Sexual reproduction, meiosis and gamete formation

Meiosis

Sexual reproduction uses a type of cell division called meiosis, which creates gametes, such as sperm and egg cells. The process of meiosis happens in the male and female reproductive organs. Just like in mitosis, a cell starts meiosis in interphase. In interphase, the DNA is copied, the cell grows and the organelles are copied too. After interphase:

  • two meiotic divisions occur which means the cell divides twice to form four gametes
  • each gamete has half the number of chromosomes found in body cells (one chromosome from each pair), and so is known as haploid

All gametes are genetically different from each other.

A diagram illustrating the process of meiosis and how it creates gametes

Comparing meiosis and mitosis

MitosisMeiosis
Diploid cells madeHaploid cells made
Used for growth and repairUsed for sexual reproduction
Cells made are genetically identical to starting cell and each otherCells made are genetically different to starting cell and each other
Two cells are producedFour cells are produced
One division occursTwo divisions occur
Interphase happens before cell divisionInterphase happens before cell division

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.

The gametes in:

  • animals are eggs (female) and sperm (male)
  • flowering plants are eggs/ovules (female) and found inside pollen (male)

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

Fertilisation in humans

Fertilisation is the fusion of the nucleus of a male gamet with the nucleus of a female gamete. In humans, each gamete has half the number of the total 46 chromosomes that the body requires. The 23 chromosomes within a gamete are referred to as a haploid.

When egg and sperm cells combine in fertilisation, they merge the two sets of chromosomes, ending up with 46 chromosomes in total. The maternal chromosomes from the egg cell and the paternal chromosomes from the sperm cell pair up.

The resultant cell is called a zygote. It is diploid as it has two copies of every chromosome - one came from the sperm cell and one came from the egg cell.

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.

The zygote will mature into an embryo. It has DNA from both the mother and the father so will have a mixture of characteristics from both parents. In this way, sexual reproduction introduces variation into a species.

The zygote grows by mitosis to form an embryo. As the embryo develops, the cells begin to differentiate.