NCERT Solutions for Class 11 Biology Chapter 10
Cell Cycle and Cell Division Class 11
Chapter 10 Cell Cycle and Cell Division Exercise Solutions
Exercise : Solutions of Questions on Page Number : 171
What is the average cell cycle span for a mammalian cell?
Distinguish cytokinesis from karyokinesis.
Cytokinesis |
Karyokinesis |
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(i) |
Cytokinesis is the biological process involving the division of a cell's cytoplasm during mitosis or meiosis. |
(i) |
Karyokinesis is the biological process involving the division of a cell's nucleus during mitosis or meiosis. |
(ii) |
Stages such as prophase, metaphase, anaphase, and telophase are not present in cytokinesis. |
(ii) |
It is divided into four stages –prophase, metaphase, anaphase, and telophase |
Q3 :
Answer :
Interphase involves a series of changes that prepare a cell for division. It is the period during which the cell experiences growth and DNA replication in an orderly manner. Interphase is divided into three phases.
(i) G1phase
(ii) S phase
(iii) G2phase
G1phase - It is the stage during which the cell grows and prepares its DNA for replication. In this phase, the cell is metabolically active.
S phase - It is the stage during which DNA synthesis occurs. In this phase, the amount of DNA (per cell) doubles, but the chromosome number remains the same.
What is G0(quiescent phase) of cell cycle?
Why is mitosis called equational division?
Name the stage of cell cycle at which one of the following events occur:
(ii) Centromere splits and chromatids separate
(iv) Crossing over between homologous chromosomes takes place
(i) Metaphase
(ii) Anaphase
(iii) Zygotene of meiosis I
(iv)
Describe the following:
Answer :
Chiasmata is the site where two non-sister chromatids of homologous chromosomes have crossed over. It represents the site of cross-over. It is formed during the diplotene stage of prophase I of meiosis.
How does cytokinesis in plant cells differ from that in animal cells?
Cytokinesis in plant cells |
Cytokinesis is animal cells |
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(i) |
The division of the cytoplasm takes place by cell plate formation. |
(i) |
The division of the cytoplasm takes place by cleavage. |
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(ii) |
Cell plate formation starts at the centre of the cell and grows outward, toward the lateral walls. |
(ii) |
Cleavage starts at the periphery and then moves inward, dividing the cell into two parts. |
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Q9 :
Answer :
(a) Spermatogenesis or the formation of sperms in human beings occurs by the process of meiosis. It results in the formation of four equal-sized daughter cells.
(b)
Distinguish anaphase of mitosis from anaphase I of meiosis.
Anaphase of mitosis |
Anaphase I of meiosis |
Anaphase is the stage during which the centromere splits and the chromatids separate. The chromosomes move apart, |
During anaphase I, the homologous chromosomes separate, while the |
toward the opposite poles. These chromosomes are genetically identical. |
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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Q11 :
Answer :
Mitosis |
Meiosis |
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1. |
In mitotic division, a single division results in two daughter cells. |
1. |
Meiotic division involves two successive divisions – meiosis Iand meiosis II. These divisions result in four daughter cells. |
2. |
Mitosis is known as equational division. This is because the daughter cells have the same diploid number of chromosomes as the parent. |
2. |
Meiosis I is known as reductional division. This is because the chromosome number is reduced to half. Meiosis II is known as equational division. This is because the sister chromatids separate and the chromosome number remains the same. |
3. |
Prophase is short and does not comprise any phase. |
3. |
Prophase I is very long and comprises 5 phases –leptotene, zygotene, pachytene, diplotene, and diakinesis. |
4. |
There is no pairing of chromosomes, crossing-over, or chiasmata-formation during prophase. |
4. |
In the zygotene stage of prophase, the pairing of chromosomes occurs. During pachytene, the crossing- over occurs. The chiasmata are formed in the diplotene |
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stage. |
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5. |
Synaptonemal complex is not formed. |
5. |
Synaptonemal complex is formed during the zygotene stage of prophase I. |
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6. |
Anaphase involves the separation of the chromatids of each chromosome. |
6. |
During anaphase I, the homologous chromosomes separate, while the chromatids remain attached at their centromeres. During anaphase II, the chromatids separate as a result of the splitting of the centromere. |
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7. |
Mitosis plays a significant role in the healing, repair, and growth of a cell. |
7. |
Meiosis brings about variation and maintains the chromosome number from generation to generation. |
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What is the significance of meiosis?
Meiosis is the process involving the reduction in the amount of genetic material. It comprises two successive nuclear and cell divisions, with a single cycle of DNA replication. As a result, at the end of meiosis II, four haploid cells are formed.
Significance of meiosis
1. Meiosis maintains the chromosome number from generation to generation. It reduces the chromosome number to half so that the process of fertilisation restores the original number in the zygote.
2. Variations are caused by the cross-over and the random distribution of homologous chromosomes between daughter cells. Variations play an important role in evolution.
3.
Discuss with your teacher about
(ii) some haploid cells in higher plants where cell-division does not occur.
(i) 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 haplontic life cycle.
(ii)
Can there be mitosis without DNA replication in S phase?
Can there be DNA replication without cell division?
Analyse the events during every stage of cell cycle and notice how the following two parameters change
(ii) Amount of DNA content (C) per cell
During meiosis, the number of chromosomes and the amount of DNA in a cell change.
(i) Number of chromosomes (N) per cell
During anaphase I of the meiotic cycle, the homologous chromosomes separate and start moving toward their respective poles. As a result, the bivalents get divided into two sister chromatids and receive half the chromosomes present in the parent cell. Therefore, the number of chromosomes reduces in anaphase I.
(ii) Amount of DNA content (C) per cell
During anaphase II of the meiotic cycle, the chromatids separate as a result of the splitting of the centromere. It is the centromere that holds together the sister chromatids of each chromosome. As a result, 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 remains the same. The DNA duplicated in the S phase gets separated in the two daughter cells during anaphase. As a result, the DNA content (C) of the two newly-formed daughter cells remains the same.