DNA and the genome

Inheritance, variation and evolution · Reproduction · note 4 of 8

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DNA and the genomeSpec 4.6.1.4

In short

DNA is the genetic material in the nucleus. It is a polymer made of two strands forming a double helix, and it is contained in chromosomes. A gene is a small section of DNA that codes for a particular sequence of amino acids, to make a specific protein. The genome is the entire genetic material of an organism.

The genetic material in the nucleus of a cell is a chemical called DNA. DNA is a polymer made up of two strands forming a double helix. The DNA is contained in structures called chromosomes.

A gene is a small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein.

Genome: The entire genetic material of an organism.
Gene: A small section of DNA on a chromosome that codes for a particular sequence of amino acids, to make a specific protein.
Chromosome: A structure in the nucleus that contains DNA.

Zoom-in sequence from a cell to its nucleus, to a chromosome, to a section of DNA double helix, with a gene shown as a short highlighted stretch of the DNA. — A gene is a short section of the DNA that makes up a chromosome in the nucleus.

Why the human genome matters

The whole human genome has now been studied, and this will have great importance for medicine in the future. You need to be able to discuss its importance. This is limited to three areas.

The search for genes linked to different types of disease. Knowing which genes are involved helps scientists understand diseases.
Understanding and treatment of inherited disorders. Finding the genes involved in a disorder is the first step towards developing treatments.
Tracing human migration patterns from the past. Comparing the genomes of different groups of people shows how they are related.

Written and checked against the AQA GCSE Biology (8461) specification · Updated October 2026

Frequently asked questions

Why is meiosis important for sexual reproduction?

Meiosis is important because it halves the number of chromosomes in gametes, so that fertilisation restores the full number. Human gametes have 23 chromosomes, and when an egg and a sperm join, the new cell has 46. Meiosis also makes all the gametes genetically different, which leads to variation in the offspring.

What is a genotype and phenotype?

A genotype is the alleles present for a characteristic, such as Bb, and a phenotype is how the characteristic is expressed, such as black fur. The alleles in the genotype operate at a molecular level to develop the characteristics seen in the phenotype. A black mouse could be BB or Bb, but a brown mouse must be bb.

What is a Punnett square used for in genetics?

A Punnett square is used to show the possible combinations of alleles from the gametes of two parents. Each box is equally likely, so it predicts the probability and ratio of the offspring's genotypes and phenotypes. For example, crossing two Bb mice gives a 3 : 1 ratio of black to brown, so a 25% chance of a brown mouse.

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