Previous year questions: Cell Cycle and Cell Division

These ten questions come from the NCERT Exemplar chapter on cell cycle and cell division, which is the only official previous year source available for Class 11 Biology. The set covers the bivalent and the prophase I sub-stages, the G0 stage, the products of meiosis, and the arithmetic that links cell numbers, generations and meiotic products. Work each item on paper first. The marks shown follow the usual weightage for the type of question in the Exemplar: one mark for a multiple choice, two marks for a very short answer, three marks for a short answer and five marks for a long answer. The chapter reference in each heading tells you exactly where the item sits.

Q1 (NCERT Exemplar, 1 mark; Chapter 10, Multiple Choice 6)

A bivalent of meiosis I consists of which of the following?

Answer: A bivalent is a pair of synapsed homologous chromosomes. Each of the two chromosomes has already replicated, so each carries two sister chromatids. Together the bivalent therefore contains four chromatids and two centromeres. Options that say two chromatids one centromere or four chromatids four centromeres are wrong because they forget replication or the pairing. The answer is four chromatids and two centromeres. To check a question of this type, count the centromeres first, since pairing does not change how many there are, then count the chromatids, which double because DNA was copied before meiosis began.

Q2 (NCERT Exemplar, 1 mark; Chapter 10, Multiple Choice 7)

Cells which are not dividing are likely to be in which stage?

Answer: Such cells have left the cycle after G1 and entered the quiescent stage called G0. They remain metabolically active but do not replicate DNA or divide unless called back. G1 and G2 belong to cycling cells and S phase is replication itself, so the answer is G0. The trap in the question is the word likely, because some G0 cells can be recalled into the cycle after an injury. The same reasoning explains why a cell that stops dividing is not dead, since G0 keeps its normal metabolism running.

Q3 (NCERT Exemplar, 1 mark; Chapter 10, Multiple Choice 9)

Identify the wrong statement about meiosis.

Answer: The wrong statement is that two cycles of DNA replication occur. Meiosis has two divisions but only one round of DNA replication, which takes place before meiosis I; interkinesis has no S phase. The other statements, pairing of homologues, four haploid products and the halving of chromosome number, are all correct. The single replication is what makes the whole process economical, because one copy of the genome supplies four haploid products. A quick way to remember the single replication is to link it with interkinesis, the stage that would have carried the second S phase if meiosis copied DNA twice, and which instead stays short and free of any synthesis.

Q4 (NCERT Exemplar, 2 marks; Chapter 10, Very Short Answer 5)

The average duplication time of E. coli is 20 minutes. How much time will two E. coli cells take to become 32 cells?

Answer: Starting from two cells, each division doubles the number: 2, then 4, 8, 16 and 32. That is four generations. Each generation takes 20 minutes, so the total is 4 x 20 = 80 minutes. Write the generations out in the exam so that the examiner can see the doubling, and remember that starting from two cells means the first division is generation one.

Q5 (NCERT Exemplar, 2 marks; Chapter 10, Very Short Answer 9)

If a tissue has 1024 cells, how many cycles of mitosis had the original single cell undergone?

Answer: Mitosis doubles the cell number at each cycle. Solve 2 raised to n equals 1024, and 1024 is 2 raised to 10. The original cell had therefore undergone 10 cycles of mitosis. Working backwards is equally quick: halve 1024 repeatedly until you reach one, and count how many halvings it took.

Q6 (NCERT Exemplar, 2 marks; Chapter 10, Very Short Answer 10)

An anther has 1200 pollen grains. How many pollen mother cells must have been there to produce them?

Answer: Each pollen mother cell undergoes meiosis and produces four pollen grains. The number of mother cells is 1200 divided by 4, which is 300. The reverse question is also common, where the number of mother cells is given and the number of pollen grains has to be found by multiplying by four, so keep the ratio of one mother cell to four pollen grains in mind.

Q7 (NCERT Exemplar, 3 marks; Chapter 10, Short Answer 4)

A cell has 32 chromosomes. It undergoes mitotic division. What will the chromosome number be during metaphase, and what will the DNA content be during anaphase?

Answer: During metaphase the cell has completed S phase, so the chromosome number is still 32 and the DNA content is 4C. During anaphase the centromeres have split and the chromatids are moving to the poles, but the cytoplasm has not yet divided. The cell at this moment contains 64 chromosomes and the DNA content is still 4C for the whole cell. Each daughter cell will receive 32 chromosomes and 2C DNA after cytokinesis. The extra chromosomes in anaphase are not a change in ploidy, because the two sets are still in one undivided cytoplasm and each pole receives a complete set once cytokinesis is over.

Q8 (NCERT Exemplar, 3 marks; Chapter 10, Short Answer 6)

Name the phase of the cell cycle at which each of these events occurs: disintegration of the nuclear membrane, appearance of the nucleolus, division of the centromere, replication of DNA.

Answer: Disintegration of the nuclear membrane marks the beginning of metaphase, though it is completed at the end of prophase. The nucleolus reappears in telophase. Division of the centromere occurs in anaphase. DNA replication takes place in the S phase of interphase. Writing the events against these four stages covers the full marks. A quick way to remember the pairs is to link the membrane events with the beginning and the end of division, and to link replication with the only phase that changes DNA content.

Q9 (NCERT Exemplar, 3 marks; Chapter 10, Short Answer 12)

Name a cell that is arrested in diplotene for months or years, and explain how it completes the cell cycle.

Answer: The primary oocyte of many vertebrates, including the human primary oocyte, is arrested in diplotene of prophase I for months or years. During the arrest the chromosomes are in the extended diplotene state with chiasmata visible. Meiosis resumes only when the oocyte is stimulated to mature, usually before ovulation, and it then completes meiosis I to produce a secondary oocyte and a first polar body. The second meiotic division is completed only after fertilisation, which finally produces the ovum and the second polar body. The long arrest is a safety device: it keeps the egg paused at a stage where the chromosomes are already recombined, and it resumes only when the signal to mature or the sperm arrives.

Q10 (NCERT Exemplar, 5 marks; Chapter 10, Long Answer 2)

Describe the various stages of meiotic prophase I and the chromosomal events during each stage.

Answer: Prophase I has five sub-stages. In leptotene the chromosomes become gradually visible as condensation continues. In zygotene homologous chromosomes pair in synapsis, the synaptonemal complex forms, and each pair becomes a bivalent or tetrad with four chromatids. In pachytene the four chromatids of each bivalent become distinct, recombination nodules appear, and crossing over between non-sister chromatids is catalysed by recombinase; recombination is complete by the end of this stage. In diplotene the synaptonemal complex dissolves and the homologues start to separate, remaining joined only at the X-shaped chiasmata, and in oocytes of some vertebrates this stage can last for months or years. In diakinesis the chiasmata move to the chromosome ends by terminalisation, the chromosomes are fully condensed, the meiotic spindle assembles, the nucleolus disappears and the nuclear envelope breaks down, so the cell passes into metaphase I. Together these events explain both the pairing that makes homologous separation possible and the crossing over that creates variation. For full marks, keep the sub-stages in order, name at least one chromosomal event for each, and finish with the transition into metaphase I.