Bihar Board - Class 11 biology - Chapter 10: Cell cycle and cell division Handwritten Notes

The cell cycle and cell division are fundamental processes that enable organisms to grow, repair damaged tissues, and reproduce. This chapter focuses on the stages of the cell cycle, the mechanisms of mitosis and meiosis, and their significance in maintaining genetic continuity and diversity.

Key Points

1. Cell Cycle

   • The cell cycle is the sequence of events in a cell's life that leads to its growth, DNA replication, and division. It consists of two main phases:
     • Interphase: The preparatory phase where the cell grows and duplicates its DNA.
       • G1 Phase: Cell growth and preparation for DNA synthesis.
       • S Phase: DNA replication occurs.
       • G2 Phase: Preparation for cell division; synthesis of proteins and organelles.
     • M Phase (Mitotic Phase): The phase of actual cell division.

2. Mitosis

   • A type of cell division that results in two genetically identical daughter cells.
   • Essential for growth, repair, and asexual reproduction.
   • Phases of Mitosis:
     • Prophase: Chromosomes condense, spindle fibers form, and the nuclear membrane breaks down.
     • Metaphase: Chromosomes align at the cell's equatorial plate.
     • Anaphase: Sister chromatids separate and move to opposite poles.
     • Telophase: Chromosomes decondense, and the nuclear membrane re-forms.
   • Cytokinesis: Division of the cytoplasm, completing the formation of two daughter cells.

3. Meiosis

   • A type of cell division that reduces the chromosome number by half, producing four haploid daughter cells.
   • Occurs in gametes (sex cells) and ensures genetic variation in sexually reproducing organisms.
   • Stages of Meiosis:
     • Meiosis I: Reductional division where homologous chromosomes separate.
       • Prophase I: Homologous chromosomes pair and exchange genetic material through crossing over.
       • Metaphase I: Homologous pairs align at the equatorial plate.
       • Anaphase I: Homologous chromosomes separate.
       • Telophase I: Two haploid cells are formed.
     • Meiosis II: Equational division where sister chromatids separate, similar to mitosis.

4. Significance of Cell Division

   • Mitosis:
     • Maintains genetic stability by producing identical cells.
     • Important for growth, tissue repair, and regeneration.
   • Meiosis:
     • Promotes genetic diversity through recombination and independent assortment.
     • Ensures the stability of the chromosome number in sexually reproducing organisms.

5. Regulation of the Cell Cycle

   • Controlled by proteins called cyclins and enzymes known as cyclin-dependent kinases (CDKs).
   • Checkpoints in the cell cycle ensure proper progression and prevent errors:
     • G1 checkpoint, G2 checkpoint, and the spindle checkpoint.

6. Errors in Cell Division

   • Abnormalities in cell division can lead to:
     • Cancer: Uncontrolled cell division due to malfunctioning cell cycle regulation.
     • Aneuploidy: Abnormal chromosome number, as seen in Down syndrome and Turner syndrome.

Conclusion

The cell cycle and cell division are vital processes for life. Mitosis ensures growth and repair by producing identical cells, while meiosis introduces genetic diversity, crucial for evolution and reproduction. Understanding these mechanisms is essential for insights into developmental biology and disease treatment.

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