(a) Describe the main stages of mitotic division. (b) State four ways in which mitosis is important to living organisms.

(a) Main stages of mitotic division

Mitosis is a type of nuclear division that produces two daughter cells each having the same number of chromosomes as the parent cell. Before it begins (interphase) the chromosomes replicate so that each consists of two chromatids joined at a centromere. Mitosis then proceeds through four main stages:

1. Prophase - The chromosomes shorten, thicken and become visible, each seen as two chromatids joined at a centromere. The centrioles move to opposite poles and spindle fibres begin to form. Towards the end, the nuclear membrane and nucleolus break down and disappear.
2. Metaphase - The spindle is fully formed. The chromosomes line up singly along the equator (metaphase plate) of the spindle, each attached to a spindle fibre by its centromere.
3. Anaphase - The centromere of each chromosome splits, and the two chromatids (now called daughter chromosomes) are pulled apart to opposite poles of the spindle by the shortening spindle fibres.
4. Telophase - The daughter chromosomes reach the poles, lengthen and become indistinct. A new nuclear membrane forms round each group and the nucleolus reappears, giving two daughter nuclei. This is followed by cytokinesis, the division of the cytoplasm into two daughter cells.

(b) Four ways in which mitosis is important to living organisms

• It brings about growth by increasing the number of body (somatic) cells.
• It is used in the repair of worn-out tissues and healing of wounds by replacing damaged or dead cells.
• It is the basis of asexual reproduction in many organisms (for example binary fission, budding and vegetative propagation).
• It ensures genetic stability, because each daughter cell receives exactly the same number and type of chromosomes as the parent cell, maintaining the chromosome number of the species.