA, UAG and UGA that do not represent aminoacids are used specifically to terminate protein synthesis. These three codons are called stop codons or termination codons (non sense codons). UAG is known as amber, UAA as ochre and UGA is sometimes called the opal codon.
• The genetic code is universal and exceptions to the universal genetic code also occur in the mitochondria from several species.

First position (5’ end)	Second position				Third position (3’ end)
	U	C	A	G
U	UUU Phe UUC Phe UUA Leu UUG Leu	UCU Ser UCC Ser UCA Ser UCG Ser	UAU Tyr UAC Tyr UAA Stop UAG Stop	UGU Cys UGC Cys UGA Stop UGG Trp	U C A G
C	CUU Leu CUC Leu CUA Leu CUG Leu	CCU Pro CCC Pro CCA Pro CCU Pro	CAU His CAC His CAA Gln CAG Gln	CGU Arg CGC Arg CGA Arg CGG Arg	U C A G
A	AUU lle AUC lle AUA lle AUG Met Start	ACU Thr ACC Thr ACA Thr ACG Thr	AAU Asn AAC Asn AAA Lys AAG Lys	AGU Ser AGC Ser AGA Arg AGG Arg	U C A G
G	GUU Val GUC Val GUA Val GUG Val	GCU Ala GCC Ala GCA Ala GCG Ala	GAU Asp GAC Asp GAA Glu GAG Glu	GGU Gly GGC Gly GGA Gly GGG Gly	U C A G