Science

Cell division

Chromosomes are made from DNA. Genes are short sections of DNA. Genetically identical cells are produced by a type of cell division called mitosis. In sexual reproduction, a male gamete fuses with a female gamete to produce a new cell. This is called fertilisation. Gametes are produced by a type of cell division called meiosis. They contain a single set of chromosomes, whereas body cells contain two sets of chromosomes.

DNA, genes and chromosomes

DNA

DNA [DNA: The material inside the nucleus of cells, carrying genetic information. DNA stands for Deoxyribonucleic Acid. ] (deoxyribose nucleic acid) molecules are large and complex. They carry the genetic code that determines the characteristics of a living thing.

Except for identical twins, each person’s DNA is unique. This is why people can be identified using DNA fingerprinting. DNA can be cut up and separated, forming a sort of 'bar code' that is different from one person to the next.

Genes

A gene [gene: The basic unit of genetic material inherited from our parents. A gene is a section of DNA which controls part of a cell's chemistry - particularly protein production. ] is a short section of DNA. Each gene codes for a specific protein by specifying the order in which amino acids must be joined together.

Chromosomes

The cell’s nucleus contains chromosomes [chromosomes: Rod shaped bodies found in the nucleus of cells that contain genetic information (DNA). ] made from long DNA molecules.

The diagram shows the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes.

Chromosome, showing gene as a section of DNA

Nucleus, chromosome and gene

Mitosis

Mitosis is the type of cell division that leads to growth or repair. When a cell divides by mitosis:

  • two new cells form
  • each cell is identical to the other one, and the cell they were formed from

Stages of mitosis

  1. parent cell
  2. chromosomes make identical copies of themselves
  3. they line up along the centre
  4. they move apart
  5. two daughter cells form with identical chromosomes to the parent cell

Gametes and fertilisation

Gender

Human body cells have 23 pairs of chromosomes in the nucleus. One of these pairs controls the inheritance of gender - whether offspring are male or female:

  • In males, the two sex chromosomes are different. They are XY.
  • In females, the two sex chromosomes are the same. They are XX.
23 pairs of chromosomes

Chromosomes from a female

23 pairs of of chromosomes

Chromosomes from a male

Gametes

Gametes are sex cells. The male gametes are the sperm, and the female gametes are the eggs.

Gametes contain one set of genetic information, while body cells contain two sets of genetic information.

Fertilisation

Fertilisation is the joining or fusion of a male gamete and a female gamete. The new cell that is formed divides over and over again by mitosis. This creates the many cells that eventually form a new individual.

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Meiosis - higher

Gametes are formed from cells in the reproductive organs by a type of cell division called meiosis.

MaleFemale
reproductive organtestesovaries
gametesspermeggs

The cells that are formed by meiosis have half as many chromosomes as the cell that formed them. Human body cells contain 23 pairs of chromosomes, while human gametes contain 23 single chromosomes.

The main features of meiosis are:

  • the chromosomes are copied
  • the cell divides twice, forming four gametes

The diagram outlines how this works.

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

Stages of meiosis

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