Meiosis

Meiosis is a type of cell division produces gametes occurs only in gamete mother cells results in variation Gamete mother cells and gametesI) GAMETE MOTHER CELLS: are diploid (2 sets of chromosomes) undergo meiosis to produce gametes II) GAMETES are haploid (1 set of chromosomes) fuse together during fertilisation to form a zygote Location of gamete mother cells and gametesMammals testes- sperm ovaries- ova Flowering plants anther- pollen ovary- ovules Process of meiosis consists of 2 consecutive divisions reduces the chromosome number by half produces 4 genetically different gametes First meiotic division:1. Chromosomes are long, thin and uncoiled. Each chromosome replicates to form 2 identical chromatids.2. Chromosomes coil becoming short and thick making them visible. Homologous chromosomes pair up.Crossing over occurs at points where the chromosomes touch- these points are called chiasmata. 3. Homologous pairs of chromosomes line up at the equator with their centromeres attached to spindle fibres.Independent assortment occurs at these points.4. Homologous pairs move to opposite poles.Each chromosome is still made up of two chromatids joined at centromere.5. Cytoplasm divides and nuclear membrane is formed.Two haploid cells have been formed.END OF FIRST MEIOTIC DIVISIONSecond meiotic division:1. The nuclear membranes disappear, chromosomes line up at equator with their centromeres attached to spindle fibres.2. The centromeres split allowing the chromatids to be pulled to opposite ends of the poles.3. Nuclear membranes reform, cytoplasm divides.END OF SECOND MEIOTIC DIVISIONResult meiotic division: 4 haploid gametes have been formed (contain half the amount of genetic material in a normal cell) Each gamete contains a different genetic combination Variation helps a species survive and adapt to a changing environment and so is important in long term evolutionary change is determined by genes, carried on chromosomes is increased by meiosis is increased at fertilisation during sexual reproduction Meiosis and VariationMeiosis increases variation by allowing new combinations of existing genetic material to be formed in gametes. This happens in 2 ways:A: Crossing Over occurs during the first meiotic division occurs between members of a homologous pair of chromosomes exchanges parts of 1 chromatid with another is random results in chromosomes with new combinations of genes results in variable gametes being formed at the end of meiosis B: Independent Assortment occurs during first meiotic division is random homologous chromosomes line up along the equator with the arrangement of any one pair unaffected by that of the others the way the chromosomes line up along the equator will determine the genetic combinations present within a particular gamete Fertilisation occurs when a male haploid gamete fuses with a female haploid gamete produces a diploid zygote which has a new combination of genes (one set from each parent) is random leads to a huge potential variability of offspring Comparison between mitosis and meiosisFeature, Mitosis, MeiosisPlace where it occurs: Body tissues, Sex organsPairing of homologous chromosomes: no, yesChiasmata formation: no, yesNumber of divisions of nucleus: 1, 2Number of daughter cells: 2, 4Ploidy of daughter cells: diploid, haploidEffect on chromosome number: none, halvedProcess providing variation: none, crossing over & independent assortment Now try the Meiosis Quiz by laldypop