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NCERT Class 11 Biology Chapter 10 Cell Cycle
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Cell Cycle
Chapter: 10
BIOLOGY
TEXTUAL QUESTIONS ANSWERS
Q.1. What is the average cell cycle span for a mammalian cell?
Ans. The average cell cycle span for a mam-malian cell is approximately 24 hours.
Q.2. Distinguish cytokinesis from kary-okinesis.
Ans. Cytokinesis is division of cytoplasm to form two daughter cells. It is different in plants and animals. Karyokinesis is division of nucleus and divided into four phases-prophase, metaphase, anaphase,and telophase.
Q.3. Describe the events taking place during interphase.
Ans. Interphase (I phase)–
1. It is called resting stage because there is no cell division.
2. It has three stages – G₁, S and G₂.
(a) G₁ Phase – First growth phase-
(i) It is phased between M phase and initiation of DNA replication.
(ii) G₁ is longest phase of interphase.
(iii) The cell is metabolically active and grows continuously.
(iv) It synthesises RNA and proteins.
(b) S phase – Synthetic phase-
(i) DNA content doubles from 1 C to 2 C or 2 C to 4 C
(ii) Number of chromosomes remains the same from 1 N to 1 N or 2 N to 2 N.
(iii) Centrioles and chromosomes replicate.
(c) G₂Phase – Second growth phase-
(i) Both cell and nucleus grow in size.
(ii) Cell organelles increase in number.
(iii) It synthesises RNA and proteins.
Q.4. What is G0 (quiescent phase) of cell cycle?
Ans. G₀ Phase – Quiescent
1. It is the stage of inactivation of cycle.
2. Cells remain metabolically active.
3. Cell function as reserve cell which can join cell cycle any time.
Q.5. Why is mitosis called equational division?
Ans. Mitosis is division in which chromosomes replicate and equally divide into two daughter nuclei. Daughter cells have the same number and type of chromosomes as present in parent cell called equational division.
Q.6. Name the stage of cell cycle at which one of the following events occur:
(i) Chromosomes are moved to spindle equator.
Ans. Metaphase.
(ii) Centromere splits and chromatids separate.
Ans. Anaphase.
(iii) Pairing between homologous chro- mosomes takes place.
Ans. (iii) Zygotene of meiosis I.
(iv) Crossing over between homologous chromosomes takes place.
Ans. Pachytene of meiosis I.
Q.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 synapsis. This occurs during zygotene stage of prophase I of meiosis.
(b) Bivalent: Pair of synapsed homologous chromosomes is called tetrad or Bivalent. They are formed during the zygotene stage of prophase I of meiosis.
(c) Chiasmata Chromosome separates except in the crossing over sites during the diplotene stage of prophase I of meiosis. This point of attachment between the chromosomes is called chiasmata.
Q.8. How does cytokinesis in plant cells differ from that in animal cells?
Ans. Cytokinesis-
(a) It is division of cytoplasm to form two daughter cells.
(b) Animal cytokinesis-
(i) It takes place by cleavage.
(ii) Cleavage spreads centripetally and divides the cells into two daughters.
(iii) Spindle plate forms dense fibrous and vesicular structure called mid body. Mid body and microfilaments causes’ centripetal cleavage of cell membrane.
(c) Plant cytokinesis-
(i) It takes place by cell plate method.
(ii) Cell plate grows centrifugally and divides the cells into two daughters.
(iii) Spindle grows in size and forms the complex called phragmoplast.
(iv) Golgi vesicles reach the phragmoplast and fuse to form cell pate and new cell membranes.
Q.9. Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.
Ans. Spermatogenesis results in the formation of four equal-sized daughter cells. Oogenesis results in the formation of four daughter cells which are unequal in size.
Q.10. Distinguish anaphase of mitosis from anaphase I of meiosis.
Ans. Anaphase of mitosis:
(i) Centromere of each chromosome divides into two.
(ii) The two chromatids separate to form daughter chromosomes.
(iii) The daughter chromosome move towards the spindle poles.
Anaphase I of meiosis:
(i) Homologous chromosome separates disjunction.
(ii) Separated chromosomes or univalent are called dyads because each has two chromatids.
Q.11. List the main differences between mitosis and meiosis.
Ans.
| Mitosis | Meiosis |
| It take place in somatic cells | It take place in reproductive cells. |
| Mitotic cell is haploid or diploid | Cell is always diploid. |
| It is single division which gives two cells. | It is a double division which gives four cells. |
| Interface occurs before each division. | Interface occurs before meiosis I only and not prior to meiosis II. |
| Number of chromosomes is same after mitosis. | Reduce to one half after meiosis. |
| Synapsis and crossing over absent. | Present |
| Chiasmata ab-sent. | Present. |
| Centromeres produce a single equatorial plate in metaphase. | Metaphase I has double equatorial plate and Metaphase II has only one. |
| Bivalents absent. | Bivalents occur in Metaphase I. |
Q.12. What is the significance of meiosis?
Ans. Significance of Meiosis:
(i) It separates the homologous chromosomes and reduces the number to one half. This is essential for sexual reproduction.
(ii) Formation of gametes.
(iii) Crossing over produces new combi- nation of genes or recombination
Q.13. Discuss with your teacher about:
(i) Haploid insects and lower plants where cell-division occurs. and
Ans. Cell division occurs in haploid insects like drones of honeybees and lower plants like gametophytes of algae, bryophytes, pteridophytes.
(ii) some haploid cells in higher plants where cell-division does not occur.
Ans. Synergids and antipodal cells in embryo sac of ovule are haploid cells in higher plants in which cell division does not occur.
Q.14. Can there be mitosis without DNA replication in S phase?
Ans. Mitosis cannot take place without DNA replication in S phase.
Q.15. Can there be DNA replication without cell division?
Ans. There can be DNA replication with-out cell division e.g. salivary glands of Drosophila has polytene chromosome.
Q.16. 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.
Ans. S phase – Number of chromosomes (N) per cell remains the same but amount of DNA content (C) per cell doubles i.e. C to 2C.
During anaphase I of the meiosis, number of chromosomes reduces to half from 2N to N and amount of DNA content per cell also reduces to half from 4C to 2C.
During anaphase II of the meiosis, number of chromosomes remains the same and amount of DNA content per cell reduces to half from 2C to C.
