DNA, Genes and the Genome

This lesson covers the structure of DNA, the relationship between genes and proteins, and the importance of the human genome project. These are key topics in the AQA GCSE Combined Science Trilogy specification (8464) — Inheritance, Variation and Evolution.

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What Is DNA?

DNA stands for deoxyribonucleic acid. It is a large molecule found in the nucleus of cells and carries the genetic code — the instructions that control how an organism develops and functions.

DNA has a distinctive structure:

• It is a polymer made up of two strands twisted into a double helix
• Each strand is made up of repeating units called nucleotides
• Each nucleotide consists of a sugar (deoxyribose), a phosphate group and a base
• The two strands are held together by complementary base pairing

Complementary Base Pairing

There are four bases in DNA. They always pair in the same way:

Base	Pairs With
Adenine (A)	Thymine (T)
Cytosine (C)	Guanine (G)

$A \longleftrightarrow T \quad \text{and} \quad C \longleftrightarrow G$A⟷TandC⟷G