Important Notes For NEET Biology – Cell Cycle and Cell Division

Cell division is essential for growth, repair, and reproduction. Multiple cell cycles transform a single cell into a multicellular organism.

Cell Cycle:

• The cell cycle consists of cell growth, DNA replication, and cell division.
• Events in a cell cycle are genetically controlled.
• Duration varies; human cell cycle averages 24 hours, while a yeast cell completes it in 90 minutes.
• The cell cycle is divided into Interphase and M phase.

Phases of the Cell Cycle:

• Interphase: Period of cell growth and DNA replication, constituting 95% of the cell cycle.
  • G1 Phase (Gap 1): Interval between mitosis and DNA replication of the next cell cycle.
  • S Phase (Synthesis): DNA replication occurs, doubling the DNA content, while chromosome number remains the same.
  • G2 Phase (Gap 2): Cell grows and prepares for mitosis, with protein synthesis.
  • G0 Phase: Inactive phase for cells that do not divide, such as heart cells.
• M Phase: Phase of mitosis (cell division), resulting in two daughter cells with the same chromosome number as the parent cell.

Mitosis

Key Points:

• Occurs mostly in diploid somatic cells of animals and both haploid and diploid cells in plants.
• Responsible for genetic continuity, growth, and repair.
• Involves four stages: Prophase, Metaphase, Anaphase, and Telophase, followed by Cytokinesis.

Stages of Mitosis:

1. Prophase:
   • Chromosomes condense and become visible.
   • Mitotic apparatus forms, and nuclear components disappear.
2. Metaphase:
   • Chromosomes align at the cell equator.
   • Chromatids attach to spindle fibres.
3. Anaphase:
   • Sister chromatids separate and move to opposite poles.
   • Chromatids now become daughter chromosomes.
4. Telophase:
   • Chromosomes cluster at poles and decondense.
   • Nuclear envelope and organelles reappear.
5. Cytokinesis:
   • Division of the cytoplasm, resulting in two daughter cells.

Meiosis

Key Points:

• Also known as reduction division, producing haploid gametes.
• Consists of two sequential divisions: Meiosis I and Meiosis II.
• Results in four haploid daughter cells.

Phases of Meiosis I:

1. Prophase I:
   • Extended stage with sub-stages: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
   • Homologous chromosomes pair and exchange genetic material.
2. Metaphase I:
   • Homologous chromosomes align at the equator.
3. Anaphase I:
   • Homologous chromosomes move to opposite poles.
4. Telophase I:
   • Nuclear membranes reappear, forming two haploid cells.

Phases of Meiosis II:

1. Prophase II:
   • Nuclear envelope disappears.
2. Metaphase II:
   • Chromosomes align at the equator.
3. Anaphase II:
   • Sister chromatids separate and move to opposite poles.
4. Telophase II:
   • Nuclear membrane reappears, resulting in four haploid daughter cells.

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