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Class XI, , Chapter 10 — Cell Cycle and Cell Division, , Biology, , , , Question 1:, , Book Name: NCERT Solutions, , What is the average cell cycle span for a mammalian cell?, , Solution 1:, 24 Hours., , Question 2:, , Distinguish cytokinesis from karyokinesis., , Solution 2:, , Differences between cytokinesis and karyokinesis are, , , , Cytokinesis, , Karyokinesis, , , , (i), , Cytokinesis is the division of, the cytoplasm of a cell., , Karyokinesis is the division of, the nucleus of a cell., , , , (ii), , It occurs at the end of M-phase,, after the nuclear division is, , It occurs during M-phase of cell, cycle before the cytokinesis, , , , , , , , , , over. begins to proceed., , , , Question 3:, Describe the events taking place during inter-phase., , Solution 3:, , The interphase, though called the resting phase, is metabolically quite active. It is the time during, which the cell prepares itself for division by undergoing both cell growth and DNA replication, in an orderly manner. The interphase is further divided into three phases:, , G1 (Gap 1) phase, , S (Synthesis) phase, , G2 (Gap 2) phase, , G1 phase corresponds to the interval between mitosis of previous cell cycle and initiation of, DNA replication. During G1 phase the cell is metabolically active and grows continuously but, does not replicate its DNA S or synthesis phase marks the period during which DNA synthesis, or replication takes place. During this time the amount DNA double per cell. In animal cells,, during the S phase DNA replication occurs in the nucleus, and the centriole duplicates in the, cytoplasm. During the G2 phase synthesis of DNA stops while cell growth continues with, synthesis of protein and RNA in preparation for mitosis., , Question 4:, What is Go (quiescent phase) of cell cycle?, , Solution 4:
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Class XI Chapter 10 — Cell Cycle and Cell Division Biology, , , , Go phase is the phase of inactivation of cell cycle due to non-availability of mitogens and energy, rich compounds. Cells in this stage remain metabolically active but no longer proliferate. 1.e.,, do not grow of differentiate unless called on to do so depending on the requirement of the, organism. E.g., Nerve and heart cells of chordates are in permanent Go phase., , Question 5:, Why is mitosis called equation division?, , Solution 5:, , Mitosis is type of cell division in which chromosomes replicate and become equally distributed, in two daughter nuclei so that the daughter cells come to have the same number and type of, chromosomes as present in parent cell. So mitosis is called as equation division., , Question 6:, , Name the stage of cell cycle at which each one of the following events occur:, (i) Chromosomes are moved to spindle equator., , (ii) Centromere splits and chromatids separate., , (111) Pairing between homologous chromo — somes takes place., , (iv) Crossing over between homologous chromosomes takes place., , Solution 6:, , (i) Metaphase, , (ii) Anaphase, , (111) Zygotene of prophase | of meiosis |, (iv) Pachytene of prophase | of meiosis |, , Question 7:, , Describe the following, , (a) Synapsis, , (b) Bivalent, , (c) Chiasmata, , Draw a diagram to illustrate your answer., , Solution 7:, , (a) Synapsis:, , During zygotene of prophase I stage homologous chromosomes start pairing together and this, process of association is called synapsis. Electron micrographs of this stage indicate that, chromosome synapsis is accompanied by the formation of complex structure called, synaptonemal complex., , b) Bivalent:, , The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a, tetrad i.e., 4 chromatids or a pair of chromosomes.
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Class XI Chapter 10 — Cell Cycle and Cell Division Biology, , , , Homologous N OTE re, , chromosomes, , (synapsed) Do not fraud, Fh, e diagvans:, , UJ * ite Ohly, Solutions., , , , , , , , , Sister, , chromatids |rera or Bivalent, , Nonsister, chromatids, Noncross-over, chromatid, Cross-over, chromatids, Noncross-over, , Crossing-over chromatid, , Fig.: Diagram showing synapsis, bivalent and chiasmata, , (c) Chiasmata:, , The beginning of diplotene is recognized by the dissolution of the synaptonemal complex and, the tendency of the synapsed homologous chromosomes of the bivalents to separate from each, other except at the sites of crossovers. These point of attachment (X-shaped structures) between, , the homologous chromosomes are called chiasmata., , Question 8:, How does cytokinesis in plant cells differ from that in animal cells?, , Solution 8:, cytokinesis and animal cytokinesis differ in following respects:, , Plant, , , , , , , , , , , , , , , , , , , , , , Plant cytokinesis Animal cytokinesis, , (i) | It usually occurs by cell plate | It takes place by cleavage., method., , (ii) | The spindle usually persists during | The spindle begins to degenerate soon, cytokinesis after anaphase., , (iii) | Central part of spindle grows in| A mid body of dense fibrous and, size and forms and interdigited | vesicular material is formed in the, complex called phragmoplast. middle., , (iv) | Vesicles derived from Golgi| The event is absent in animal, apparatus reach the equator of the | cytokinesis., phragmoplast and fuse to form cell, plate and new cell membranes., , (v) | Cell plate grows centrifugally. Cleavage progresses centripetally., , (vi) | The new cell membrane is derived | The new cell membrane is usually, from vesicles of Golgi apparatus. _| derived from endoplasmic reticulum.
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Class XI Chapter 10 — Cell Cycle and Cell Division Biology, , , , Question 9:, Find examples where the four daughter cells from meiosis are equal in size and where they are, found unequal in size., , Solution 9:, , During formation of male gametes (i.e., spermatozoa) in a typical mammal (i.e., human being),, the four daughter cells formed from meiosis are equal in size. On the other hand, during, formation of female gamete (i.e., ovum), in a typical mamma (i.e., human being), the four, daughter cells unequal in size., , Question 10:, Can there be DNA replication without cell division?, , Solution 10:, , Yes. Endomitosis is the multiplication of chromosomes present in a set in a set in nucleus, without karyokinesis and cytokinesis resulting in numerous copies within each cell. It is of 2, types., , Polyteny: Here chromosomes divide and redivide without separation of chromatids so that such, chromosomes become multistranded with many copies of DNA. Such polytene (many stranded), chromosomes remain in permanent prophase stage and do not undergo cell cycle e.g., polytene, (salivary glands) chromosome of Drosophila 512 — 1024 chromatids. Here number of sets of, chromosomes does not change., , Polyploidy (endoduplication): Here all chromosomes in a set divide and its chromatids, separate but nucleus does not divide. This results in an increase in number of sets of, chromosomes in the nucleus (4x, 8x.....). This increase in sets of chromosomes is called, polyploidy. It can be induced by colchicines and granosan. These chromosomes are normal and, undergo cell cycle., , Question 11:, List the main differences between mitosis and meiosis., , Solution 11:, , , , Mitosis Meiosis, , (1) It occurs in all somatic cells and | It occurs in reproductive cells at specific ties., may continue throughout life, (ii) | It involves a single division,| It involves two successive divisions,, resulting in two daughter cells | resulting in four daughter cells., , , , , , , , only., (iii) | Subsequent mitotic divisions are | Two meiotic divisions are dissimilar, first is, similar to the earlier ones reductional while the second is equational., , , , (iv) | Prophase is relatively short and | Prophase I is very long and elaborate,, simple. comprising 5 subphases.
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Class XI Chapter 10 — Cell Cycle and Cell Division Biology, (v) | There is no_ pairing of | Homologous chromosomes pair and often, homologous chromosomes. undergo crossing over in prophase I., , (vi) | Chromatids are — genetically | Chromatids may differ genetically from the, similar to chromosomes they | chromosomes they arise from due to crossing, , arise from. over., , , , (vil), , No synaptonemal complex forms, , Synaptonemal complex forms between, synapsed homologous chromosomes., , , , Chromosomes do not unfold, and, no transcription and _ protein, synthesis occur in prophase., , (viii), , Chromosomes unfold, an transcription and, protein synthesis may occur in diplotene of, prophase | (oocytes of certain animals)., , , , (ix) | Daughter cells have diploid, , number (2N) of chromosomes, , Daughter cells have haploid number (N) of, chromosomes unlike the parent cell., , , , , , , , , , like the parent cell., , , , Question 12:, Distinguish anaphase of mitosis from anaphase | of meiosis., , Solution 12:, , Anaphase of mitosis: it is the phase of shortest duration. APC (anaphase promoting complex), develops. It degenerates proteins binding the two chromatids in the region of centromere. As a, result, the centromere of each chromosome divides. This converts the two chromatids into, daughter chromosomes each being attached to the spindle pole of its side by independent, chromosomal fibre. The chromosomes move towards the spindle poles with the centromeres, projecting towards the poles and the limbs trailing behind. There is corresponding shortening of, chromosome fibres. The two pole ward moving chromosomes of each type remain attached to, each other by interzonal fibres. Ultimately, two groups of chromosomes come to lie at the, spindle poles., , aie fel rays,, iy ; Aster, , “25> Centrioles!, , , , , Centromere, Chromosome, , ‘ontinuous fibre, Discontinuous fib:, , Fig.: Mitotic enaphase., Anaphase | of meiosis: Chiasmata disappear completely and the homologous chromosomes, separate. The process is called disjunction. The separated chromosomes (univalents) show, , divergent chromatids and are called dyads. They move towards the spindle poles and ultimately, form two groups of haploid chromosomes.
