How genes control proteins

Inheritance · Chromosomes, genes and proteins · note 3 of 4

Spec 17.1.5, 17.1.6, 17.1.7, 17.1.8

Supplement

Supplement: only for the Extended papers (2 and 4). Core students can skip it. What the labels mean

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How genes control proteinsSpec 17.1.5, 17.1.6, 17.1.7, 17.1.8

In short

The sequence of bases in a gene determines the sequence of amino acids used to make a specific protein. The gene stays in the nucleus, and a copy called messenger RNA (mRNA) is made. The mRNA moves to the cytoplasm and passes through a ribosome, which assembles amino acids into the protein in the order set by the bases.

A gene is a length of DNA. DNA contains a sequence of bases. The sequence of bases in a gene determines the sequence of amino acids used to make a specific protein.

Amino acid sequence and protein shape

A protein is a chain of amino acids. Different sequences of amino acids make chains that fold into different shapes. Each protein has a shape that suits its job. If the sequence changes, the shape may change, and the protein may no longer work.

DNA controls cell function

DNA controls cell function by controlling the production of proteins. Examples of proteins include:

enzymes, which control the chemical reactions in a cell
membrane carriers, which move substances across the cell membrane
receptors for neurotransmitters, which detect chemicals released at synapses

How a protein is made

The gene for a protein stays in the nucleus. A copy of the gene is made instead. This copy is called messenger RNA (mRNA).

The gene coding for the protein remains in the nucleus.
mRNA is made in the nucleus as a copy of the gene.
The mRNA molecule moves out of the nucleus to the cytoplasm.
The mRNA passes through a ribosome.
The ribosome assembles amino acids into a protein molecule.
The specific sequence of amino acids is determined by the sequence of bases in the mRNA.

A cell with a nucleus: the gene stays in the nucleus, mRNA is made in the nucleus as a copy of the gene, the mRNA moves out of the nucleus to the cytoplasm and passes through a ribosome, which assembles amino acids into a protein. — The gene stays in the nucleus; an mRNA copy carries the code to a ribosome in the cytoplasm.

Exam tip:

Learn the order: gene in nucleus, mRNA copy made, mRNA to cytoplasm, ribosome assembles amino acids. You do not need the details of transcription or translation.

Written and checked against the Cambridge IGCSE Biology (0610) specification · Updated October 2026

Frequently asked questions

What is the difference between a gene and an allele?

A gene is a length of DNA that codes for a protein, while an allele is an alternative form of a gene. For example, the gene for flower colour may have one allele for red flowers and another for white flowers. Different alleles have slightly different DNA, so they code for slightly different proteins.

How is the sex of a baby determined?

The sex of a baby is determined by which sperm fertilises the egg. Females are XX, so every egg carries an X chromosome. Males are XY, so half the sperm carry X and half carry Y. An X sperm gives a girl (XX) and a Y sperm gives a boy (XY), a 1 : 1 chance.

How do genes control the making of proteins?

Supplement Genes control protein production through their sequence of bases, which determines the sequence of amino acids in a protein. The gene stays in the nucleus, and a copy called mRNA is made. The mRNA moves to the cytoplasm and passes through a ribosome, which assembles amino acids into the protein in the correct order.

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