Meiosis and Sexual Reproduction

Meiosis and Sexual Reproduction

• Sexual reproduction involves fertilization, where two haploid gametes combine to form a diploid zygote.
• Meiosis is the process that produces haploid cells from an original diploid cell.

Stages of Meiosis

• Meiosis involves one round of DNA replication followed by two nuclear divisions.

• Meiosis I reduces the chromosome number.

  • In prophase I, homologous pairs of sister chromatids form tetrads (bivalents) through synapsis.
  • Crossing over, the exchange of genetic material between non-sister chromatids, occurs at chiasmata.
  • In metaphase I, bivalents align randomly along the metaphase plate.
  • In anaphase I, homologous chromosomes separate and move to opposite poles.
  • In telophase I and cytokinesis, the chromosomes decondense, and the nuclear envelope reforms, producing two haploid daughter cells.

• Meiosis II divides the sister chromatids.

  • Prophase II: Sister chromatids condense, and the spindle starts to form. The nuclear envelope begins to break down.
  • Prometaphase II: The nuclear envelope completely breaks down. Sister chromatids attach to the spindle via kinetochore microtubules.
  • Metaphase II: Sister chromatids align along the metaphase plate.
  • Anaphase II: Sister chromatids separate and individual chromosomes move toward opposite poles.
  • Telophase II and cytokinesis: Chromosomes decondense, and the nuclear envelope reforms. Cleavage furrows separate the two cells into four haploid daughter cells.

Meiosis vs. Mitosis

• Mitosis produces two genetically identical diploid daughter cells.
• Meiosis produces four genetically different haploid daughter cells.

Mechanisms for Genetic Variation

• Crossing over in prophase I shuffles genetic material between homologous chromosomes.
• Random alignment of homologous chromosomes in metaphase I leads to different combinations of maternal and paternal chromosomes in daughter cells.
• Random fertilization further increases genetic variation, as any sperm can fertilize any egg.

Changes in Chromosome Number

• Euploidy: The normal number of chromosomes. Diploid (2n) organisms have two sets of chromosomes.
• Polyploidy: Organisms with more than two complete sets of chromosomes. This is common in some plants, but rare in animals.
• Triploidy (3n), Tetraploidy (4n).
• Aneuploidy: An abnormal number of particular chromosomes. This can result from nondisjunction (failure of chromosomes to separate properly during meiosis).

Nondisjunction

• Nondisjunction is the failure of chromosomes to separate properly during meiosis, which can result in gametes with extra or missing chromosomes.

Examples of Aneuploidy

• Trisomy 21 (Down syndrome): Three copies of chromosome 21.
• Monosomy: Missing one copy of a chromosome.