Sexual Reproduction and Meiosis

Sexual Reproduction and Meiosis

Sexual Reproduction

  • Sexual reproduction entails the combination of genetic material from two parents, resulting in offspring with a mix of characteristics from both.
  • This process creates genetic variation within species, which is crucial for evolution.
  • Sexual reproduction involves the gametes or sex cells, namely the sperm from the father and the egg from the mother.
  • Each gamete carries a single set of chromosomes (haploid) to be combined in the offspring.
  • This haploid number is restored to the diploid number (two sets of chromosomes) during fertilisation when the sperm and egg combine.

Meiosis

  • Meiosis is a two-part cell division process that results in four daughter cells, each with half the number of chromosomes as the parent cell.
  • This is how haploid gametes are produced for sexual reproduction.
  • Meiosis I is where the first division occurs, creating two haploid cells.
  • Meiosis II resembles mitosis, where these two cells divide once again generating four haploid daughter cells.
  • Crossing-over during Prophase I of meiosis, where segments of DNA are swapped between homologous chromosomes, contributes significantly to genetic variation.

Stages of Meiosis

  • Prophase I: In this longest phase, the nuclear envelope breaks down, and homologous chromosomes pair up. Crossing-over occurs here.
  • Metaphase I: The paired chromosomes align at the metaphase plate in the middle of the cell.
  • Anaphase I: The spindle fibres pull apart the chromosome pairs towards opposite poles, each pole getting one from each pair. This is called real separation or disjunction.
  • Telophase I and Cytokinesis: The nuclear membrane forms around the chromosomes at each pole. Cytokinesis divides the cytoplasm giving rise to two haploid daughter cells.
  • Prophase II, Metaphase II, Anaphase II, Telophase II and Cytokinesis: These are very similar to the respective stages in mitosis - chromosomes align at the equator, sister chromatids separate, and the nuclear membrane reforms around the separated chromatids. After cytokinesis, four haploid daughter cells are produced.

Importance of Meiosis and Sexual Reproduction

  • Meiosis and sexual reproduction create genetic diversity within populations, allowing for natural selection and evolution.
  • They help in the removal and masking of deleterious mutations from a population.
  • Sexual reproduction enables the inheritance of traits that might increase an organism’s survival and reproduction chances, referred to as adaptive evolution.

This section concludes the key points on sexual reproduction and meiosis. It’s necessary to understand how sexual reproduction contributes to genetic variation and how meiosis enables this process.