EXERCISES

Q1. What is the average cell cycle span for a mammalian cell?
Answer: The average cell cycle span for a mammalian cell, especially human cells in culture, is approximately 24 hours.

Q2. Distinguish cytokinesis from karyokinesis.
Answer:
Karyokinesis is the division of the nucleus, whereas cytokinesis is the division of the cytoplasm. Karyokinesis occurs first and is followed by cytokinesis to complete cell division.

Q3. Describe the events taking place during interphase.
Answer: Interphase is the period between two successive M phases. It is not a resting stage but a phase of active preparation for cell division involving cell growth and DNA replication. It has three stages:

1. G₁ phase (Gap 1): The cell grows actively, synthesizes proteins, and duplicates organelles but does not replicate DNA.
2. S phase (Synthesis): DNA replication occurs, doubling the DNA content from 2C to 4C, while chromosome number remains 2n. Centriole duplication also occurs.
3. G₂ phase (Gap 2): The cell continues to grow, synthesizes mitotic proteins, and prepares for mitosis.

Q4. What is G₀ (quiescent phase) of cell cycle?
Answer: The G₀ phase is an inactive stage where cells exit the cell cycle after G₁ and stop dividing. They remain metabolically active but do not proliferate unless needed, such as during tissue repair. Examples include heart cells and some nerve cells in adult animals.

Q5. Why is mitosis called equational division?
Answer: Mitosis is called equational division because it produces two daughter cells with the same chromosome number and identical genetic material as the parent cell. The diploid parent cell gives rise to diploid daughter cells, maintaining genetic consistency.

Q6. . Name the stage of cell cycle at which one of the following events occur:
(i) Chromosomes are moved to spindle equator.
(ii) Centromere splits and chromatids separate.
(iii) Pairing between homologous chromosomes takes place.
(iv) Crossing over between homologous chromosomes takes place.
Answer:
(i) Chromosomes are moved to spindle equator — Metaphase
(ii) Centromere splits and chromatids separate — Anaphase
(iii) Pairing between homologous chromosomes takes place — Zygotene (Prophase I)
(iv) Crossing over between homologous chromosomes takes place — Pachytene (Prophase I)

Q7. Describe the following:
(a) synapsis (b) bivalent (c) chiasmata
Draw a diagram to illustrate your answer.
Answer:
(a) Synapsis: The process during zygotene (Prophase I) where homologous chromosomes pair to form a structure called the synaptonemal complex.
(b) Bivalent: The structure formed when two homologous chromosomes pair during synapsis. It consists of four chromatids and is visible during pachytene.
(c) Chiasmata: X-shaped structures seen during diplotene (Prophase I) representing the sites of crossing over between homologous chromosomes.

Q8. How does cytokinesis in plant cells differ from that in animal cells?
Answer: Cell division ends with cytokinesis, the process where the cytoplasm divides to form two new cells. Although this happens in both plant and animal cells, the method differs because plant cells have a rigid wall while animal cells do not.

1. In animal cells, cytokinesis occurs by the formation of a cleavage furrow that deepens inward and pinches the cell into two daughter cells, whereas in plant cells, it happens by the formation of a cell plate that starts in the center and grows outward to join the cell walls.
2. In animal cells, the division is inward-directed, caused by the contraction of the plasma membrane, whereas in plant cells, it is outward-directed, due to the deposition of new cell wall materials along the growing cell plate.
3. Animal cells can form a cleavage furrow easily because they lack a rigid cell wall and have a flexible plasma membrane, whereas plant cells have a rigid cell wall, so cytokinesis takes place by cell plate formation instead of membrane constriction.

Q9. Find examples where the four daughter cells from meiosis are equal in size and where they are unequal in size.
Answer: During meiosis, the size of the four daughter cells produced may either be equal or unequal depending on the type of gamete formation.

1. When the four daughter cells are equal in size, it occurs during spermatogenesis in animals and microsporogenesis in plants, whereas in these processes, all four resulting cells become functional gametes such as sperms or pollen grains.
2. When the four daughter cells are unequal in size, it takes place during oogenesis in animals and megasporogenesis in plants, whereas in these cases, only one large functional cell (the ovum or functional megaspore) is formed along with smaller, non-functional polar bodies.

Q10. Distinguish anaphase of mitosis from anaphase I of meiosis.
Answer: The events of anaphase in mitosis and meiosis I differ mainly in the type of chromosomes that separate and the resulting genetic composition of daughter cells.

1. In anaphase of mitosis, the centromeres split, and sister chromatids separate and move to opposite poles, whereas in anaphase I of meiosis, the centromeres do not split, and only homologous chromosomes separate while sister chromatids remain together.

2. In mitosis, the chromosome number remains diploid (2n) in the daughter cells, whereas in meiosis I, the chromosome number is reduced to haploid (n).

3. The chromosomes moving to opposite poles in mitosis are genetically identical, whereas those separated during meiosis I are genetically different because of crossing over that occurs during prophase I.

Q11. List the main differences between mitosis and meiosis.
Answer: Mitosis and meiosis are two types of cell divisions that differ in their purpose, process, and outcome. Mitosis maintains the chromosome number, whereas meiosis reduces it to half for gamete formation.

Q12. What is the significance of meiosis?
Answer: Meiosis is essential for sexual reproduction. It reduces the chromosome number by half, ensuring the species’ chromosome number is maintained generation after generation. It also introduces genetic variation through crossing over and recombination, which are important for evolution and adaptation.

Q13. Discuss with your teacher about (i) haploid insects and lower plants where cell-division occurs, and (ii) some haploid cells in higher plants where cell-division does not occur.
Answer:
(i) In lower plants like mosses and ferns, the gametophyte stage is haploid and divides by mitosis to form gametes. Insects like male honey bees are haploid and produce sperms by mitosis.
(ii) In higher plants, certain haploid cells such as male gametes (sperm nuclei) and the egg cell do not divide further. They remain haploid until fertilisation occurs.

Q14. Can there be mitosis without DNA replication in ‘S’ phase?
Answer: No, mitosis cannot occur without prior DNA replication. The S phase is essential for duplicating the genome so that each daughter cell receives a complete set of chromosomes during mitosis.

Q15. Can there be DNA replication without cell division?
Answer: Yes, sometimes DNA replication occurs without cytokinesis. This results in a multinucleate condition called a syncytium. An example is the liquid endosperm in a coconut, where nuclei divide without cytoplasmic division.

Q16. Analyse the events during every stage of cell cycle and notice how the following two parameters change (i) number of chromosomes (N) per cell (ii) amount of DNA content (C) per cell.
Answer: