Meiosis

Meiosis is the process of copying, pairing and separating chromosomes to make reproductive cells such as eggs and sperm. Organisms that reproduce asexually use a form of cell division called mitosis to reproduce. *See Mitosis* Meiosis generally means a parent cell creating daughter cells. Because the cells divide twice, they end up with only 23 chromosomes, rather than the requisite 46 that are in other cells. These cells are gametes, or sex cells. This means when they pair up with during the fertilisation process, the resulting zygote has the correct total of 46 chromosomes. If an embryo has the wrong number of chromosomes, it will either suffer spontaneous abortion (miscarriage) or be born with a genetic defect such as down syndrome.

Cells are grouped in two very broad groups: Prokaryotes and Eukaryotes. Prokaryotes are cells in which all the genetic material floats within the cell in a spaghetti-like mass called a nucleiod. Eukaryotes, on the other hand, have a nucleus that contains the DNA at the cell’s centre. Eukaryotes also have organelles, such as mitochondria, chloroplasts, lysomes, the endoplasmic reticulum and the Golgi apparatus.

Eukaryotic DNA is grouped in a nucleus, in which the DNA is wrapped around histones (proteins which alter gene expression). This forms a rod like structure known as a chromosome. A chromatid is one of the strands that a chromosome splits into during cell division. When the maternal and paternal chromatids pair during prophase I to exchange DNA, they form and X-shaped structure, referenced throughout the text.

Meiosis takes place in two stages, that each have four phases: The prophase, metaphase, anaphase and telophase. Meiosis stage I: The duplicated chromatids have by now fused together, and become joined at the centrometre, forming an X shape.

  • Prophase I: The sister chromatids from the maternal chromosomes find an pair up with the corresponding chromatid from the paternal chromosome. During this pairing, the two chromatids exchange small bits of their DNA to create diverse daughter cells. This keeps any offspring healthy and genetically diverse.
  •  Metaphase I: Meiotic spindles, a type of thin protein filament, emerge from the centriole at either end of the cell. They reach out across the cell and attach themselves to the centre point of the X-shaped chromatids.
  • Anaphase I: The spindles start pulling away from the chromatids, back towards the centriole, and in doing so pulls them away from each other, and towards the ends of the cell.
  • Telophase I: The chromatids reach the ends of the cell, and the cell splits in two, leaving two daughter cells with one X-shaped chromatid each.

Meiosis stage II: Now, the two daughter cells divide again, this time creating identical copies, but with only half of the X-shaped chromatid in each cell.

  • Prophase II: The nuclear membrane of the cell disintegrates, and the spindles begin forming again
  • Metaphase II: The meiotic spindles latch onto the centre point of the chromatid, lining them up with cell’s centre.
  • Anaphase II: The spindles begin pulling the chromatid apart, until one half is on either side of the cell.
  • Telophase II: The nuclear membrane reforms, creating two separate cells. The process has created a total of four daughter cells with 23 chromosomes.

Each cell is unique, as each has a different percentage of paternal and maternal genetic material from when the chromatids exchanged chromosomal information during prophase I. It is important that the cells have diverse genes, otherwise over time, inbreeding and a lack of diversity would occur if all cells made exact copies of themselves to pass on to their offspring.

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