NCERT Solutions For Class 11 Biology Chapter 10 - Cell Cycle and Cell Division

Q1. What is the average cell cycle span for a mammalian cell?

Answer:

The average cell cycle span for a mammalian cell is 24 hours.

Q2. Distinguish cytokinesis from karyokinesis.

Answer:

The differences between cytokinesis and karyokinesis are as follows:

Q3. Describe the events taking place during the interphase.

Answer:

Interphase is the stage at which the cell makes preparations for division. It consists of three phases:
G1 phase: The cell increases in size and synthesises RNA, proteins, and molecules required for DNA replication. The cell determines whether to proceed in the cycle or enter a resting phase (G0).
S phase: The replication of DNA takes place, doubling the DNA but not changing the number of chromosomes. Duplicating begins in the centrosome.
G2 phase: The cell gets ready for mitosis by producing additional RNA, proteins, and organelles.

Q4. What is the Go (quiescent phase) of the cell cycle?

Answer:

G₀ is a resting phase in which the cell halts dividing but doesn't die. These cells may re-enter the cycle if necessary. Example: Cardiac Muscle cells.

Q5. Why is mitosis called equational division?

Answer:

Mitosis is referred to as equational division since it gives rise to two daughter cells with the same number of chromosomes as the parent cell.

Q6. Name the stage of the cell cycle at which one of the following events occurs:

(i) Chromosomes are moved to the spindle equator.

Answer:

During metaphase, the chromosomes move to the spindle equator and align themselves there.

Q6. Name the stage of the cell cycle at which one of the following events occurs:

(ii) Centromere splits and chromatids separate.

Answer:

During anaphase, the centromere splits, and chromatids are separated. These chromatids, after separation, move to the opposite poles.

Q6. Name the stage of the cell cycle at which one of the following events occurs:

(iii) Pairing between homologous chromosomes takes place.

Answer:

The pairing between homologous chromosomes takes place during zygotene of prophase I of meiosis I. As a result of this pairing, bivalents are formed.

Q6. Name the stage of the cell cycle at which one of the following events occurs:

(iv) Crossing over between homologous chromosomes takes place.

Answer:

Crossing over between homologous chromosomes takes place during the pachytene of prophase I of meiosis I.

Q7. Describe the following:

(a) synapsis

Draw a diagram to illustrate your answer.

Answer:

Synapsis- The process of the pairing of homologous chromosomes during the zygotene stage of prophase I of meiosis I is called synapsis. During synapse, a synaptonemal complex is formed by the homologous chromosomes, which are referred to as a bivalent or a tetrad.

Q7. Describe the following:

(b) bivalent

Draw a diagram to illustrate your answer.

Answer:

Bivalent- it refers to the structure formed by a pair of synapsed homologous chromosomes.

Q7. Describe the following:

(c) chiasmata

Draw a diagram to illustrate your answer.

Answer:

Chiasmata- These are points of attachment between homologous chromosomes during their separation in diplotene to the metaphase I stage of meiosis. Chiasmata are initially formed in the regions of crossing over between nonsister chromatids, but later on, tend to shift sideways.

Q8. How does cytokinesis in plant cells differ from that in animal cells?

Answer:

The process of cytokinesis differs in plant and animal cells

Q9. Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.

Answer:

• In the formation of a male gamete (spermatogenesis), all four sperm cells are equal in size.
• During the process of female gamete development (oogenesis), a large egg cell and three smaller polar bodies are produced.

Q10. Distinguish the anaphase of mitosis from anaphase I of meiosis.

Answer:

Q11. List the main differences between mitosis and meiosis.

Answer: The major differences between mitosis and meiosis are discussed below in the table:

Answer:

Meiosis is significant because:

• It produces gametes for sexual reproduction.

• It maintains the chromosome number stable in the species.

• It creates genetic variation by rearranging chromosomes.

• It enables new characteristics to develop, helping in evolution.

• It occasionally causes beneficial genetic mutations.

Q13. Discuss with your teacher about

(i) haploid insects and lower plants, where cell division occurs

Answer:

In some lower plants and haploid insects, such as drones of a honey bee, meiosis occurs in the zygote. This type of meiosis is called zygotic meiosis, whereas this type of life cycle is called haplontic life cycle.

(ii) some haploid cells in higher plants where cell division does not occur.

Answer:

Synergids and antipodals are haploid cells in higher plants where cell division does not occur.

Q14. Can there be mitosis without DNA replication in 'S' phase?

Answer:

No, mitosis cannot occur without DNA replication during the S phase. Without copying DNA, the daughter cells will contain fewer chromosomes than required.

Q15. Can there be DNA replication without cell division?

Answer:

Yes, DNA can replicate without cell division. For instance, in polyploidy, chromosomes replicate many times without the cell dividing, resulting in excess DNA.

Q16. Analyse the events during every stage of the cell cycle and notice how the following two parameters change

(i) number of chromosomes (N) per cell

Answer:

The number of chromosomes decreases during anaphase I of meiosis I, when homologous chromosomes separate, resulting in daughter cells being haploid.

Q16. Analyse the events during every stage of the cell cycle and notice how the following two parameters change

(ii) amount of DNA content (C) per cell

Answer:

DNA content doubles during the S phase since replication takes place. It reduces in anaphase II of meiosis, as sister chromatids move apart and halve the DNA content.