Notes on Sexual Reproduction and Meiosis

Overview of Sexual Reproduction

• Sexual reproduction involves the transfer of genes from parent organisms to offspring.
• Begins when two gametes unite to form an embryo.
• The embryo is genetically unique and grows into an adult.
• The adult can pass on genetic information to its own offspring through gametes.

Formation of Gametes: Meiosis

• Meiosis is the process through which gametes are formed.
• Germline cells undergo meiosis to produce gametes.
• Germline cells are diploid (two copies of each chromosome).
• Through meiosis, haploid gametes (one copy of each chromosome) are produced.
• Haploid gametes fuse to form a diploid embryo.

Stages of Cell Cycle in Germline Cells

• Similar to mitosis, germline cells pass through the interphase stages: G1, S, and G2.
• DNA duplication occurs during the S phase.
• Duplicated chromosomes are called sister chromatids.
• Sister chromatids remain attached until the second cell division in meiosis.

Meiosis: Two Cell Division Events

• Meiosis I:

  • Prophase I:
    • DNA condenses to form chromosomes.
    • Sister chromatids join at the centromere.
    • Homologous chromosomes undergo synapsis.
    • Crossing over occurs, exchanging chromosomal material, leading to genetic diversity.
    • Nuclear membrane breaks down, centrosomes migrate, microtubules attach to chromosomes.
  • Metaphase I:
    • Synapsed chromosomes align at the cell equator randomly.
  • Anaphase I:
    • Homologous chromosomes separate; sister chromatids remain attached.
  • Telophase I and Cytokinesis:
    • Cell divides into two unique daughter cells.

• Meiosis II:

  • Prophase II:
    • Chromosomes condense, nuclear envelope breaks down, spindle apparatus forms.
    • No synapsis or crossing over.
  • Metaphase II:
    • Chromosomes align randomly at the equator of the cell.
  • Anaphase II:
    • Sister chromatids are pulled apart.
    • Cell elongates as microtubules elongate.
  • Telophase II and Cytokinesis:
    • Nuclear membrane reforms, cytoplasm divides.
    • Results in four unique haploid daughter cells (gametes).

Outcome of Meiosis

• Four haploid gametes are produced from one germline cell.
• Gametes from different parents can fuse to form a diploid embryo.
• This embryo grows through mitosis cycles.