Genetic code

From EvoWiki

The genetic code is the way in which DNA nucleotide triplets map to amino acids. When DNA is translated into the polypeptide (protein) molecules three DNA letters (nucleotides) represent one polypeptide letter (amino acid). Each of the three letter words of DNA are called codons. There are four different nucleotides (A, C, G and T) in DNA, which are capable of producing 64 different three letter combinations. Most species' cells only require 20 amino acids, so many amino acids are represented by more than one codon, a state known as the degeneracy of the genetic code.

In protein biosynthesis, DNA is transcribed to messenger RNA (mRNA) before it is translated into polypeptide. In RNA the letter T is replaced by a different nucleotide letter, U. The table below uses the mRNA rather than the DNA codons:

*Table 1.* The Genetic Code
		U	C	A	G
		2nd base
1st base	U	UUU Phenylalanine UUC Phenylalanine UUA Leucine UUG Leucine, Start	UCU Serine UCC Serine UCA Serine UCG Serine	UAU Tyrosine UAC Tyrosine UAA Ochre (Stop) UAG Amber (Stop)	UGU Cysteine UGC Cysteine UGA Opal (Stop) UGG Tryptophan
	C	CUU Leucine CUC Leucine CUA Leucine CUG Leucine, Start	CCU Proline CCC Proline CCA Proline CCG Proline	CAU Histidine CAC Histidine CAA Glutamine CAG Glutamine	CGU Arginine CGC Arginine CGA Arginine CGG Arginine
	A	AUU Isoleucine, Start² AUC Isoleucine AUA Isoleucine AUG Methionine, Start¹	ACU Threonine ACC Threonine ACA Threonine ACG Threonine	AAU Asparagine AAC Asparagine AAA Lysine AAG Lysine	AGU Serine AGC Serine AGA Arginine AGG Arginine
	G	GUU Valine GUC Valine GUA Valine GUG Valine, Start²	GCU Alanine GCC Alanine GCA Alanine GCG Alanine	GAU Aspartic acid GAC Aspartic acid GAA Glutamic acid GAG Glutamic acid	GGU Glycine GGC Glycine GGA Glycine GGG Glycine

¹The codon AUG both codes for methionine and serves as an initiation site: the first AUG in an mRNA's coding region is where translation into protein begins.
²This is a start codon for prokaryotes only.