The genetic code and the gene

Membranes, Proteins, DNA and Gene Expression · DNA, the genetic code and gene expression · note 4 of 7

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The genetic code and the geneSpec 2.11, 2.12

In short

A gene is a sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain. The genetic code is a triplet code, in which three bases code for one amino acid; it is non-overlapping, so each base is read once, and degenerate, so most amino acids have more than one triplet.

A gene is a sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain. The order of bases in the gene therefore determines the order of amino acids in the polypeptide, and so the protein's structure and function.

Features of the genetic code

Triplet code: a sequence of three bases (a triplet in DNA; a codon in mRNA) codes for one amino acid. There are four bases, so there are 4³ = 64 possible triplets, and these are enough to code for the 20 amino acids. (A doublet code would give only 4² = 16 combinations, too few.)
Non-overlapping: each base is part of only one triplet, so the sequence is read in separate groups of three, one after another.
Degenerate: most amino acids are coded for by more than one triplet. For example, several codons code for glycine. This means that some changes in the base sequence do not change the amino acid.
Some triplets are not amino acids: the codon AUG is a start codon (it also codes for methionine) and three codons (UAA, UAG, UGA) are stop codons, which end translation.

Reading a sequence in triplets

A section of the template strand of DNA reads TAC GGG TTT. State the number of amino acids coded for by these nine bases, and write the sequence of the mRNA transcribed from it.

The code is a non-overlapping triplet code, so divide the bases into groups of three: 9 ÷ 3 = 3 triplets.
Each triplet codes for one amino acid: 3 amino acids.
mRNA bases are complementary to the template: A–U, T–A, C–G, G–C.
TAC → AUG; GGG → CCC; TTT → AAA.

Answer: 3 amino acids. mRNA: AUG CCC AAA.

Common mistake:

'Degenerate' does not mean the code is damaged. It means there is more than one codon for an amino acid. 'Non-overlapping' does not mean there are gaps between triplets.

Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026

Frequently asked questions

What is the difference between DNA and RNA?

DNA is a double helix made of two polynucleotide strands, with deoxyribose and the bases A, T, C and G. RNA is a single polynucleotide strand with ribose, and uracil instead of thymine. DNA stores genetic information in the nucleus; mRNA, tRNA and rRNA are involved in protein synthesis.

What does semi-conservative replication mean?

In semi-conservative replication each new DNA molecule contains one original strand and one new strand. The helix unwinds, each strand acts as a template, and DNA polymerase joins complementary nucleotides to it. The Meselson and Stahl experiment, using heavy and light nitrogen, gave evidence for it.

What is the genetic code and why is it degenerate?

The genetic code is the set of rules by which base triplets code for amino acids. It is a non-overlapping triplet code. It is degenerate because there are 64 possible triplets but only 20 amino acids, so most amino acids are coded for by more than one triplet.

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