Two forms of division:

For growth, repair, replacement and asexual reproduction.

The daughter cells are identical to the parent.

For the formation of gametes, and ensures that variation occurs.

The daughter cells have half the number of chromatids than that of the parents.

Before division, the nucleus enlarges, there are chromosome in the nucleus(23 pairs in humans). Mitosis is the behaviour of these chromosomes during cell division. Stages of mitosis:

The chromosomes will react more readily to stains and chemical fixatives. They shorten and thicken - by coiling up tightly. The nuclear membrane dissolves, leaving the chromosomes suspended in the cytoplasm; and at the same time the one or more nucleoli disappear.

Small bodies called centrioles lie just outside the nucleus. At this stage they move away from each other and go to opposite ends of the cell. From each centriole comes cytoplasmic fibres which meet and join at the centre of the nucleus. This system of fibres makes a web-like structure called a spindle. The chromosomes attach themselves to the "equatorial" region of the spindle. Each chromosome consists of two strands called chromatids, joined at the centromere. In forming two chromatids the chromosome doesn't split but reproduces itself exactly. This replication has already occurred at prophase, but is more evident during metaphase.

The two chromatids separate at the centromere, and go to opposite ends of the nucleus(towards spindles).

The chromatids, now chromosomes become less distinct, they probably become uncoiled and therefore are thinner. The one or more nucleoli reappear, and the nuclear membrane forms around each group of daughter chromosomes. There are now two nuclei in the cell. Now the cytoplasm between the 2 two nuclei will constrict, and two cells are formed.

Meiosis takes place in testis and ovary cells in mammals, and in the stamen and ovary cells in plants.

One diploid(2N) cell with 2 chromosomes gives rise to FOUR haploid(N) cells.

Pairing - The chromatin network shrinks into distinct threads, the chromosomes - which shorten the thicken. Homologous chromosomes move towards each other and PAIR, lying side by side.

Crossing Over - The chromatids intertwine and form points of connection, CHIASMATA. At each point there is a possibility of crossing over. Each pair of homologous chromosomes consist of 4 chromatids = a tetrad. Nuclear membrane dissapears. Spindle of protein fibres forms, from one centrosome to the other, across the cell. The homologous chromosomes repel one another, remaining attached at the chiasmata.

The chromosomes arrange themselves in PAIRS on the equator of the spindle. The centromeres do NOT divide. A short spindle fibres becomes attached to each centromere.

The spindle fibres shrink - drawing the chromosomes of a homologous pair apart. At the end of anaphase, there is one of each pair of chromosomes at each pole of the cell.

The nucleus may reform, but generally it goes directly into the second meiotic division.

Centromere - A region of a chromosome that becomes associated with the spindle fibres during mitosis and meiosis.

Chiasmata - The cross-shaped arrangement of the chromatids which is formed at their point of exchange during crossing over.

Chromatid - Each of the longitudinal subunits of a duplicated chromosome that become visible during mitosis and meiosis.

Chromomere - One of many bead-like structures visible along a chromosome during prophase of meisos and mitosis.

Crossing Over - A process of exchange between homologous chromosomes which may give rise to new combinations of characters. It takes place by the breaking and rejoining of chromatids and leads to the formation of chiasmata.

Diploid - Cells, phases of the life cycle, and the organisms which are characterised by having two sets of chromosomes.

Gamete - A mature reproductive cell which is capable of fusing with a similar cell of opposite sex to give a zygote.

Homologous Chromosomes - Chromosomes that are identical in their shape, size and the content and distribution of their genes.

Meiosis - The reduction division of the nucleus in which the zygotic number of chromosomes is reduced to the gametic number.

Segregation - Separation of pairs of unlike alleles(A and a) into different cells as a result of meiosis.

Somatic Cells - "Body cells" which arn't destined to become a gamte.

Spindle - A barrel-shaped structure made of microtubules that is formed in a cell during division of the nucleus, and which serves to align and move the chromosomes at metaphase and anaphase.

Zygote - A cell formed by the fusion of two gametes.