Sexual reproduction, meiosis and gamete formation

Sexual reproduction

Two parents are needed in sexual reproduction. During this process the nuclei of the male and female gametes are fused in order to create a zygote. This process is known as fertilisation. Gametes contain half the number of chromosomes of all other cells in the organism. This means they are haploid. When the male and female gametes combine in fertilisation they create an embryo with the full complement of chromosomes (diploid). Cells which are diploid have two sets of chromosomes - for most organisms this means the cells have one set of chromosomes from their mother and one set from their father.

The gametes in animals are sperm (male) and eggs (female).

The gametes in flowering plants are pollen (male) and ovules (female).

A sperm fertilising an egg
Plant reproduction: a pollen tube comes out of the ovary. Pollen grains are above the pollen tube.

The offspring produced in sexual reproduction are genetically different to each other and to their parents. This process results in variation within a population because it involves the mixing of genetic information.

Meiosis

This short video explains the role of meiotic cell division

Sexual reproduction uses the process of meiosis, which creates gametes. The process of meiosis happens in the male and female reproductive organs.

As a cell divides to form gametes:

  • A copy of all of the genetic information is made.
  • The cell divides twice to form four gametes, each with a single set of chromosomes haploid. This means the chromosome number has halved.
  • All gametes are genetically different from each other.
A diagram illustrating the process of meiosis and how it creates gametes

Fertilisation

Gametes have half the total number of chromosomes that the organism needs to develop and are referred to as haploid. For example, humans need 46 chromosomes to develop, therefore a human gamete has 23 chromosomes. Fertilisation is the fusion of the nucleus of a male gamete with the nucleus of a female gamete.

When the two gametes combine, they merge the two sets of chromosome to create a cell with the total number of chromosomes needed to develop, known as a diploid cell. In humans when the haploid sperm and egg cell join in fertilisation the resulting zygote has a total of 46 chromosomes the correct number to develop. By having gametes which are haploid, when the gametes combine, diploid cells are maintained. Also, the mixing of chromosomes in fertilisation is a source of genetic variation.

Fertilisation produces a zygote, which will mature into an embryo. The number of cells increases by mitosis, and as the embryo develops, the cells begin to differentiate (or specialise).

Egg (23 chromosomes) combines with sperm (23 chromosomes).  Fertilisation occurs creating a zygote with 46 chromosomes in 23 pairs. This matures into embryo with 46 chromosomes in 23 pairs.