DNA and cell division

Cromozones are made from DNAGenes are Small 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.

DNADNA (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.

GenesA gene 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.

Chromosones

Chromosomes The cell’s nucleus contains chromosomes made from long DNA molecules. The diagram shows the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes.

MitosisThe chromosomes contain the cell’s genetic information. They are normally found in pairs in body cells. Body cells divide by a process called mitosis.

The diagram summarises how this happens. Note that you do not need to know or understand the different stages in mitosis for the examination.

When a body cell divides by mitosis: The genetic material is copied The cell divides once to form two genetically identical body cells

Mitosis occurs during growth or to produce replacement cells.

Gametes Cells in the reproductive organs (testes and ovaries in humans) divide to form gametes. Gametes are sex cells: Male gametes are sperm (produced in the testes) Female gametes are eggs (produced in the ovaries)

Gametes contain one set of genetic information, while body cells contain two sets of genetic information.The type of cell division that produces gametes is called meiosis.

FertilisationFertilisation is the joining or fusion of a male gamete and a female gamete.When fertilisation happens, a single body cell with new pairs of chromosomes is formed.The new cell then divides over and over again by mitosis. This creates the many cells that eventually form a new individual.

Differentation  Most types of animal cells differentiate at an early stage - they become specialised for a particular function and cannot change into different types of cell. In mature animals, cell division is mainly restricted to replacement and repair. However, many plant cells keep the ability to differentiate.

Stem CellsStem cells can be made to differentiate to form different types of cell, such as nerve cells.Human stem cells can come from human embryos or from adult bone marrow.They are able to develop into any kind of human cell. Note that you do not need to know any details of how this happens for the examination.

Stem Cell TreatmentStem cells have the potential to be used in new treatments for conditions such as Parkinson's disease and paralysis.There are social and ethical issues concerning the use of human embryonic stem cells in medical research and treatments.

DNA and Cell division

Mitosis

Gametes and fertilisation

Differentiation and stem cells