Cell Division
NCERT Solution
Question 1: What is the average cell cycle span for a mammalian cell?
Answer: 24 hours
Question 2: Distinguish cytokinesis from karyokinesis.
Answer: Division of cytoplasm takes place during cytokinesis, while division of nucleus takes place during karyokinesis.
Question 3: Describe the events taking place during interphase.
Answer: The interphase is divided into three phases, which are as follows:
- G1 phase (Gap 1): During this phase, the cell is metabolically active and continuously grows.
- S phase (Synthesis): During this phase, DNA synthesis or replication takes place. The amount of DNA becomes double during this phase, but the number of chromosomes remains the same.
- G2 phase (Gap 2): During this phase, protein synthesis takes place.
Question 4: What is G0 (quiescent phase) of cell cycle?
Answer: Quiescent Stage (G0): Cells which do not divide further, exit G1 phase to enter an inactive stage. This stage is called quiescent stage (G0) of the cell cycle. The cells in this stage remain metabolically active but do not undergo division. But these cells can resume division as and when required.
Question 5: Why is mitosis called equational division?
Answer: The number of chromosomes in daughter cells is same as in mother cell, so mitosis is called equational division.
Question 6: Name the stage of cell cycle at which one of the following events occur:
- Chromosomes are moved to spindle equator.
- Centromere splits and chromatids separate.
- Pairing between homologous chromosomes takes place.
- Crossing over between homologous chromosomes takes place.
Answer: (a) Metaphase, (b) Anaphase, (c) Zygotene, (d) Pachytene
Question 7: Describe the following: (a) synapsis (b) bivalent (c) chiasmata Draw a diagram to illustrate your answer.
Answer: The pairing of chromosomes during zygotene is called synapsis. The synaptonemal complex formed by a pair of homologous chromosomes is called bivalent. The X-shaped structure, formed during crossing over is called chiasmata.
Question 8: How does cytokinesis in plant cells differ from that in animal cells?
Answer: In animal cell, a furrow appears in the plasma membrane. The furrow gradually deepens and finally joins in the centre. Thus, the cytoplasm is divided into two parts. In plant cells, cell wall formation begins in the centre. This grows outwards to meet the existing lateral walls and thus, the cytoplasm is divided into two parts.
Question 9: Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.
Answer: The four daughter cells formed after microsporogenesis in flowering plants are equal in size. The four daughter cells formed after megasporogenesis in flowering plants are unequal in size.
Question 10: Distinguish anaphase of mitosis from anaphase I of meiosis.
Answer: The centromere splits during anphase of mitosis, while it does not split during anaphase I of meiosis.
Question 11: List the main differences between mitosis and meiosis.
Answer:
| Difference Between Mitosis and Meiosis | |
|---|---|
| Mitosis | Meiosis |
| This type of division takes place in somatic cells. | This type of division takes place in gametic cells. |
| Two daughter cells are formed. | Four daughter cells are formed. |
| Number of chromosomes remains diploid in daughter cells. | Number of chromosomes becomes haploid in daughter cells. |
| Mitosis is necessary for growth and repair. | Meiosis is necessary for sexual reproduction. |
| Crossing over does not take place. | Crossing over takes place. |
Question 12: What is the significance of meiosis?
Answer: Significance of Meiosis:
- Conservation of specific chromosome number of each species is achieved across successive generations in sexually reproducing organisms through meiosis.
- Meiosis helps in increasing the genetic variations in the population of organisms from one generation to the next.
Question 13: Discuss with your teacher about
(a)Haploid insects and lower plants where cell-division occurs, and
Answer: Male bees, wasps and ants are haploid as they are produced from unfertilized eggs.
(b)Some haploid cells in higher plants where cell-division does not occur.
Answer: Cell division does not happen in synergids and antipodal cells; in the ovule.
Question 14: Can there be mitosis without DNA replication in ‘S’ phase?
Answer: There cannot be mitosis without DNA replication, because additional DNAs are required for the formation of new cells.
Question 15: Can there be DNA replication without cell division?
Answer: DNA replication can take place without cells division; as in case of formation of new mitochondria and chloroplasts.
Question 16: Analyse the events during every stage of cell cycle and notice how the following two parameters change
(a)Number of chromosomes (N) per cell
Answer: Number of chromosomes becomes half after meiosis.
(b)Amount of DNA content (C) per cell
Answer: Amount of DNA becomes double after S phase.