Genes and the Genetic Code

A gene is a fundamental unit of heredity, but its definition at the molecular level is more precise than simply "a section of DNA." This lesson explores what genes are, how the genetic code works, and the structural features of eukaryotic genes — all essential knowledge for AQA A-Level Biology.

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Defining a Gene

Key Definition: A gene is a sequence of DNA nucleotides that codes for a functional polypeptide (or a functional RNA molecule such as tRNA or rRNA).

Important clarifications:

• Not all DNA codes for proteins. In the human genome, only about 1.5% of DNA is protein-coding.
• Some genes code for functional RNA molecules (e.g., ribosomal RNA, transfer RNA) that are never translated into protein.
• Each gene occupies a specific position on a chromosome called its locus (plural: loci).
• Different versions of a gene at the same locus are called alleles.

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The Triplet Code

The sequence of bases in DNA is read in groups of three called triplets (or codons when referring to mRNA). Each triplet codes for one amino acid.

Properties of the Genetic Code

Property	Meaning	Significance
Triplet	Three bases code for one amino acid	With 4 bases taken 3 at a time, there are 4³ = 64 possible codons — enough to code for all 20 amino acids
Degenerate (redundant)	Most amino acids are coded for by more than one codon	Provides some protection against the effects of point mutations (e.g., a change in the third base of a codon may still code for the same amino acid)
Non-overlapping	Each base is part of only one codon	A change in one base affects only one amino acid, not multiple
Universal	The same codons code for the same amino acids in almost all organisms	Supports the theory of a common ancestor; enables recombinant DNA technology (a human gene will produce the same protein in a bacterium)
Has start and stop signals	AUG is the start codon (methionine); UAA, UAG, UGA are stop codons	Defines the reading frame and the beginning and end of translation

Exam Tip: The universality of the genetic code is a key piece of evidence for evolution from a common ancestor. It also explains why genes can be transferred between species in genetic engineering and still produce functional proteins.

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Reading Frames

The sequence of bases can be read in three possible reading frames, depending on where reading begins. For example, the sequence AUGCGAUUC could be read as:

• Frame 1: AUG | CGA | UUC → Met-Arg-Phe
• Frame 2: A | UGC | GAU | UC → different amino acids
• Frame 3: AU | GCG | AUU | C → different amino acids

The start codon (AUG) establishes the correct reading frame. An insertion or deletion mutation shifts the reading frame downstream of the mutation, potentially altering every subsequent amino acid — this is called a frameshift mutation.

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Introns and Exons

Eukaryotic genes contain two types of sequences:

• Exons — coding sequences that are expressed. Exons are retained in the mature mRNA and are translated into amino acids.
• Introns — non-coding sequences that intervene between exons. Introns are transcribed into pre-mRNA but are removed (spliced out) during mRNA processing.

Key Points

• Prokaryotic genes generally do not contain introns — their genes are continuous coding sequences.
• The presence of introns allows alternative splicing, where different combinations of exons can be joined together to produce different mRNA molecules (and therefore different proteins) from the same gene.
• Introns may also contain regulatory sequences that influence gene expression.
• Some introns are thought to be remnants of ancient genetic elements ("junk DNA"), but many have been shown to have regulatory functions.