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Cell Cycle and Cell Division

Biology · Class 11NCERT pp. 120–1282 sets × 25 questions

Key facts and formulas for Cell Cycle and Cell Division, three worked sample questions, and two full practice sets. Every question has a hint, a reason for every option and a step-by-step solution, with a calculator or animation where one helps. English and Hinglish.

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Key facts for this chapter

IdeaRule or fact
Order of the cell cycleG1 → S → G2 → M. Interphase (G1, S, G2) is over 95 per cent of a 24 hour cycle.
S phaseDNA doubles (2C to 4C) but chromosome number stays 2n; each chromosome gets 2 chromatids. Centrosome also duplicates.
G0 and controlCells leave the cycle from G1 into G0. CDKs work only when bound to cyclins. Spindle checkpoint holds anaphase until all kinetochores attach.
Mitosis stagesProphase: condensation, envelope and nucleolus vanish. Metaphase: plate. Anaphase: centromeres split. Telophase: nucleus reforms.
CytokinesisAnimals: furrow from the edge inwards. Plants: cell-plate from the centre outwards. Karyokinesis without cytokinesis gives a syncytium.
Prophase ILeptotene, zygotene (synapsis), pachytene (crossing over), diplotene (chiasmata), diakinesis (terminalisation).
Meiosis counts46 chromosomes make 23 bivalents. DNA: 4C after S, 2C after meiosis I, 1C after meiosis II. Interkinesis has no replication.
Anaphase I vs IIAnaphase I separates homologues with centromeres intact. Anaphase II splits centromeres and separates sister chromatids.
Chromosome typesMetacentric V, submetacentric L, acrocentric J, telocentric I. Short arm p, long arm q. Secondary constriction carries rRNA genes.
Gametes and variationOne spermatocyte gives 4 sperms; one oocyte gives 1 ovum. Meiosis is zygotic, gametic or sporic. Replication is semiconservative.

Three sample questions

Q1. Which is the correct order of the phases of the cell cycle?
  1. G1 → G2 → S → M
  2. S → G1 → G2 → M
  3. G1 → S → G2 → M
  4. M → S → G1 → G2
Hint and solution

Hint: DNA copying happens in the middle of interphase, between the two gap phases. Division comes last.

  1. Cell cycle = interphase + M phase — The cycle has a long interphase followed by the short M phase where division occurs.
  2. Interphase: G1 → S → G2 — G1 is the first gap and growth phase, S is the synthesis phase where DNA is copied, and G2 is the second gap before division.
  3. Then M phase — After G2 the cell enters mitosis, so the full order is G1, S, G2, M.
  4. Answer: G1 → S → G2 → M — This matches the third choice. Option C.
Answer: C. DNA is copied in S, between the two gaps. M phase then follows G2.
Q2. A diploid cell has 2n = 14. Just after S phase it has
  1. 28 chromosomes and 28 chromatids
  2. 14 chromosomes and 28 chromatids
  3. 14 chromosomes and 14 chromatids
  4. 7 chromosomes and 14 chromatids
Hint and solution

Hint: S phase copies DNA but the copies stay attached at the centromere. Do you count the attached copies as new chromosomes?

  1. 2n = 14 chromosomes before S — A diploid cell with 2n equal to 14 starts with 14 chromosomes, each with one chromatid.
  2. S phase: DNA doubles — Each chromosome is replicated into two sister chromatids held together at the centromere.
  3. Chromatids = 14 × 2 = 28 — Chromosome number is counted by centromeres, so it stays 14 while chromatids become 28.
  4. 14 chromosomes, 28 chromatids — So the answer is 14 chromosomes and 28 chromatids. Option B.
Answer: B. S phase doubles DNA, so each of the 14 chromosomes has 2 chromatids: 28 chromatids. The count stays 14.
Q3. In a 24 hour human cell cycle, M phase takes about 1 hour. The share of the cycle spent in interphase is about
  1. about 50 per cent
  2. more than 95 per cent
  3. about 75 per cent
  4. less than 10 per cent
Hint and solution

Hint: Subtract the M phase from the whole cycle, then divide by the whole cycle.

  1. Interphase = 24 − 1 = 23 hours — The cycle is 24 hours and M phase takes 1 hour, so everything else is interphase.
  2. Share = 23 ÷ 24 × 100 — Divide interphase time by total time and convert to per cent.
  3. 23 ÷ 24 × 100 ≈ 95.8 per cent — The result is just under 96 per cent, which is above 95 per cent.
  4. More than 95 per cent — So interphase is the longest part of the cycle. Option B.
Answer: B. 23 of 24 hours is about 95.8 per cent.

How to use this chapter

Read the NCERT pages first, then attempt Set 1 without the hints. Note every wrong answer in a mistake list, read the reason for each option, and retry Set 2 two days later. A topic is solid when you can say why each wrong option is wrong.

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