Cell Cycle and Cell Division Notes: Class 11 Biology Chapter 10
Class 11 Biology Chapter 10 Cell Cycle and Cell Division covers the phases of the cell cycle, and the processes of mitosis and meiosis, including their significance.
Overview of Topics & Subtopics (Click to Jump)
1. The Cell Cycle
Short Note:
• The cell cycle consists of Interphase (G1, S, G2 phases — growth and DNA replication) and M Phase (Mitosis) — actual cell division.
• S Phase (Synthesis): DNA replication occurs, and chromosome number remains the same, but DNA content doubles.
2. Mitosis
Short Note:
• Mitosis is equational division, producing two daughter cells genetically identical to the parent cell (2n → 2n).
• Phases: Prophase (chromatin condenses into chromosomes), Metaphase (chromosomes align at the equatorial plate), Anaphase (sister chromatids separate and move to opposite poles), Telophase (chromosomes decondense, nuclear envelope reforms), followed by cytokinesis.
3. Meiosis
Short Note:
• Meiosis is reductional division occurring in germ cells, producing four haploid daughter cells from one diploid parent cell (2n → n), essential for sexual reproduction.
• Consists of two sequential divisions: Meiosis I (reductional, includes crossing over/recombination during prophase I) and Meiosis II (equational, similar to mitosis).
NCERT Textbook Exercise Questions
What is the significance of mitosis?
Answer: Mitosis is important for growth of multicellular organisms, replacement of dying/damaged cells, cell repair, and asexual reproduction in some organisms; it produces genetically identical daughter cells, maintaining the chromosome number constant across generations of cells.
Distinguish between mitosis and meiosis.
Answer: Mitosis occurs in somatic cells, involves one division producing two diploid daughter cells identical to the parent, and does not involve crossing over. Meiosis occurs in germ cells, involves two successive divisions producing four haploid daughter cells with genetic variation, and includes crossing over during prophase I.
What is the significance of the S phase of the cell cycle?
Answer: The S (synthesis) phase is significant because DNA replication occurs during this phase, doubling the DNA content of the cell while the chromosome number remains unchanged; this ensures that each daughter cell receives a complete and identical set of genetic material after cell division.
Why is meiosis called a reductional division?
Answer: Meiosis is called a reductional division because it reduces the chromosome number by half — a diploid (2n) parent cell produces four haploid (n) daughter cells, which is essential to maintain a constant chromosome number across generations when gametes fuse during sexual reproduction.
