NCERT Solutions for Class 11 Biology Chapter 10: Cell Cycle and Cell Division

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

    Ans. The average cell cycle span for a mammalian cell is about 24 hours. In which the cell remains in the G1 phase for about 10 hours and the duration of S phase or the Synthetic phase varies with respect to the total DNA present in a cell.

    2. Distinguish cytokinesis from karyokinesis.

    Ans. Karyokinesis is the first step in M-phase of cell cycle where the nucleus divides into two daughter nuclei. It is usually followed by cytokinesis. In this process, the DNA condenses and the chromosomal material divides equally into two daughter nuclei.

    Cytokinesis, on the other hand, is defined as the division of the cytoplasm during the M-phase of the cell cycle. It is the second step in M-phase and cannot occur without karyokinesis. It is also the final stage of mitosis, where the cytoplasm and other cell organelles divides between the two daughter cells.

    3. Describe the events taking place during interphase.

    Ans. Interphase is the longest phase of cell division. During which, cell prepares itself for mitotic division by undergoing both cell growth and DNA replication in an orderly manner.

    This phase is divided into three sub-phases:
    G1-phase, S-phase and G2-phase.

    G1-phase: During this phase, the cell is metabolically active and continuously grows but does not replicate its DNA.

    S-phase or Synthesis phase: During this phase, DNA synthesis or replication takes place. As a result, the amount of DNA per cell doubles but the number of chromosomes remains the same. In animal cells, DNA replication begins in the nucleus and the centrioles duplicate in the cytoplasm.

    G2-phase: In this phase proteins are synthesised in preparation for mitosis and cell growth continues.

    Cell Cycle and Cell Divisionans3

    4. What is G0 (quiescent phase) of cell cycle?

    Ans. Some cells do not divide further and exist in G1-phase to enter in an inactive stage called quiescent phase (G0) of the cell cycle.

    During G0 or quiescent phase, cells remain metabolically active, but do not proliferate unless called to do so. Such cells are used for
    replacing the cells lost during an injury.

    5. Why is mitosis called equational division?

    Ans. Mitosis is the process of cell division wherein the chromosomes replicate and get equally distributed into two daughter cells. The chromosome number in each daughter cell is equal to that in the parent cell, i.e., diploid. Hence, mitosis is known as equational division.

    6. Name the stage of cell cycle at which one of the following events occur:

    (a) Chromosomes are moved to spindle equator.

    (b) Centromere splits and chromatids separate.

    (c) Pairing between homologous chromosomes takes place.

    (d) Crossing over between homologous chromosomes takes place.

    Ans. (a) Metaphase

    (b) Anaphase

    (c) Zygotene of meiosis I

    (d) Pachytene of meiosis I

    7. Describe the following:

    (a) synapsis (b) bivalent (c) chiasmata

    Draw a diagram to illustrate your answer.

    Ans. (a) Synapsis: The pairing of homologous chromosomes is called as synapsis. This occurs during the second stage of prophase I or Zygotene.

    Cell Cycle and Cell Divisionans7_1

    (b) Bivalent: Bivalent or tetrad is pairing of synapsed homologous chromosomes. They are formed during the Zygotene stage of prophase I of meiosis.

    Cell Cycle and Cell Divisionans(b)

    (c) Chiasmata: Chiasmata is the site where two non-sister chromatids have crossed over. It represents the site of cross-over. It is formed during the diplotene stage of prophase I of meiosis.

    Cell Cycle and Cell Divisionans(c)

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

    Ans. Cytokinesis in plants and animals is different from each other due to presence of cell wall in plant cells. Cytokinesis in animal cells occurs by furrowing of the cytoplasm. While in plant cells, cytokinesis is initiated with the formation of a cell plate in the middle of the cell and the cell plate extends on either side from the middle until it finally divides the cell into two daughter halves.

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

    Ans. (a) Spermatogenesis or the formation of sperms in male occurs by the process of meiosis. It results in the formation of four equal-sized daughter cells.

    (b) Oogenesis or the formation of the ovum in female occurs by the process of meiosis but it results in the formation of four daughter cells which are unequal in size, i.e., a big mature ovum and 3 small polar bodies.

    10. Distinguish anaphase of mitosis from anaphase I of meiosis.

    Ans. Anaphase of mitosis: Anaphase is the stage during which, the centromere splits and the chromatids separate. The chromosomes move apart, towards the opposite poles. These chromosomes are genetically identical.

    Anaphase-I of meiosis: During anaphase-I, the homologous chromosomes separate, while the chromatids remain attached at their centromeres. Hence, in anaphase-I, the chromosomes of each bivalent pair separate, while the sister chromatids remain together.

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    11. List the main differences between mitosis and meiosis.

    Ans. Differences between Mitosis and Meiosis.

    Mitosis Meiosis
    1. Mitosis takes place in somatic cells and as a result, growth occurs. This takes place usually in reproductive cells and as a result of this character passes from one generation into the other.
    2. Completed in one stage. Completed in two stages.
    3. Crossing over does not take place. Crossing over takes place in which exchange of chromatid segments occurs.
    4. Synapsis does not take place at metaphase. Synapsis between homologous chromosomes takes place (bivalent stage).
    5. At metaphase, the centromere is towards the equatorial plate and ends of chromosomes towards the poles. Centromere divides. At metaphase-I, the centromere is towards poles and ends of chromosomes towards the equatorial plate. Centromere does not divide.
    6. Chromatids are longer and thin. Chromatids are shorter and thick.
    7. Cytokinesis follows karyokinesis. After Telophase I, cytokinesis does not take place always (may or may not occur).

    12. What is the significance of meiosis?

    Ans. The significance of meiosis are as follows:

    (a) Formation of gametes: Gametes formed during meiosis are essential for sexual reproduction which are responsible for passing characters from one generation to other.

    (b) Genetic information: Meiosis switches on the genetic information for the development of gametes.

    (c) Maintenance of chromosomal number: Meiosis maintains the fixed number of chromosomes in sexually reproducing organisms.

    (d) Assortment of chromosomes: In meiosis, paternal and maternal chromosomes assort independently. It causes reshuffling of chromosomes and the traits are controlled by them.

    (e) Crossing over: It introduces new combina-tion of traits or variations.

    (f) Mutation: Mutations take place due to irregularities in meiotic division.

    13. Discuss with your teacher about:

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

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

    Ans. (a) In some insects and lower plants, fertilization is immediately followed by zygotic meiosis, which leads to the production of haploid organisms. This type of life cycle is known as the haplontic life cycle.

    (b) The phenomenon of polyploidy can be observed in some haploid cells as in higher plants in which cell division does not occur. Polyploidy can be artificially induced in plants by applying colchicine to cell culture.

    14. Can there be mitosis without DNA replication in ‘S’ phase?

    Ans. There are two important events that take place during S-phase – one is the synthesis or replication of DNA for maintaining the chromosomal number in the daughter cells and the other is the duplication of the centriole. Hence mitosis can’t take place without DNA replication in S-phase as without it, there will be a reduction in the number of chromosomes of daughter cells.

    15. Can there be DNA replication without cell division?

    Ans. Yes, DNA replication can take place without cell division. In order to prepare for cell division, DNA replication is necessary. Because cell division is the succeeding logical step that occurs post cell division. For example, polytene chromosomes found in drosophila is a result of DNA replication without cell division.

    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

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

    Ans. During meiosis, the number of chromosomes and the amount of DNA in a cell change.

    (a) Number of chromosomes(N) per cell: During anaphase I of meiosis, the homologous chromosomes separate and start moving towards their respective poles. As a result, the bivalents get divided into two sister chromatids and receive half number of chromosomes present in the parent cell. Therefore, the number of chromosomes reduces during anaphase I.

    (b) Amount of DNA content (C) per cell: During anaphase II of meiosis, the chromatids separate as a result of the splitting of the centromere. The chromatids move toward their respective poles. Therefore, at each pole, a haploid number of chromosomes and a haploid amount of DNA are present.
    During mitosis, the number of chromosomes in the cell remains same. The DNA duplicated in the S-phase gets separated in two daughter cells during anaphase. As a result, the DNA content (C) of the two newly-formed daughter cells remains the same.

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