What Is the Genetic Code? Degenerate and Universal Explained
Learn what the genetic code means in AQA A Level Biology, why it is degenerate and universal, and how silent mutations and DNA triplets relate to mRNA codons.
The genetic code is the set of rules that links a sequence of bases in DNA to the sequence of amino acids in a polypeptide. It is degenerate because more than one base triplet can code for the same amino acid, and universal because a given triplet codes for the same amino acid in almost all organisms.
How the code works
A triplet is a sequence of three DNA bases. During transcription, the information in DNA is copied into messenger RNA (mRNA). A group of three bases on mRNA is called a codon. During translation, ribosomes read the codons to assemble amino acids in the order specified by the gene. The code is also non-overlapping: each base is read as part of one codon, not several.
Why degeneration matters
Several codons can specify the same amino acid. For example, the mRNA codons UUU\mathrm{UUU}UUU and UUC\mathrm{UUC}UUC both specify phenylalanine. If a base substitution changes one codon to the other, the amino acid stays the same. This is a silent mutation: the base sequence has changed, but the amino acid sequence has not. Degeneracy does not mean that every substitution is silent; some change the amino acid.
Why universality matters
The code is shared across almost all organisms. This helps explain why a gene transferred from one species to another can be used to make the same polypeptide in the recipient, provided the gene is expressed.
Exam mistake to avoid: do not call an mRNA codon a DNA triplet, or say that a silent mutation means no mutation occurred. The DNA sequence has changed even when the polypeptide’s amino acid sequence has not.
