Cell Cycle and Cell Division – Class 11 Biology Notes
Introduction to Cell Cycle and Cell Division
The cell cycle is a series of events that lead to the growth and division of a cell. It consists of two main phases: interphase (the phase of preparation for division) and mitotic phase (M phase), where actual cell division occurs. Cell division is essential for growth, development, and reproduction in multicellular organisms. There are two types of cell division: mitosis (for somatic cell division) and meiosis (for the production of gametes).
1. The Cell Cycle
The cell cycle is divided into two main phases:
- Interphase (Preparation Phase)
- M Phase (Mitotic Phase)
1.1. Interphase
Interphase is the phase where the cell spends most of its time. It prepares the cell for division by replicating DNA and increasing the size of its organelles.
- G1 Phase (Gap 1): The cell grows in size and synthesizes proteins necessary for DNA replication. The cell also carries out its normal metabolic functions.
- S Phase (Synthesis): DNA replication occurs, doubling the number of chromosomes to prepare for division. Each chromosome is now made up of two sister chromatids.
- G2 Phase (Gap 2): The cell continues to grow and synthesizes proteins required for cell division. It also checks for any errors in DNA replication before proceeding to mitosis.
1.2. M Phase (Mitotic Phase)
The M phase is when the cell actually divides into two daughter cells. It consists of two main stages:
- Mitosis: Division of the nucleus into two genetically identical nuclei.
- Cytokinesis: Division of the cytoplasm to form two distinct daughter cells.
Stages of Mitosis:
- Prophase:
- Chromosomes condense and become visible as sister chromatids.
- The nuclear envelope begins to break down.
- The spindle fibers begin to form from the centrioles (in animal cells) or spindle poles (in plant cells).
- Metaphase:
- Chromosomes align at the metaphase plate (center of the cell).
- Spindle fibers attach to the centromeres of the chromosomes.
- Anaphase:
- The sister chromatids are pulled apart to opposite poles of the cell.
- The centromere splits, and the chromatids separate.
- Telophase:
- Chromatids reach the opposite poles and begin to de-condense back into chromatin.
- The nuclear envelope re-forms around each set of chromosomes.
- Cytokinesis:
- The cytoplasm divides, forming two separate daughter cells.
- In animal cells, a cleavage furrow forms, and in plant cells, a cell plate forms to separate the two cells.
2. Meiosis
Meiosis is a special type of cell division that occurs only in germ cells (sperm and egg cells) and results in the formation of four genetically different daughter cells, each with half the number of chromosomes of the parent cell. Meiosis consists of two sequential divisions: Meiosis I and Meiosis II.
2.1. Meiosis I
- Prophase I: Chromosomes condense, and homologous chromosomes pair up (synapsis). These homologous chromosomes exchange genetic material through crossing-over, which introduces genetic variation.
- Metaphase I: The homologous chromosome pairs align at the metaphase plate.
- Anaphase I: Homologous chromosomes are pulled apart to opposite poles of the cell. Unlike mitosis, the sister chromatids remain attached.
- Telophase I and Cytokinesis: The cell divides into two daughter cells, each with half the chromosome number (haploid).
2.2. Meiosis II
Meiosis II is similar to mitosis but involves the division of haploid cells (formed in Meiosis I).
- Prophase II: Chromosomes re-condense, and a new spindle apparatus forms in each of the two daughter cells.
- Metaphase II: Chromosomes align at the metaphase plate in both cells.
- Anaphase II: The sister chromatids are separated and pulled to opposite poles.
- Telophase II and Cytokinesis: The two cells divide, resulting in four non-identical haploid daughter cells.
3. Regulation of the Cell Cycle
The cell cycle is tightly regulated by a series of checkpoints to ensure that each phase is completed correctly before moving to the next. Some key regulators include:
- Cyclins: Proteins that regulate the progression of the cell cycle by activating cyclin-dependent kinases (CDKs).
- CDKs (Cyclin-Dependent Kinases): Enzymes that, when activated by cyclins, phosphorylate target proteins to drive the cell cycle forward.
- Checkpoints:
- G1 Checkpoint: Checks if the cell has enough resources and if the DNA is intact before entering the S phase.
- G2 Checkpoint: Ensures that DNA replication has been completed without errors before entering mitosis.
- Metaphase Checkpoint: Ensures that all chromosomes are properly aligned at the metaphase plate before anaphase begins.
4. Differences Between Mitosis and Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Sexual reproduction (formation of gametes) |
| Chromosome Number | Same as parent (diploid) | Half the parent’s chromosome number (haploid) |
| Number of Divisions | One (1) | Two (2) |
| Daughter Cells | Two identical cells | Four non-identical cells |
| Genetic Variation | No genetic variation | Genetic variation due to crossing-over and independent assortment |
| Type of Cells | Somatic (body) cells | Germ cells (sperm and egg) |
MCQs – Cell Cycle and Cell Division
1. During which phase of the cell cycle does DNA replication occur?
a) G1 phase
b) S phase
c) G2 phase
d) M phase
Answer: b) S phase
2. Which of the following is the correct sequence of stages in mitosis?
a) Telophase → Anaphase → Metaphase → Prophase
b) Prophase → Metaphase → Anaphase → Telophase
c) Anaphase → Telophase → Prophase → Metaphase
d) Metaphase → Prophase → Anaphase → Telophase
Answer: b) Prophase → Metaphase → Anaphase → Telophase
3. Which of the following events occurs during prophase I of meiosis?
a) Chromosomes condense
b) Chromosomes align at the metaphase plate
c) Sister chromatids are separated
d) Crossing-over occurs between homologous chromosomes
Answer: d) Crossing-over occurs between homologous chromosomes
4. The end result of meiosis is:
a) Two diploid daughter cells
b) Four haploid daughter cells
c) Four diploid daughter cells
d) Two haploid daughter cells
Answer: b) Four haploid daughter cells
5. Which checkpoint checks the integrity of the DNA before the cell enters mitosis?
a) G1 checkpoint
b) G2 checkpoint
c) Metaphase checkpoint
d) Anaphase checkpoint
Answer: b) G2 checkpoint
6. Which of the following is the correct number of chromosomes in the daughter cells after mitosis, assuming the parent cell had 46 chromosomes?
a) 23
b) 46
c) 92
d) 4
Answer: b) 46
7. During which phase of the cell cycle does the cell prepare for mitosis by synthesizing proteins and organelles?
a) G1 phase
b) S phase
c) G2 phase
d) M phase
Answer: a) G1 phase
8. In which phase of meiosis do homologous chromosomes separate?
a) Anaphase I
b) Anaphase II
c) Metaphase I
d) Telophase II
Answer: a) Anaphase I