Cell Cycle and Cell Division

What to remember

• The cell cycle has two parts: interphase (G1, S, G2) and the M phase. Interphase is the longest part. DNA is replicated in the S phase, so DNA content doubles (2C to 4C) while the chromosome number stays the same.
• Mitosis is equational division. It gives two genetically identical daughter cells with the same chromosome number as the parent. Its stages are prophase, metaphase, anaphase and telophase.
• Meiosis has one round of DNA replication and two divisions. It gives four haploid cells. Homologous chromosomes pair in zygotene, cross over in pachytene, and separate in anaphase I. Sister chromatids separate in anaphase II.
• Cytokinesis is by a furrow in animal cells and by a cell plate in plant cells.

The cell cycle

All cells divide to produce new cells. A cell grows, copies its DNA and then divides. This sequence of events is the cell cycle. The duration of the cycle differs among organisms. A human cell in culture divides about once in 24 hours. Yeast completes a cycle in about 90 minutes.

The cell cycle has two basic phases.

Interphase has three stages.

Cell division is not the only event in the cycle. Cell growth goes on during G1 and G2. The cycle can be pictured as G1 → S → G2 → M → G1. After M, each daughter cell enters G1 again and may either continue the cycle or move into G0.

G0 phase (quiescent stage). Some cells do not divide further. They leave G1 and enter an inactive stage called G0. Cells in G0 remain metabolically active but do not proliferate unless called on to do so. Some cells in adult animals, such as heart cells, do not divide. Many other cells divide only when needed, to replace cells lost to injury or death.

M phase: karyokinesis and cytokinesis

The M phase begins with karyokinesis, the division of the nucleus, and ends with cytokinesis, the division of the cytoplasm. In mitosis, a parent cell divides into two daughter cells with the same number of chromosomes. For this reason mitosis is called equational division. It is seen in somatic cells. Karyokinesis is divided into four stages: prophase, metaphase, anaphase and telophase.

Stages of mitosis

Cytokinesis. In an animal cell, a furrow appears in the plasma membrane. It deepens and joins in the centre, dividing the cell into two. In a plant cell, the wall is rigid, so a furrow is not possible. A cell plate forms at the centre and grows outward to meet the existing lateral walls. It forms from Golgi vesicles and becomes the middle lamella between the two cells. Organelles such as mitochondria and plastids are shared between the two daughter cells during cytokinesis.

In some organisms, karyokinesis is not followed by cytokinesis. This gives a multinucleate cell called a syncytium. An example is the liquid endosperm of coconut.

Significance of mitosis

• It produces genetically identical daughter cells with the same chromosome number.
• It brings growth. As cells grow, the ratio of nucleus to cytoplasm is disturbed, so cells divide to restore it.
• It replaces and repairs cells, such as those of the skin surface, gut lining and blood.
• It allows continuous growth in plants, as meristematic cells at the apices and cambium divide repeatedly.

Meiosis

Meiosis is the division that produces gametes in sexually reproducing organisms. It reduces the chromosome number by half, so it is called reductional division. It involves one cycle of DNA replication followed by two successive nuclear and cell divisions, meiosis I and meiosis II. It produces four haploid cells from one diploid cell. Meiosis I starts with a long and complex prophase I.

Prophase I has five substages.

Meiosis I (rest of the division)

Interkinesis is the short stage between meiosis I and meiosis II. DNA is not replicated.

Meiosis II is similar to mitosis.

Significance of meiosis

• It keeps the chromosome number constant from generation to generation in sexually reproducing species. Without it, the number would double in each generation.
• It increases genetic variability, through crossing over, and this variation is important in evolution.

Counting chromosomes and DNA through the divisions

Take a human cell with 2n = 46 chromosomes and DNA content 2C in G1. The table shows how the numbers change. A chromosome with two chromatids is still counted as one chromosome. The chromosome number rises only when the centromere splits in anaphase.

Some terms to keep clear:

• Homologous chromosomes are a pair, one from each parent, with the same genes in the same order. They pair only in meiosis.
• Sister chromatids are the two copies of one chromosome, joined at the centromere. They form during S phase.
• Non-sister chromatids belong to different chromosomes of a homologous pair. Crossing over occurs between them.
• Spindle fibres are microtubules. They attach to the kinetochores and pull the chromosomes or chromatids towards the poles.

In an animal cell, the centrosome duplicates in interphase. In prophase, the two centrosomes move to opposite poles and the spindle forms between them. In plant cells, there are no centrioles, yet a spindle still forms.

Mitosis and meiosis compared

NEET traps

• 1. G1 has no DNA replication. DNA is replicated in S. G2 has 4C DNA.
• 2. DNA content doubles in S, but chromosome number does not change.
• 3. G0 is a resting stage that is metabolically active. It is not part of the interphase sequence G1 to S to G2.
• 4. In anaphase of mitosis, sister chromatids separate. In anaphase I of meiosis, homologous chromosomes separate. In anaphase II, sister chromatids separate.
• 5. Synapsis occurs in zygotene. Crossing over occurs in pachytene. Chiasmata are seen in diplotene.
• 6. Terminalisation of chiasmata occurs in diakinesis.
• 7. Interkinesis has no DNA replication.
• 8. The best stage to count and study chromosomes is metaphase.
• 9. Animal cytokinesis uses a furrow. Plant cytokinesis uses a cell plate.
• 10. Meiosis I is reductional, since the chromosome number is halved. Meiosis II is equational.

One-liners

• 1. A human cell cycle takes about 24 hours.
• 2. A yeast cell cycle takes about 90 minutes.
• 3. M phase of a human cell lasts about 1 hour.
• 4. Interphase is the longest part of the cell cycle.
• 5. DNA replicates in the S phase.
• 6. Quiescent cells are in the G0 phase.
• 7. Mitosis is an equational division.
• 8. The nuclear envelope breaks down at the end of prophase.
• 9. Chromosomes line up at the equatorial plate in metaphase.
• 10. The synaptonemal complex forms in zygotene.
• 11. A bivalent is also called a tetrad.
• 12. Recombination nodules appear in pachytene.
• 13. Chiasmata are X-shaped structures seen in diplotene.
• 14. Meiosis gives four haploid cells.
• 15. A syncytium is a multinucleate cell formed when karyokinesis is not followed by cytokinesis.

Practice questions

1. A typical human cell in culture completes one cell cycle in about

   1. 1 hour
   2. 12 days
   3. 24 hours
   4. 90 minutes
   Answer

   C. 24 hours

   A human cell divides about once in 24 hours. Yeast takes about 90 minutes.

2. A yeast cell completes its cell cycle in about

   1. 24 hours
   2. 48 hours
   3. 10 minutes
   4. 90 minutes
   Answer

   D. 90 minutes

   Yeast takes about 90 minutes for one cell cycle.

3. DNA replication in the cell cycle takes place in the

   1. G1 phase
   2. S phase
   3. M phase
   4. G2 phase
   Answer

   B. S phase

   DNA is synthesised in the S phase, so DNA content doubles from 2C to 4C.

4. The inactive stage in which cells leave the cell cycle but stay metabolically active is called the

   1. G0 phase
   2. S phase
   3. interkinesis
   4. G2 phase
   Answer

   A. G0 phase

   Cells that do not divide further exit G1 and enter G0, the quiescent stage.

5. The best stage to study the number and shape of chromosomes is

   1. interphase
   2. telophase
   3. metaphase
   4. prophase
   Answer

   C. metaphase

   At metaphase, chromosomes are fully condensed and lie on the equatorial plate.

6. In mitosis, the centromere splits and chromatids separate during

   1. metaphase
   2. anaphase
   3. telophase
   4. prophase
   Answer

   B. anaphase

   Centromeres split at anaphase and sister chromatids move to opposite poles.

7. In plant cells, cytokinesis takes place by

   1. cell plate formation
   2. budding
   3. furrow formation
   4. fragmentation
   Answer

   A. cell plate formation

   Plant cytokinesis uses a cell plate that grows outward from the centre.

8. The synaptonemal complex is formed during

   1. leptotene
   2. pachytene
   3. diplotene
   4. zygotene
   Answer

   D. zygotene

   Pairing of homologous chromosomes in zygotene is accompanied by the synaptonemal complex.

9. Crossing over occurs during which stage of prophase I?

   1. Pachytene
   2. Zygotene
   3. Diakinesis
   4. Leptotene
   Answer

   A. Pachytene

   Recombination nodules appear and crossing over takes place in pachytene.

10. X-shaped chiasmata become visible during

    1. leptotene
    2. pachytene
    3. diplotene
    4. zygotene
    Answer

    C. diplotene

    In diplotene the synaptonemal complex dissolves and chiasmata become visible.

11. Terminalisation of chiasmata takes place in

    1. diplotene
    2. pachytene
    3. interkinesis
    4. diakinesis
    Answer

    D. diakinesis

    In diakinesis the chiasmata move towards the ends of the chromosomes.

12. A pair of synapsed homologous chromosomes is called a

    1. chromatid
    2. bivalent
    3. kinetochore
    4. dyad
    Answer

    B. bivalent

    A bivalent, also called a tetrad, is formed by a pair of synapsed homologous chromosomes.

13. The short stage between meiosis I and meiosis II is called

    1. diakinesis
    2. telophase II
    3. interkinesis
    4. G0 phase
    Answer

    C. interkinesis

    Interkinesis is short and has no DNA replication.

14. The enzyme involved in crossing over is

    1. RuBisCO
    2. amylase
    3. ligase
    4. recombinase
    Answer

    D. recombinase

    Recombinase takes part in the recombination in pachytene.

15. How many cells are formed from one diploid cell at the end of meiosis?

    1. two
    2. four
    3. eight
    4. one
    Answer

    B. four

    Meiosis I and II give four haploid cells.

16. When karyokinesis is not followed by cytokinesis, the result is a

    1. multinucleate syncytium
    2. dyad of cells
    3. cell plate
    4. bivalent
    Answer

    A. multinucleate syncytium

    A syncytium forms, as in the liquid endosperm of coconut.

17. Given below are two statements: Statement I: DNA is replicated in the G1 phase. Statement II: The DNA content of a cell doubles in the S phase. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    D. Statement I is incorrect but Statement II is correct

    DNA replicates in the S phase, not in G1.

18. Given below are two statements: Statement I: Mitosis is an equational division. Statement II: Mitosis takes place in somatic cells. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    Both are correct. Daughter cells have the same chromosome number as the parent.

19. Given below are two statements: Statement I: In metaphase I, bivalents are aligned on the equatorial plate. Statement II: In anaphase I, sister chromatids separate. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    In anaphase I homologous chromosomes separate. Sister chromatids separate in anaphase II.

20. Given below are two statements: Statement I: Meiosis involves two rounds of DNA replication. Statement II: Meiosis produces four haploid cells. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    D. Statement I is incorrect but Statement II is correct

    Meiosis has one round of DNA replication followed by two divisions.

21. Given below are two statements: Statement I: DNA replication takes place during interkinesis. Statement II: Synapsis of homologous chromosomes takes place in prophase II. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    B. Both Statement I and Statement II are incorrect

    No DNA replicates in interkinesis. Synapsis occurs in zygotene of prophase I.

22. Which of the following statements are correct? 1. Cells in G0 are metabolically active. 2. Heart cells of adult animals do not appear to divide. 3. Interphase lasts for less than 5 per cent of the cell cycle.

    1. 1 and 2 only
    2. 2 and 3 only
    3. 1 and 3 only
    4. 1, 2 and 3
    Answer

    A. 1 and 2 only

    Interphase lasts for more than 95 per cent of the cell cycle.

23. Which of the following statements are correct? 1. The cell plate forms from Golgi vesicles. 2. A furrow forms during animal cytokinesis. 3. Plant cell cytokinesis takes place by a furrow.

    1. 1 and 2 only
    2. 2 and 3 only
    3. 1 and 3 only
    4. 1 only
    Answer

    A. 1 and 2 only

    Plant cytokinesis uses a cell plate, not a furrow.

24. Given below are two statements: Statement I: Chiasmata become visible in diplotene. Statement II: The synaptonemal complex dissolves in diplotene. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    A. Both Statement I and Statement II are correct

    Both are correct.

25. Given below are two statements: Statement I: Telophase I gives a dyad of cells. Statement II: Telophase I gives four cells. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    C. Statement I is correct but Statement II is incorrect

    Four cells form only after telophase II.

26. Given below are two statements: Statement I: Crossing over occurs between sister chromatids of the same chromosome. Statement II: Crossing over occurs between non-sister chromatids of homologous chromosomes. In the light of the above statements, choose the correct answer from the options given below.

    1. Both Statement I and Statement II are correct
    2. Both Statement I and Statement II are incorrect
    3. Statement I is correct but Statement II is incorrect
    4. Statement I is incorrect but Statement II is correct
    Answer

    D. Statement I is incorrect but Statement II is correct

    Crossing over involves non-sister chromatids of homologous chromosomes.

27. Assertion (A): Plant cells divide by cell plate formation. Reason (R): The rigid cell wall of plant cells does not allow a furrow to form. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    A. Both A and R are true and R is the correct explanation of A

    The rigid wall prevents furrowing, so a cell plate grows outward from the centre.

28. Assertion (A): Meiosis keeps the chromosome number constant in successive generations. Reason (R): Meiosis halves the chromosome number in the gametes. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    A. Both A and R are true and R is the correct explanation of A

    Halved gametes restore the diploid number at fertilisation.

29. Assertion (A): Metaphase is the best stage to study chromosome morphology. Reason (R): At metaphase, the chromosomes decondense and lose their individuality. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    C. A is true but R is false

    At metaphase the chromosomes are fully condensed. Decondensation happens in telophase.

30. Assertion (A): The DNA content of a cell doubles in the S phase. Reason (R): The chromosome number doubles in the S phase. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    C. A is true but R is false

    The chromosome number stays the same, since sister chromatids stay joined.

31. Assertion (A): Mitosis produces genetically different daughter cells. Reason (R): Mitosis gives daughter cells with the same chromosome number as the parent. Choose the correct answer from the options given below.

    1. Both A and R are true and R is the correct explanation of A
    2. Both A and R are true but R is not the correct explanation of A
    3. A is true but R is false
    4. A is false but R is true
    Answer

    D. A is false but R is true

    Mitosis gives genetically identical daughter cells.

32. Match List I with List II. List I: a. Leptotene b. Zygotene c. Pachytene d. Diplotene List II: i. Crossing over at recombination nodules ii. Chiasmata become visible iii. Synapsis with synaptonemal complex iv. Chromosomes become visible and condense Choose the correct answer from the options given below.

    1. a-i, b-iv, c-iii, d-ii
    2. a-iv, b-iii, c-i, d-ii
    3. a-ii, b-i, c-iii, d-iv
    4. a-ii, b-iii, c-iv, d-i
    Answer

    B. a-iv, b-iii, c-i, d-ii

    Each substage is matched to its key event.

33. Match List I with List II. List I: a. G1 phase b. S phase c. G2 phase d. G0 phase List II: i. DNA replication ii. Protein synthesis for mitosis iii. Quiescent stage iv. Cell grows with no DNA replication Choose the correct answer from the options given below.

    1. a-iv, b-iii, c-ii, d-i
    2. a-iv, b-i, c-ii, d-iii
    3. a-ii, b-i, c-iii, d-iv
    4. a-iv, b-ii, c-i, d-iii
    Answer

    B. a-iv, b-i, c-ii, d-iii

    Each phase is matched to its event.

34. Match List I with List II. List I: a. Prophase b. Metaphase c. Anaphase d. Telophase List II: i. Nuclear envelope reforms ii. Chromatin condenses into chromosomes iii. Chromosomes at the equatorial plate iv. Centromere splits Choose the correct answer from the options given below.

    1. a-i, b-iii, c-iv, d-ii
    2. a-i, b-iv, c-iii, d-ii
    3. a-ii, b-iii, c-iv, d-i
    4. a-ii, b-iv, c-iii, d-i
    Answer

    C. a-ii, b-iii, c-iv, d-i

    Each mitotic stage is matched to its key event.

35. Match List I with List II. List I: a. Diakinesis b. Interkinesis c. Metaphase I d. Telophase I List II: i. No DNA replication ii. Terminalisation of chiasmata iii. Dyad of cells iv. Bivalents on the equatorial plate Choose the correct answer from the options given below.

    1. a-iv, b-ii, c-i, d-iii
    2. a-iv, b-iii, c-ii, d-i
    3. a-iii, b-ii, c-iv, d-i
    4. a-ii, b-i, c-iv, d-iii
    Answer

    D. a-ii, b-i, c-iv, d-iii

    Each stage is matched to its feature.

36. Match List I with List II. List I: a. Karyokinesis b. Cytokinesis c. Synapsis d. Chiasma List II: i. Division of the cytoplasm ii. Division of the nucleus iii. X-shaped site of crossing over iv. Pairing of homologous chromosomes Choose the correct answer from the options given below.

    1. a-i, b-iii, c-ii, d-iv
    2. a-ii, b-i, c-iv, d-iii
    3. a-ii, b-i, c-iii, d-iv
    4. a-iii, b-i, c-ii, d-iv
    Answer

    B. a-ii, b-i, c-iv, d-iii

    Each term is matched to its meaning.

37. Match List I with List II. List I: a. Human cell cycle b. Yeast cell cycle c. M phase of a human cell d. DNA content after S phase of a 2C cell List II: i. About 90 minutes ii. 4C iii. About 24 hours iv. About 1 hour Choose the correct answer from the options given below.

    1. a-iv, b-ii, c-iii, d-i
    2. a-iii, b-i, c-ii, d-iv
    3. a-iii, b-i, c-iv, d-ii
    4. a-iv, b-ii, c-i, d-iii
    Answer

    C. a-iii, b-i, c-iv, d-ii

    Each item is matched to its value.

38. A cell with 2n = 20 chromosomes is at metaphase of mitosis. The total number of chromatids in the cell is

    1. 10
    2. 20
    3. 80
    4. 40
    Answer

    D. 40

    There are 20 chromosomes, each with two chromatids.

39. A human cell with 46 chromosomes undergoes meiosis. Each cell formed at the end of meiosis I has

    1. 92 chromosomes
    2. 23 chromosomes
    3. 46 chromosomes
    4. 12 chromosomes
    Answer

    B. 23 chromosomes

    Meiosis I halves the chromosome number (homologous chromosomes separate).

40. A cell with 2n = 8 is at anaphase of mitosis. The total number of chromosomes in the whole cell at this stage is

    1. 16
    2. 8
    3. 4
    4. 32
    Answer

    A. 16

    Each of the 8 chromosomes splits into two, so 16 separate chromatids are counted as chromosomes.

41. A student sees chromosomes paired as bivalents on the equatorial plate. The stage is

    1. diakinesis
    2. metaphase of mitosis
    3. metaphase I
    4. metaphase II
    Answer

    C. metaphase I

    Bivalents on the equator are seen in metaphase I.

42. A drug blocks DNA replication in dividing cells. Which phase of the cell cycle can these cells NOT complete?

    1. Cytokinesis
    2. G1 phase
    3. G0 phase
    4. S phase
    Answer

    D. S phase

    DNA is replicated in S phase.

43. The liquid endosperm of coconut contains many free nuclei in a common cytoplasm. This arises because

    1. meiosis stops at interkinesis
    2. karyokinesis is not followed by cytokinesis
    3. DNA replication is absent
    4. cytokinesis occurs without karyokinesis
    Answer

    B. karyokinesis is not followed by cytokinesis

    Repeated nuclear divisions without cytokinesis give a multinucleate syncytium.

44. In the oocytes of some vertebrates, prophase I can last for months or years. The substage that is prolonged is

    1. zygotene
    2. leptotene
    3. diplotene
    4. pachytene
    Answer

    C. diplotene

    Diplotene can be prolonged in the oocytes of some vertebrates.