Chromosomes, Mitosis and the Cell Cycle

This lesson covers the cell cycle, chromosomes and the process of mitosis as required by the AQA GCSE Combined Science Trilogy specification (8464). You need to describe the stages of the cell cycle, explain the importance of mitosis and understand what chromosomes are.

---

Chromosomes

Chromosomes are thread-like structures found in the nucleus of eukaryotic cells. They are made of tightly coiled DNA (deoxyribonucleic acid).

Key Facts About Chromosomes

• Each chromosome consists of a single, very long molecule of DNA wrapped around proteins.
• Human body cells contain 46 chromosomes, arranged in 23 pairs. Each pair consists of one chromosome from the mother and one from the father.
• The number of chromosomes in a body cell is called the diploid number (represented as 2n). For humans, 2n = 46.
• Sex cells (gametes — sperm and egg cells) contain half the normal number: 23 chromosomes. This is the haploid number (represented as n).

Cell Type	Number of Chromosomes	Ploidy
Human body cell	46 (23 pairs)	Diploid (2n)
Human gamete (sperm or egg)	23	Haploid (n)