Meiosis

• In sexual reproduction two gametes join together at fertilisation to form a zygote. The zygote then divides and developes into a new organism.
• Meiosis is a type of cell division that happens in the reproductive organs to produce gametes.
• Meiosis involves a reduction division. Cells that divide by meiosis have the full number of chromosomes to start with, but the cells that are formed from meiosis have half the number. Cells with half the normal number of chromosomes are called haploid cells.
• Cells formed by meiosis are all genetically diifferent because each new cell ends up with a different combination of chromosomes.
• Meiosis involves two divisions: Meiosis 1 and Meiosis 2. Meiosis 1 is the reduction division (it halves the chromosome number).
• Similar to Mitosis, Meiosis 1 and 2 are each split into prophase, metaphase, anaphase and telophase stages.
• Prophase 1 - The chromosomes condense, getting shorter and fatter. The chromosomes arrange themselves into homologous pairs and crossing over occurs. Centrioles start movinf to opposite ends of the cell, forming the spindle fibres. The nuclear envelope breaks down.
• Metaphase 1 - The homologus pairs attach to the spindle fibres by their centromeres are line up along the centre of the cell.
• Anaphase 1 - The spindles contract, separating the homologus pairs - one chromosome goes to each end of the cell.
• Telophase - A nuclear envelope forms around each group of chromosomes.
• Cytokinesis - The division of the cytoplasm and two haploid daughter cells are produced.
• Meiosis 2 - The two daughter cells undergo prophase 2, Metaphase 2, Anaphase 2, Telophase 2 and Cytokinesis - whish are a lot like the stages in mitosis.
• In Anaphase 2, the pairs of sister chromatids are separated - each new daughter cell inherits one chromatid from each chromosome. Four haploid daughter cells are produced - These are the gametes.
• During Prophase 1, homologous pairs of chromosomes come together and pair up. The chromatids twist around each other and bits of chromatids swap over. The chromatids still contain the same genes but now have a different combination of alleles.
• The crossing over of chromatids in meiosis 1 means that each of the 4 daughter cells formed from meiosis contains chromatids with different alleles.
• Each Homologous pair of chromosomes are made up of one chromosome from each parent. When these line up and are separated it randomises which pair ends up in which daughter cell. Which means that each combination is completely random.