Cell division


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In humans all reproduction is sexual. It involves joining together cells from each parent with half the normal number of chromosomes to make a new cell containing both parents' genetic material.

The cells from each parent that combine to form the zygote are called gametes. In humans the male gamete is called sperm, and the female gamete is called an egg. When the gametes join they form a cell called a zygote. Human sperm and eggs contain 23 chromosomes. Human zygotes contain 46 chromosomes

The type of cell division that produces gametes with half the normal chromosome number is called meiosis.

Gametes contain different genetic information to each other and to the parent cell.

Meiosis is responsible for causing genetic variation.

the female egg and the male sperm fuse to create a zygote cell which then turns into an embryo. 23 chromosomes from the male and female each make 46 chromosomes in 23 pairs

Process of fertilisation

These are helpful terms to learn:

  • gamete – cell with half the normal number of chromosomes, and only used for sexual reproduction
  • zygote – cell formed when two gametes combine
  • fertilisation – term to describe the joining of two gametes
  • haploid – having half the normal number of chromosomes
  • diploid – having the normal number of chromosomes


All gametes are haploid. They also have other adaptations to increase the chances of fertilisation and successful development of the embryo.

Sperm cells need to move to the egg cell. They have a tail to propel them, and many mitochondria to provide energy. The front of the sperm contains enzymes to digest the egg membrane.

The egg contains a large food store to support the developing zygote until it can get food via the placenta.


Meiosis is the type of cell division used in sexual reproduction. It occurs only in the testes and ovaries. It produces cells that are different to each other, and to the parent cell. The cells produced contain half the normal number of chromosomes.

chromosomes divide, similar chromosomes pair up, sections of DNA get swapped, pairs of chromosomes divide,  chromosomes divide

Stages of meiosis


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