Meiosis

• Meiosis is a type of cell division that results in four non-identical daughter cells, each with half the number of chromosomes of the parent cell. This process is critical for sexual reproduction.
• This process involves two rounds of division: meiosis I and meiosis II.
• At the start of meiosis, the DNA in the cell is duplicated during the S phase of the interphase cycle.
• Stage one, “meiosis I”, separates the pairs of homologous chromosomes.
• In prophase I of meiosis I, each chromosome pairs with its homologous chromosome to form a structure known as a bivalent or tetrad. This is also the stage where crossing over occurs. Crossing over is the exchange of genetic material between homologous chromosomes, which contributes to genetic variation.
• Metaphase I involves the arrangement of chromosome pairs along the cell’s equator, where they are attached to the spindle fibres.
• Anaphase I sees the separate homologous chromosomes move to opposite poles of the cell.
• Telophase I and cytokinesis completes the division of the cell into two cells, each with a haploid set of chromosomes in duplicated form.
• Meiosis II then separates the sister chromatids. The phases of meiosis II - prophase II, metaphase II, anaphase II, telophase II and cytokinesis - are similar to the phases in mitosis, but the outcome is the creation of four haploid daughter cells, genetically distinct from the parent cell.
• One key difference between mitosis and meiosis involves the genetic composition of the resulting cells. Mitosis results in the production of two identical cells, both diploid and genetically identical. In contrast, meiosis produces four genetically unique haploid cells.
• Additionally, meiosis is only seen in germ cells, which are the cells that give rise to gametes (sperm and egg cells), while mitosis occurs in all cells of the body.
• The key purpose of meiosis is to maintain a constant number of chromosomes within a species, from generation to generation. It also provides genetic variation within a species, which is important for evolution and adaptation.
• Understanding meiosis also enables understanding of genetic disorders, many of which occur due to errors in this process. For example, Down syndrome occurs due to non-disjunction (failure of chromosome pairs to separate correctly) during meiosis.

It’s crucial to remember all these steps and the role of meiosis in genetic variation for effective revision. Visual diagrams or flowcharts can be helpful tools to aid in memorising this process.