B2.7.1 Cell division (part 1)
In body cells the chromosomes are normally found in pairs. Body cells divide by mitosis.
The chromosomes contain the genetic information.
When a body cell divides by mitosis:
- copies of the genetic material are made
- then the cell divides once to form two genetically identical body cells.
Mitosis occurs during growth or to produce replacement cells.
Body cells have two sets of chromosomes; sex cells (gametes) have only one set.
Cells in reproductive organs – testes and ovaries in humans – divide to form gametes.
The type of cell division in which a cell divides to form gametes is called meiosis.
B2.7.1 Cell division (part 2)
When a cell divides to form gametes:
- copies of the genetic information are made
- then the cell divides twice to form four gametes, each with a single set of chromosomes.
When gametes join at fertilisation, a single body cell with new pairs of chromosomes is formed. A new individual then develops by this cell repeatedly dividing by mitosis.
Most types of animal cells differentiate at an early stage whereas many plant cells retain the ability to differentiate throughout life. In mature animals, cell division is mainly restricted to repair and replacement.
Cells from human embryos and adult bone marrow, called stem cells, can be made to differentiate into many different types of cells, eg nerve cells.
Human stem cells have the ability to develop into any kind of human cell.
Treatment with stem cells may be able to help conditions such as paralysis.
The cells of the offspring produced by asexual reproduction are produced by mitosis from the parental cells. They contain the same alleles as the parents.
B2.7.2 Genetic variation
Sexual reproduction gives rise to variation because, when gametes fuse, one of each pair of alleles comes from each parent.
In human body cells, one of the 23 pairs of chromosomes carries the genes that determine sex. In females the sex chromosomes are the same (XX); in males the sex chromosomes are different (XY).
Some characteristics are controlled by a single gene. Each gene may have different forms called alleles.
An allele that controls the development of a characteristic when it is present on only one of the chromosomes is a dominant allele. An allele that controls the development of characteristics only if the dominant allele is not present is a recessive allele.
Chromosomes are made up of large molecules of DNA (deoxyribo nucleic acid).
A gene is a small section of DNA. Each gene codes for a particular combination of amino acids which makes a specific protein.
Each person (apart from identical twins) has unique DNA. This can be used to identify individuals in a process known as DNA fingerprinting.
B2.7.3 Genetic disorders
Some disorders are inherited.
Polydactyly – having extra fingers or toes – is caused by a dominant allele of a gene and can therefore be passed on by only one parent who has the disorder.
Cystic fibrosis (a disorder of cell membranes) must be inherited from both parents. The parents may be carriers of the disorder without actually having the disorder themselves. It is caused by a recessive allele of a gene and can therefore be passed on by parents, neither of whom has the disorder.
Embryos can be screened for the alleles that cause these and other genetic disorders.