Cell division

• Interphase - first growth phase. Organelles are synthesised and biochemicals produced
• Synthesis phase - DNA is replicated
• Second growth phase - energy stores are increased
• Prophase - chromosomes become visible, shorten and thicken. Spindle develops. Nuclear envelop disintergrates
• Metaphase - chromosomes are arranged on the equator
• Anaphase - chromosome migrate to opposite poles
• Telophase - spindle disintergrates. Nuclear envelop develops

Tumor - mass of cells that have divided too many times

• Benign tumor - slow growing. Stay within one tissue, usually are not life threatening but may cause damage e.g. by pressing on important nerve or blood vessel
• Malignant tumor - grow rapidly. Invade surrounding tissues and they also have cells that break off from the main tumor, these can spread around the body in blood or lymph vessels and form secondary tumor

Proto-oncogenes - a gene that stops cells dividing by mitosis too often, work by;

1) Some proto-oncogenes code for receptor proteins in the cell membrane, when these are activated by a specific growth factor they activate the genes that stimulate cell division

2) Produce the growth factor that stimulate cell division

Oncogenes - a mutation of proto-oncogenes, cause uncontrolled cell division by either;

1) Producing a different form of the receptor protein taht stimulates cell divion, even when growth factor is not present

2) Produce uncontrolled amounts of growth factor

Haploid - cells contain half the number of chromosomes i.e. 23

Diploid - cells contain full number of chromosomes i.e. 46

• Interphase - DNA replicates
• First meiotic division - cells divide resulting in two daughter cells
• Second meiotic division - cells divide resulting in four daughter cells

Meiosis occurs in gametes which means full diploid number is restored at fertilisation

Mistakes in meiosis

Non-disjunction - failure of chromosomes separating properly during meiosis. Resulting in gametes containing the wrong number of chromosomes

• Non-disjunction of chromosome 21 during meiosis will result in Down's syndrome, meaning individual will have 47 chromosomes instead of 46