The genetic code is the set of rules by which the base sequence of DNA (and mRNA) is converted into the amino acid sequence of a polypeptide.Its key features are:• Triplet code: a sequence of three bases codes for one amino acid. On mRNA each triplet is called a codon. With four bases there are 43 = 64 possible codons, more than enough for the 20 amino acids. A doublet code (42 = 16) would not be enough.
• Degenerate: most amino acids are coded for by more than one codon. For example, glycine is coded by GGU, GGC, GGA and GGG.
• Non-overlapping: each base is read as part of only one codon, so the sequence is read in consecutive groups of three.
• Universal: the same codons code for the same amino acids in almost all organisms, which is evidence for common ancestry and lets genes work when transferred between species.
• Start and stop codons: AUG is the start codon (it also codes for methionine). UAA, UAG and UGA are stop codons that code for no amino acid and end the polypeptide.Worked example: a coding sequence has 300 bases, not counting the stop codon. 300 ÷ 3 = 100 codons, so it codes for 100 amino acids.
• Degenerate: most amino acids are coded for by more than one codon. For example, glycine is coded by GGU, GGC, GGA and GGG.
• Non-overlapping: each base is read as part of only one codon, so the sequence is read in consecutive groups of three.
• Universal: the same codons code for the same amino acids in almost all organisms, which is evidence for common ancestry and lets genes work when transferred between species.
• Start and stop codons: AUG is the start codon (it also codes for methionine). UAA, UAG and UGA are stop codons that code for no amino acid and end the polypeptide.Worked example: a coding sequence has 300 bases, not counting the stop codon. 300 ÷ 3 = 100 codons, so it codes for 100 amino acids.