• DNA is located in the chromosomes which coil and are therefore visible during cell division.
• The chromosome complement (karyotype) of most cells consists of homologous pairs of chromosomes.
• Individuals of the same species have the same karyotype.. However, karyotype varies between species both in the number and the morphology of the chromosomes.
• Each chromosome of a species carries a particular set of genes controlling the characteristics of that species.

The chromosomes are uncoiled and therefore cannot be seen. DNA replication takes place during interphase.

chromosomes start to coil, spindle starts to form. (Question: find out what the spindle consists of)

Chromosomes visible, each longitudinally divided into identical chromatids held together at the kinetochores (centromeres)

nuclear envelope breaks down and nucleolus disappears. Chromosomes continue to coil and become shorter and thicker.

Chromosomes align at the equator of the spindle. They are attached to the spindle by their kinetochores.

The two chromatids of each chromosome separate and go to opposite poles.

Chromatids move towards the poles, the kinetochores lead the way.

Chromosome movement continues. Cleavage furrow begins to form.

Chromosome movement complete.

Cleavage completed, chromosomes uncoil, nuclear envelope and nucleoli reappear.

Two daughter cells in the G1 stage of interphase.

In mitosis the rates of cell division vary.

most bacterial cells 15-20 minutes

most mammalian cells 12-40 hours

Evidence from autoradiography shows that chromatids are formed during late interphase.

In meiosis diploid cells divide to produce four daughter cells each of which is haploid. It is the type of cell division requires to produce gametes.

The homologous chromosomes pair up, each chromosome consists of two chromatids. Chiasmata and crossing over occurs. The homologous chromosomes exchange portions of chromosome (see separate notes for details of this and why it is important).

The chromosomes line up in pairs at the spindles equator. Chiasmata may continue.

The homologous chromosomes separate to opposite poles while the chromatids of each chromosome stay together.

4. The daughter nuclei from the first division contain a chromosome from each homologous pair. No DNA synthesis or chromosome duplication has taken place (telophase 1)

5. In the second meiotic division the chromosomes behave as in mitosis. One chromatid of from each chromosome goes to each of the poles (metaphase II, anaphase II).

Two spindles have formed in the one cell.

6. Thus each cell entering meiosis produces four daughter cells. Each homologous chromosome segregates independently of the other.

e.g. A enters the same daughter cell as B or b with equal frequency. Thus another (Aa, Bb, XY) cell entering meiosis could produce daughters of abY, ABY aBX, AbY with an equal probability.