Cell Cycle and Cell DivisionNEET Botany · Class 11 · NCERT Chapter 10

Solution

The amount of DNA is 4C during the G2 phase, after DNA replication in the S phase and before the onset of mitosis. NCERT XI chapter Cell Cycle and Cell Division states that DNA content doubles during the S phase and remains at 4C during G2, so option (c) is correct.

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In the 'S' phase of the cell cycle, DNA replication occurs, doubling the amount of DNA in each cell. NCERT XI chapter Cell Cycle and Cell Division states that DNA synthesis is a key event in the S phase, so option (a) is correct.

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Kinetochore of chromosomes facilitates the attachment of spindle fibre (chromosomal fibre) and the poles. www.vedantu.com 51

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In pachytene recombination nodule is formed after which crossing over occurs.

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Synapsis is pairing of homologous chromosomes which occurs during meiosis but it is absent in mitosis.

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Polyploidy cells have a chromosome number that is more than double the haploid number.

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The correct sequence of events during mitosis would be as follows (i) Condensation of DNA so that chromosomes become visible occurs during early to mid-prophase. (ii) N uclear membrane disassembly begins at late prophase or transition to metaphase. (iii) Arrangement of chromosomes at equator occurs during metaphase, called congression. (iv) Centromere division or splitting occurs during anaphase forming daughter chromosomes. (v) Segregation also occurs during anaphase as daughter chromosomes separate and move to opposite poles. (vi) Telophase leads to formation of two daughter nuclei.

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Anaphase Promoting Complex (APC) is a protein necessary for separation of daughter chromosomes during anaphase. If APC is defective then the chromosomes will fail to segregate during anaphase.

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During diplotene, the homologous chromosomes begin to separate from each other, though they remain connected at chiasmata. NCERT XI chapter Cell Cycle and Cell Division describes this as the stage where chiasmata formation is completed and homologous chromosomes start to repel each other, so option (b) is correct.

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Factual- NCERT

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Cornea is white fibrous connective tissue and very sensitive due to good nerve supply

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Ref. XII NCERT Chapter- 06, Page No. 114 If GGU is removed from the given mRNA, one amino acid coded by this codon is not incorporated in the polypeptide, rest all codon will not change.

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The synaptonemal complex dissolves during diplotene, leading to the separation of homologous chromosomes while they remain connected at chiasmata. NCERT XI chapter Cell Cycle and Cell Division describes this event in the prophase I of meiosis, so option (c) is correct.

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During the G1 phase, the cell is metabolically active and grows, but DNA replication does not occur. NCERT XI chapter Cell Cycle and Cell Division states that DNA synthesis occurs in the S phase, not G1, so option (c) is correct.

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Cells can exit the cell cycle and enter the quiescent stage (G 0 ) at the end of the G 1 phase. NCERT XI chapter Cell Cycle and Cell Division describes G 0 as a stage where cells are not preparing for division and can remain for long periods, so option (b) is correct.

Solution

- Zygotene is characterized by the formation of the synaptonemal complex (synapsis).
- Pachytene involves crossing over between homologous chromosomes.
- Diplotene is marked by the appearance of chiasmata and the beginning of chromosome desynapsis.
- Diakinesis is the stage where chiasmata move to the ends of the chromosomes (terminalization).

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The division of the centromere occurs during anaphase II, when sister chromatids separate and move to opposite poles. NCERT XI chapter Cell Cycle and Cell Division describes this event as a key feature of anaphase II, so option (c) is correct.

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When the centromere is located in the middle of the chromosome, resulting in two equal arms, the chromosome is classified as metacentric. NCERT XI chapter Cell Cycle and Cell Division describes this classification, so option (a) is correct.

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- S phase: DNA replication occurs (iv)
- G2 phase: Proteins are synthesized in preparation for mitosis (i)
- Quiescent stage: Cells in an inactive phase (ii)
- G1 phase: Interval between mitosis and initiation of DNA replication (iii)

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During the S phase of mitosis, DNA replication occurs, doubling the number of chromosomes. If the number of chromosomes at G1 phase is 8, after S phase, the number of chromosomes will be 16, so option (b) is correct.

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Diakinesis is characterized by the terminalization of chiasmata, where the chiasmata move to the ends of the chromosomes. This is a distinctive feature of the late prophase I stage, as described in NCERT XI chapter Cell Cycle and Cell Division, so option (c) is correct.

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The centriole undergoes duplication during the S-phase of the cell cycle, when DNA replication also occurs. NCERT XI chapter Cell Cycle and Cell Division states that centriole duplication is a part of the S-phase, so option (a) is correct.

Solution

Pairing of homologous chromosomes occurs during prophase I of meiosis. Coiling and condensation of chromatids, spindle fibres attachment to the kinetochores a nd movement of centrioles towards opposite poles occur in both mitosis and meiosis. - 43 - NEET (UG)-2022 (Code-Q1)

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Pachytene stage of meiosis is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non sister chromatids of homologous chromosomes. - 48 - NEET (UG)-2022 (Code-Q1)

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Spindle fibres attach to the kinetochores of chromosomes. Kinetochores are the disc shaped structures present on sides of primary constriction or centromere of chromosomes.

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Recombination nodules appear during pachytene, a substage of prophase I in meiosis. NCERT XI chapter Cell Cycle and Cell Division describes pachytene as the stage where crossing over occurs, leading to genetic recombination, so option (c) is correct.

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DNA replication occurs during the S phase of the cell cycle, when the genetic material is duplicated. NCERT XI chapter Cell Cycle and Cell Division specifies that the S phase is dedicated to DNA synthesis, so option (c) is correct.

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The division of the centromere occurs during anaphase II, when sister chromatids separate and move to opposite poles. NCERT XI chapter Cell Cycle and Cell Division describes this event as a key feature of anaphase II in meiosis, so option (d) is correct.

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Statement I is incorrect because cells in G₀ phase are metabolically active but have exited the cell cycle. Statement II is correct as centrosome duplication occurs during the S phase, as per NCERT XI chapter Cell Cycle and Cell Division. Therefore, option (a) is correct.

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• During leptotene stage the chromosomes become gradually visible under the light microscope. • The beginning of diplotene is recognised by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the site of crossover. Thus both statement I and II are correct.

Solution

(A) Diakinesis – Completion of terminalisation of chiasmata (B) Pachytene – Appearance of recombination nodules (C) Zygotene – Synaptonemal complex formation (D) Leptotene – Chromosomes look like thin threads A-II, B-IV, C-I, D-III - 65 - NEET (UG)-2024 (Code-Q1)

Solution

The correct sequence of stages of cell division is ) Gap 1 phase Synthesis phase Gap 2 pha (E) ( se C) (A →→ Karyokinesis Cytokinesis (D) (B) →→ The correct sequence will be → E → C → A → D → B

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During mitosis, spindle fibre get attach to the kinetochores of the chromosome and help in the separation of the chromosome.