6.2.1—Gene

A gene is a defined sequence of nucleotides in a DNA molecule that contains the information for producing a specific polypeptide. In the wider idea of gene expression, a DNA sequence is used to make an RNA message and, for a protein-coding gene, that message specifies an amino-acid sequence.

• Information source: The ordered DNA bases in the gene carry the sequence information; the gene is a section of a DNA molecule, not the whole chromosome.
• Expression chain: DNA base sequence → transcribed RNA information → codons read in order → amino-acid sequence → polypeptide.
• Product boundary: The polypeptide sequence is the immediate product specified by this syllabus objective. Its folding and interactions can give the final protein its structure and function.
• Expression boundary: Having a gene in the DNA does not mean it is being used at the same level in every cell or situation; gene expression refers to when and how the information is transcribed and used.

Because the DNA base order is copied into an RNA message and decoded into amino-acid order, changing the gene sequence can change the instructions available for the polypeptide. The gene supplies information; it is not itself the RNA message, amino-acid chain or finished protein.

A gene is not the entire chromosome, all of the DNA in a cell, or a protein molecule. It is a nucleotide sequence within DNA that can be expressed to specify a polypeptide; the later transcription and translation steps explain how that information reaches the product.