A gene is a base sequence of DNA that codes for: the amino acid sequence…

The idea

A gene is a length of DNA whose base sequence carries information. For most genes that information is read, by way of messenger RNA, as the sequence of amino acids in a polypeptide, which is its primary structure and which determines how the chain folds and therefore what it can do. But the definition deliberately has a second half. Some genes are transcribed into RNA molecules that are never translated and whose job is done by the RNA itself: ribosomal RNA, which combines with proteins to form ribosomes and is produced at the nucleolus, and transfer RNAs, which carry amino acids to the ribosome. These are called functional RNAs. So a gene is not the same thing as a sequence that codes for a protein, and a stretch of DNA coding only for a transfer RNA is still a gene. Alleles are different versions of the same gene, that is different base sequences at the same locus.

What earns the marks

• A gene being a sequence of DNA bases.
• Stating that it codes for the amino acid sequence, or primary structure, of a polypeptide.
• Recognising that a gene may instead code for a functional RNA, naming ribosomal RNA or transfer RNA.
• Associating the production of ribosomal RNA with the nucleolus.
• An allele being a different base sequence of the same gene.

Common mistakes and how to put them right

• Defining a gene only as a section of DNA that codes for a protein, so functional RNA is left out.

• Saying a gene codes for a characteristic rather than for a polypeptide.

• Using gene and allele interchangeably.

• Saying the gene itself leaves the nucleus to be translated at a ribosome.

• Describing transfer RNA and ribosomal RNA as being made of protein.

Exam tips

• Learn the definition with both halves; a one-sided definition loses the mark.
• Write polypeptide rather than protein where the gene codes for a single chain.
• Use base sequence rather than genetic information, which examiners often ignore.