Cell Division Mitosis

Mitosis is just one part of the cell cycle The Mitotic (M) phase is the shortest part of the cell cycle (Cytokinesis may be included in this phase) Mitosis is a continuum, but can be separated into 4 phases

Mitosis Prophase Metaphase Anaphase Telophase

Prophase Chromatin condenses and becomes visible in the light microscope as chromosomes. The nucleolus disappears. Centrioles begin moving to opposite ends of the cell Mitotic spindle begins to form

Metaphase Nuclear membrane dissolves Spindle fibres align the chromosomes along the middle of the cell.

Anaphase The paired chromatids separate at the kinetochores and move to opposite sides of the cell. Speed of 1μm/min

Telophase Chromatids arrive at opposite poles of cell New membranes form around the daughter nuclei. The chromosomes disperse and are no longer visible under the light microscope. The spindle fibres disperse Cytokinesis may also begin during this stage.

Cytokinesis In animal cells, cytokinesis results when a fibre ring composed of a protein called actin around the centre of the cell contracts pinching the cell into two daughter cells, each with one nucleus. In plant cells, the rigid wall requires that a cell plate be synthesized between the two daughter cells.

Meiosis

Meiosis is another form of cell division that is associated with reproduction in many organisms In humans, meiosis is responsible for the formation of the reproductive cells or gametes SPERM CELL EGG CELL In humans, these are the egg and sperm cells Within the human ovaries and testes, gametes are produced by meiosis and this process halves the chromosome number Human body cells are DIPLOID as they possess two sets of chromosomes (23 pairs) Human gametes are described as being HAPLOID as they possess only one set of chromosomes (23 chromosomes) If the gametes were diploid then the number of chromosomes would double at every generation after fertilisation

MEIOSIS MITOSIS Diploid body cell Two diploid daughter cells The nucleus divides twice Four haploid, genetically different gametes are produced Meiosis is important as it ensures that, when the gametes fuse at fertilisation, the normal diploid number of chromosomes is maintained; meiosis is also an important source of genetic variation

Use Word Handout to go over each stage and variation

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16 Significance of Meiosis Introduces some genetic variation Maintains genetic stability Each haploid (n) gamate carries only 1 gene for a particular characteristic Crossing over leads to new combinations of genes Independent assortment of chromosomes mixes up combinations of chromosomes