Cell cycle and cell division - Notes | Class 11 | Part 3: Meiosis

Meiosis

• It is the division of diploid germ cells that reduces the chromosome number by half, forming haploid daughter cells (gametes).
• It occurs during gametogenesis and leads to the haploid phase in the life cycle of sexually reproducing organisms.
• Fertilization restores the diploid phase.

Key Features of Meiosis

• Involves two cycles (Meiosis I and Meiosis II) but only a single cycle of DNA replication.
• Involves pairing of homologous chromosomes and recombination between their non-sister chromatids.
• Meiosis I begins after replication of parental chromosomes to form identical sister chromatids at the S phase.
• Four haploid cells are formed at the end of Meiosis II.

Meiosis I	Meiosis II
Prophase I	Prophase II
Metaphase I	Metaphase II
Anaphase I	Anaphase II
Telophase I	Telophase II

Meiosis I

Prophase I:

It is typically longer and more complex, including five phases based on chromosomal behavior: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.

• a. Leptotene:
  • Chromatin fibers become long, slender chromosomes.
  • Nucleus enlarges.
• b. Zygotene:
  • Chromosomes become more condensed.
  • Similar chromosomes pair together (synapsis) via the synaptonemal complex.
  • Paired chromosomes are called homologous chromosomes
  • Each pair is called a bivalent.
• c. Pachytene:
  • A longer phase.
  • Bivalent chromosomes split into similar chromatids, forming tetrads.
  • Recombination nodules appear, enabling crossing over, leading to genetic recombination.
  • Crossing over: Exchange of genetic material between non-sister chromatids of homologous chromosomes, facilitated by the enzyme recombinase.
  • Recombination is completed by the end of pachytene.
• d. Diplotene:
  • The synaptonemal complex dissolves.
  • Recombined homologous chromosomes separate except at chiasmata (X-shaped structures).
  • In oocytes of some vertebrates, diplotene lasts months or years.
• e. Diakinesis:
  • Terminalization of chiasmata occurs.
  • Chromosomes are fully condensed.
  • Meiotic spindle fibers prepare homologous chromosomes for separation.
  • Nucleolus and nuclear envelope disappear.

Metaphase I:

• Spindle formation is completed.
• Chromosomes align on the equatorial plate.
• Microtubules from the spindle attach to homologous chromosome pairs.

Anaphase I:

• Homologous chromosomes separate, while sister chromatids remain associated at their centromeres.

Telophase I:

• Nuclear membrane and nucleolus reappear, forming two haploid daughter nuclei (diad).
• Cytokinesis may or may not occur.
• A short stage, interkinesis, follows without DNA replication.

Meiosis II

It resembles mitosis and includes:

Prophase II:

• Initiated after cytokinesis.
• Chromosomes become compact.
• Nucleolus and nuclear membrane disappear in both nuclei.

Metaphase II:

• Chromosomes align at the equator.
• Microtubules from opposite spindle poles attach to kinetochores of sister chromatids.

Anaphase II:

• Centromeres of each chromosome split, allowing sister chromatids to move toward opposite poles via microtubule shortening.

Telophase II:

• Chromosomes are enclosed by a nuclear envelope.
• Cytokinesis forms a tetrad of four haploid daughter cells.

Meiosis I

Meiosis II

Significance of Meiosis

• Conserves chromosome number of each species.
• Causes genetic variation via crossing over, important for evolution.

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Topic 1: Cell Cycle
Topic 2: Mitosis
Topic 3: Meiosis

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