Cell Division: Mitosis and Meiosis
- Biology
- Ages 14–18 · Grades 9–12
What you’ll find out
- Mitosis produces two genetically identical cells for growth, repair and asexual reproduction.
- The cell cycle is controlled by checkpoints; failures can lead to cancer.
- Meiosis halves the chromosome number to make gametes and creates variation by crossing over and independent assortment.
No sign-up and no real names. In the lesson you check your understanding as you read, and practise until it sticks.
Mitosis: growth and repair
Many-celled organisms grow, and repair damaged tissues, by producing new cells. They do this through a type of cell division called mitosis. Before dividing, a cell copies every one of its chromosomes. Each chromosome then consists of two identical copies joined together. The chromosomes line up across the middle of the cell. Fibres pull the copies apart to opposite ends. The cell then splits in two. Each of the two daughter cells receives a complete, identical set of chromosomes. Mitosis happens millions of times a day, for example to heal a cut or replace worn-out skin. It can be watched under a microscope.
Meiosis: making sex cells
Sex cells, or gametes, such as eggs and sperm, are made by a different kind of division called meiosis. In meiosis, a cell divides twice, producing four cells. Each has only half the usual number of chromosomes: 23 in humans instead of 46. When a sperm fertilises an egg, the full number of 46 is restored. Meiosis also creates variety. Matching chromosomes swap sections. They are then shared out between the new cells in random combinations. As a result, every gamete is genetically different. This is why children of the same parents differ, unless they are identical twins.
Curriculum links: NGSS HS-LS1-4 · NGSS HS-LS3-2 · England KS4: Biology · Cambridge IGCSE: Biology · NCERT Biology, Class 11