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CBSE Class 11 Biology · 10 questions · 24 marks
A single fertilised egg becomes a trillion-celled organism only because cells can divide, and this chapter explains the orderly sequence of steps — interphase, mitosis and, in germ cells, meiosis — that makes division safe rather than chaotic. Mitosis copies a cell's chromosomes exactly and hands one identical set to each daughter, sustaining growth and repair; meiosis instead halves the chromosome number and shuffles genes, which is what makes sexual reproduction and genetic variation possible.
DNA replication in a dividing cell takes place during which phase of the cell cycle?
Answer
S phase is correct — during synthesis phase the DNA content per cell doubles as each chromosome is replicated, though the chromosome number itself remains unchanged at this stage since the two copies stay joined as sister chromatids.
Chromosomes are aligned at the equatorial plate, allowing their morphology to be studied best, during:
Answer
Metaphase is correct — at the metaphase plate every chromosome is maximally condensed and held under tension by spindle fibres from both poles, giving the clearest view of chromosome number and shape, which is why this stage is used for karyotyping.
Crossing over between non-sister chromatids of homologous chromosomes occurs during which sub-stage of meiotic prophase I?
Answer
Pachytene is correct — after the homologous chromosomes have fully paired (synapsis) during zygotene, pachytene is when the crossover events actually occur at points called chiasmata, exchanging segments of genetic material between chromatids.
In a plant cell, division of the cytoplasm during telophase is achieved by the formation of a:
Answer
Cell plate is correct — because the rigid cell wall prevents the membrane from pinching inward as in animal cells, plant cells build a cell plate at the equator that grows outward and fuses with the existing wall, partitioning the cell in two.
Assertion (A): Meiosis is described as a reductional division. Reason (R): One round of DNA replication is followed by two successive rounds of division, so the chromosome number is halved from diploid to haploid.
Answer
Both A and R are true and R is the correct explanation of A — because DNA is copied only once but the nucleus divides twice, the four resulting daughter cells each carry half the chromosome number of the parent cell, exactly the reduction needed before fertilisation restores the diploid number.
What is the G0 phase, and why do some cells enter it?
Answer
G0 is a quiescent phase that a cell can enter from G1 instead of continuing through the rest of the cell cycle; the cell remains metabolically active and carries out its normal physiological functions but does not proceed to DNA replication or division. Cells enter G0 when they are fully differentiated and have no further need to divide, such as mature neurons and cardiac muscle cells, or when external conditions or signals do not favour further proliferation; some G0 cells can re-enter the active cycle later if appropriately stimulated.
Differentiate between anaphase of mitosis and anaphase I of meiosis in terms of what separates and moves to the poles.
Answer
In anaphase of mitosis, the centromere joining the two sister chromatids of every chromosome splits, and the two sister chromatids of each chromosome separate and move to opposite poles as independent daughter chromosomes; the two poles therefore end up with identical, complete sets of chromosomes. In anaphase I of meiosis the centromeres do not split; instead, whole homologous chromosomes, each still made of two sister chromatids joined at an undivided centromere, separate from each other and move to opposite poles. This is why anaphase I achieves the reduction in chromosome number, while the actual separation of sister chromatids is postponed to anaphase II, which resembles ordinary mitotic anaphase.
Describe the stages of mitosis in sequence, explaining the key event that characterises each stage.
Answer
Mitosis proceeds through four stages following interphase. Prophase begins with the condensation of the loosely packed chromatin fibres into visibly distinct chromosomes, each consisting of two identical sister chromatids held together at the centromere as a result of the DNA replication that occurred earlier in S phase; simultaneously the centrioles (in animal cells) move to opposite poles helping to organise the spindle, and by late prophase the nucleolus and nuclear envelope have begun to disintegrate. Metaphase follows once the nuclear envelope has completely disassembled: spindle fibres from both poles attach to the kinetochore of each chromosome, and through a process of alignment the chromosomes come to lie precisely along the equatorial plate of the cell, held in place under tension from both poles, which makes this the best stage for studying chromosome number and structure. Anaphase begins abruptly when the centromere of every chromosome splits simultaneously, freeing the two sister chromatids, which are then pulled apart as independent chromosomes toward opposite poles by the shortening of the spindle fibres, so that identical sets move to each end of the cell. Telophase reverses several events of prophase: the chromosomes, having reached the poles, begin to decondense back into diffuse chromatin, the nuclear envelope re-forms around each group to create two daughter nuclei, and the nucleolus reappears within each; the spindle apparatus disassembles, and cytokinesis (division of the cytoplasm, by a cleavage furrow in animal cells or a cell plate in plant cells) usually begins during this stage, ultimately producing two genetically identical daughter cells.
Explain why meiosis is significant for sexually reproducing organisms, covering chromosome number and genetic variation.
Answer
Meiosis is significant for two closely related reasons. The first concerns chromosome number: every generation of a sexually reproducing species is created by the fusion of a male and a female gamete during fertilisation. If gametes were produced by ordinary mitosis they would remain diploid, and fusing two diploid gametes would double the chromosome number in every successive generation, which is biologically impossible to sustain. Because meiosis performs one round of DNA replication followed by two successive nuclear divisions, it converts a diploid germ cell into four haploid gametes; when two haploid gametes then fuse at fertilisation, the diploid chromosome number characteristic of the species is exactly restored, keeping the chromosome number constant across generations. The second reason concerns genetic variation, which is generated at two distinct points in meiosis. During pachytene of prophase I, non-sister chromatids of paired homologous chromosomes undergo crossing over at points called chiasmata, physically exchanging segments of DNA and creating chromatids with new combinations of alleles that did not exist in either parent chromosome. Independently, during anaphase I, the orientation in which each pair of homologous chromosomes lines up on the metaphase plate is random with respect to every other pair, so the maternal and paternal chromosomes are distributed to the daughter cells in many possible combinations, a phenomenon called independent assortment. Together, crossing over and independent assortment ensure that the gametes produced by any individual are genetically diverse, and it is this variation, generated afresh in every generation, that supplies the raw material on which natural selection can act, making meiosis fundamental to evolution as well as to reproduction.
A researcher examines root-tip cells of an onion (2n = 16) under a microscope after squashing and staining a sample. In one field of view she sees a cell with 16 distinctly separate, single-chromatid chromosomes clustered tightly at each of two opposite poles, connected by a thin remnant of spindle fibres, with the nuclear envelope not yet reformed. (a) Which stage of the cell cycle is this cell in, and what feature identifies it? (b) How many chromosomes will each of the two resulting daughter cells contain once division is complete, and why? (c) Is this cell undergoing mitosis or meiosis? Give one reason based on the chromosome count described. (d) Name the immediate next stage this cell will enter and describe one event that will occur in it.
Answer
(a) The cell is in anaphase. The identifying feature is that single-chromatid chromosomes (formed after the centromeres split) are already separated into two clusters moving toward opposite poles, connected by the remaining spindle fibres, while the nuclear envelope has not yet re-formed around either group. (b) Each daughter cell will contain 16 chromosomes, the same as the original onion root-tip cell, because this is a somatic cell and each pole has received one complete set of the 16 single-chromatid chromosomes that resulted from splitting of the centromeres. (c) This is mitosis, not meiosis, because each pole receives the full diploid number of 16 chromosomes; had this been anaphase I of meiosis, each pole would receive only 8 chromosomes (the haploid number), since whole homologous chromosomes rather than sister chromatids separate at that stage. (d) The cell will next enter telophase, during which the chromosomes at each pole will decondense back into diffuse chromatin, the nuclear envelope and nucleolus will reappear around each set, and cytokinesis will begin, forming a cell plate since this is a plant cell, ultimately producing two genetically identical daughter cells.
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