Mitosis results in the production of additional cells for growth, repair and replacement.
Most animal cells differentiate at an early stage but most plant cells have the ability to differentiate into different types of speciallised cells.
Stem cells are unspecialised cells found in human embryos and in some adult tissues such as the bone marrow. Stem cells have the potential to differentiate into two types of specialised cells.
Cells which divide to form gametes undergo meiosis. Gametes have a single set of genetic information, whereas body cells have two sets. Fertilisation results in the formation of a cell with new pairs of chromosomes so sexual reproduction gives rise to variation.
Some characteristics are conrolled by a single gene. Each gene may have different forms called alleles. An allele which controls the development of a characteristic when it is present on only one of the chromosomes is a dominant allele is not present is a recessive allele.
Chromosomes are made up of large molecules of DNA. A gene is a small section of DNA.
In human cells, one of the 23 pairs of chromosomes carries the genes which determine sex. In females, the sex chromosomes are the same (XX) in males the sex chromosomes are different (XY).
Some disorders are inherited. Huntingtons disease is caused by a dominant allele of a gene. Cystic fibrosis is caused by a recessive allele and must be inherited from both parents. Embryos can be screened for the alleles that cause these other genetic disorders.