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Meiosis keeps the chromosome number constant from one generation to the next. It generates genetic variation in the gametes & therefore the zygotes that they produce. In 2 ways: crossing over during prophase1 & independent assortment in both metaphases. In the long term, if a species is to survive in a constantly changing environment & to colonise new environments, sources of variation are essential.

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Cytokinesis takes place producing 4 haploid daughter cells.

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At the poles, the chromatids lengthen & can no longer be distinguished in the microscope. The spindle disintegrates & the nuclear envelope & nucleoli re-form.

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The centromeres divide & the spindle fibres shorten, pulling the chromatids to opposite poles.

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The division of cytoplasm occurs making two haploid cells. The cell begins to cleavage.

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Chromosomes in each bivalent separate & spindle fibres shorten, one of each pair is pulled to opposite poles. Each pole receives one of each homologous chromosome from each pair of chromosomes. Because of the assortment in metaphase, there is a random mixture of parental & maternal chromosomes.

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The chromosomes reach the poles. In some species the nuclear envelope reforms around the haploid groups of chromosomes & the chromosomes condense. In many species the chromosomes stay in their condensed form.

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Homologous chromosomes align at the equator because of the spindle fibres. Independent assortment - a combination of maternal & paternal chromosomes face each pole & the combination of chromosomes that goes to each daughter cell at meiosis 1 is random the respect with which parent they came from.

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The chromosomes line up on the equator, with each chromosome attached to a spindle fibre by its centriole. Independent assortment happens because the chromatids of the chromosomes can face either pole.

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The centrioles separate & organise a new spindle at right angles to the old spindle.

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It is sometimes describes as resembling mitosis, as there is no pairing of homologous chromosomes at metaphase & it's the chromatids rather than homologous chromosomes that separate at anaphase.

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At the stage paternal & maternal chromosomes come together in homologous pairs. This pairing is called synapsis & each pair is a bivalent. Chromosomes condense, homologous chromosomes pair up & chromatids wrap around each other at points called chiasmata/chiasma(crossing over, homologs trade equivalent DNA).

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Meiosis takes place in the reproductive organs of plants, animals & some protoctistans. It results in the formation of 4 genetically different haploid cells(gametes/germ cells). When 2 haploid cells fuse at fertilisation, the zygote formed restores the diploid condition.

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The division of cytoplasm to make 2 cells. Occurs by constriction of parent cell around the equator from outside inwards(in animal cells). In plant cells, droplets of cell wall material, a cell plate, form across the equator of the parent cell from the centre outwards & they extend and join to form new cell wall.

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Length of the cell cycle is controlled by genes. If the gene is damaged, cells may fail to divide, divide too often or at the wrong time. Genes control the cell cycle like a brake, if the gene is mutated the brake is damaged, & cell cycles may continually happen. If this happens a tumour can form. The genes are called 'tumour supressor genes'. These genes have the potential to cause cancer, if they become mutated of infected. Before they are altered they are called proto-oncogenes. Once they are altered & able to cause cancer, they're called oncogenes.

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A very rapid stage. Centromere sprees & spindle fibres shorten, pulling the now spirited chromatids to the poles, centromere first.

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Chromatids reached poles(now called chromosomes). Chromosomes lengthen, spindle fibres breakdown, nuclear envelope reforms & nucleolus reappears.

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Produces complete offspring that are genetically identical to the parent. Takes place in unicellular organisms such as yeast & bacteria, also some insects(greenfly). Also takes place in some flowering plants, occurs when organs such as bulbs/runners/tubers produce large numbers of identical offspring in a short time. They also reproduce sexually. There is no genetic variation between individuals formed from asexual reproduction as they are genetically identical.

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Chromosome number - Each chromosome in the daughter cells is an exact replica of those in the parental cell. The genetically identical daughters give genetic stability.
Growth - growth & repair of tissues & cells(body cells). Some tissues such as skin in mammals is constantly being worn away, mitosis replaces these cells. Mitosis occurs in plants in small groups of cells, in the root & shoot apex, called meristems.

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Longest phase of the cycle, with most metabolic activity. Newly formed cell grows & replicates organelles(that were previously lost). DNA replicates, proteins are synthesised which requires ATP. Chromatin dispersed throughout the nucleus, so you cannot see it through a microscope.

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Mitosis produces two daughter cells that are genetically identical to the parent cell & each other. Dividing cells undergo a regular pattern known as the cell cycle. Cells are used for growth & repair.

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Chromosomes attach to the spindle fibres at their centromeres & align on the equator.

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The longest phase of mitosis. Chromosomes condos - shorter & thicker becoming distinguished as a pair of chromatin. Centrioles separate & move to opposite ends(poles) of the cell. Protein microtubules form, creating the spindle extending from pole to pole. Towards the end, nuclear envelope disintegrates & nucleolus disappears.

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Normal human body cells have 46 chromosomes.
In human cells there are 23 chromosomes in a haploid cell. There are 46 chromosomes in a diploid cell(somatic cell). The chromosomes in a homologous pair(matching) are identical in size, shape & carry the sam gene loci, with genes for the same characteristics. One chromes of each pair comes from each parent. Pairs of sex chromosomes, X and Y are different sizes & not homologous pairs. The number of complete sets of chromosomes in an organism is its ploidy number. Organisms with more than 2 complete sets of chromosomes are described as polyploid.

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They are made of DNA & a protein called histone. The DNA molecule is a double helix, running the length of the chromosome, with sections along it called genes. Chromosomes are only visible when chromatin condenses before cell division, each molecule of DNA replicates itself. The two copies of chromosomes are sister chromatids & they lie parallel along their length, joined at the centromere.