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The genetic code

Of course, not all nucleotide sequence substitutions lead to amino acid substitutions in protein-coding genes. There is redundancy in the genetic code. Table 1 is a list of the codons in the universal genetic code.3 Notice that there are only two amino acids, methionine and tryptophan, that have a single codon. All the rest have at least two. Serine, arginine, and leucine have six.


Table 1: The universal genetic code.
  Amino   Amino   Amino   Amino
Codon Acid Codon Acid Codon Acid Codon Acid
UUU Phe UCU Ser UAU Tyr UGU Cys
UUC Phe UCC Ser UAC Tyr UGC Cys
UUA Leu UCA Ser UAA Stop UGA Stop
UUG Leu UCG Ser UAG Stop UGG Trp
               
CUU Leu CCU Pro CAU His CGU Arg
CUC Leu CCC Pro CAC His CGC Arg
CUA Leu CCA Pro CAA Gln CGA Arg
CUG Leu CCG Pro CAG Gln CGG Arg
               
AUU Ile ACU Thr AAU Asn AGU Ser
AUC Ile ACC Thr AAC Asn AGC Ser
AUA Ile ACA Thr AAA Lys AGA Arg
AUG Met ACG Thr AAG Lys AGG Arg
               
GUU Val GCU Ala GAU Asp GGU Gly
GUC Val GCC Ala GAC Asp GGC Gly
GUA Val GCA Ala GAA Glu GGA Gly
GUG Val GCG Ala GAG Glu GGG Gly


Moreover, most of the redundancy is in the third position, where we can distinguish 2-fold from 4-fold redundant sites (Table 2). 2-fold redundant sites are those at which either one of two nucleotides can be present in a codon for a single amino acid. 4-fold redundant sites are those at which any of the four nucleotides can be present in a codon for a single amino acid. In some cases there is redundancy in the first codon position, e.g, both AGA and CGA are codons for arginine. Thus, many nucleotide substitutions at third positions do not lead to amino acid substitutions, and some nucleotide substitutions at first positions do not lead to amino acid substitutions. But every nucleotide substitution at a second codon position leads to an amino acid substitution. Nucleotide substitutions that do not lead to amino acid substitutions are referred to as synonymous substitutions, because the codons involved are synonymous, i.e., code for the same amino acid. Nucleotide substitutions that do lead to amino acid substituions are non-synonymous substitutions.


Table 2: Examples of 4-fold and 2-fold redundancy in the 3rd position of the universal genetic code.
  Amino  
Codon Acid Redundancy
CCU Pro 4-fold
CCC    
CCA    
CCG    
AAU Asn 2-fold
AAC    
AAA Lys 2-fold
AAG    



next up previous
Next: Rates of synonymous and Up: Patterns of nucleotide and Previous: Introduction
Kent Holsinger 2012-10-30