the cell division

New cells are needed throughout life. These are for growth, to replace damaged cells and repair worn out tissues. Normal human body cells are diploid – they have two of each chromosome. When new cells are made, these 46 chromosomes (in other organisms the number is different) are copied exactly in a process called mitosis.

Humans are made of millions of cells. This has a number of benefits:

• cells can be specialised to do particular tasks
• groups of cells can function as organs making a more efficient but complex organism
• the organism can grow very large

Mitosis is the type of cell division used for growth and repair. Mitosis occurs wherever new cells are needed. It produces two cells that are identical to each other, and the parent cell.

Higher tier only

In mitosis each chromosome is copied exactly. The new chromosomes are moved to opposite sides of the cell, before the cell divides leaving one complete set of 46 chromosomes in each of the two new cells.

Diagram of the stages of mitosis

Constant cell division ensures that cells never get too large. The larger the cell becomes, the smaller its surface area to volume ratio. Objects with a small surface area to volume ratio find it difficult to maintain exchange of materials with their environment. Large cells could run out of oxygen, and accumulate too much waste, such as carbon dioxide. For this reason it's more efficient for large organisms to be multicellular, rather than single-celled.

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.

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.