Meiosis

• Asexual reproduction can be successful at creating new individuals but leaves the population vulnerable to changes in their environment as the offspring are genetical identical to their parents so there is limited genetic variation (except mutation)
• Sexual reproduction is the production of a new individual resulting from the fusion of gametes. Sexual reproduction produces new individuals that are not genetically identical to their parents. Some disadvantages of sexual reproduction is that it can be hard to find a mate in the wild and it also uses bodily resources. However it creates genetic variation that can lead to a better chance of surviving and going on to reproduce - contributed to natural selection and evolution
• A cell containing two full sets of chromosomes is called diploid (2n)
• Haploid nuclei contain one set of chromosomes amd are usually within specialised cells called gametes
• Sexual reproduction occurs when the two haploid nuclei fuse to form a new diploid cell called a zygote. This is called fertilisation
• The sex organs in more complex organisms are called gonads. The female animal gonads are the ovaries and the male animal gonads are the testes. The female plant gonad are the ovaries and the male plant gonad is the anther
• Meiosis is a reduction division and occurs in the sex organs.
• In animals, the gametes are formed directly from meiosis. In plants, meiosis forms special male cells called microspores and female cells called megaspores, which then develop into the gametes.
• In meiosis two nuclear divisions produce four haploid daughter cells, each with its own unique combination of genetic material. In meiosis, the contents of the cell, in particular, the DNA are replicated when the cell is in interphase
• In prophase 1 of meiosis, each chromosome appears in the condensed form with two chromatids. Homologous pairs of chromosomes associate with each other. Crossing over occurs.
• In metaphase 1, the spindle forms and the pairs of chromosomes line up on the metaphase plate
• In anaphase 1, the centromeres do not divide. One chromosome from each homologous paid moves to each end of the cell. As a result, the chromosome number in each cell is half that of the original
• In telophase 1, the nuclear membrane reforms and the cells begin to divide. In some cells, this continues to full cytokinesis and there may be a period of brief or prolonged interphase. During this interphase, there is no further replication of the DNA
• In prophase 1, new spindles are formed
• In metaphase 2, the chromosomes still made up of pairs of chromosomes line up on the metaphase plate
• In anaphase 2, the centromeres now divide and the chromatids move to opposite ends of the cell
• In telophase 2, the nuclear envelopes reform, cytokinesis gives 4 daughter cells
• Before meiosis starts, the chromosomes replicate to form chromatids joined by a centromere as in mitosis. In prophase 1 of meiosis, the two chromosomes of each pair, known as homologous pairs, stay close together. At this stage, crossing over (recombination) introduces genetic variation as the chromatids may break and recombine.
• The cell then goes into second division after telophase 1 without any further replication of the chromosomes.
• In telophase 2, cytokinesis occurs producing four haploid daughter cells, each with half of the chromosomes number of the original parent cell. These daughter cells later develop into gametes
• Meiosis reduces the chromosome number in gametes from diploid to haploid. This means that sexual reproduction is possible without each following generation having more and more genetic material
• Generation in meiosis is introduced by meiosis:

  • crossing over (recombination)
  • independent assortment (random assortment)
• Crossing over occurs in prophase 1 of meiosis when large, multi-enzyme complexes ‘cut and join’ bits of the maternal and paternal chromatids together. The point where the chromatids break are called chiasmata. These lead to exchange of genetic material which leads to added genetic variation. Errors in this process lead to mutation which leads to new combinations into the genetic makeup of a species
• Independent assortment happens because the maternal and paternal chromosomes are distributed into the gametes completely at random. Each gamete receives 23 chromosome, and any from 0-23 and come from the maternal and paternal chromosomes. There are more than 8 million possible genetic combinations within the sperm or egg. This guarantees huge variety in the gametes. This random assortment occurs at metaphase 1
• Crossing over