IB Biology Topic D1.2.8: Features of the Genetic Code Notes

The genetic code is a triplet, degenerate, and near-universal set of rules

1. The genetic code is the set of rules by which the base sequence in DNA is translated into a sequence of amino acids.
2. It has three defining features: it is a triplet code, it is degenerate, and it is near-universal.

Codons are read in triplets because pairs cannot cover 20 amino acids

1. DNA is built from four bases: adenine (A), thymine (T), cytosine (C), and guanine (G).
2. Proteins, however, are built from 20 different amino acids.
3. The number of bases read per amino acid must therefore generate at least 20 distinct combinations.
   1. One base per amino acid gives only $4^1 = 4$ combinations, far too few.
   2. Two bases per amino acid give $4^2 = 16$ combinations, still short of 20.
   3. Three bases per amino acid give $4^3 = 64$ combinations, more than enough to specify all 20.

A unit of three bases is called a codon, so the genetic code is a triplet code.

Degeneracy means several codons can specify the same amino acid

1. The code is degenerate because most amino acids are coded by more than one codon.
2. Error tolerance is the main benefit, since many base substitutions do not change the amino acid produced.
3. Protein function is therefore protected against many point mutations.

Degeneracy is organised, not random

Each codon still has one specific meaning, so degeneracy adds redundancy without adding ambiguity.

Near-universality is evidence of common ancestry and enables biotechnology

1. The code is near-universal, meaning the same codons specify the same amino acids in almost all organisms, from bacteria to humans.
2. Common ancestry is strongly suggested, because a shared code points to a single origin for all life.
3. Gene transfer between species is possible, because a gene from one organism is read the same way in another.
   1. Human insulin, for example, is produced in bacteria carrying the human insulin gene.

Rare exceptions explain why it is near-universal, not fully universal

1. A few organisms and organelles read some codons differently from the standard code.
2. In some mitochondria, for example, the codon UGA codes for tryptophan instead of acting as a stop codon.