NEET Biology Cell Cycle and Cell Division MCQs Set 21

Answer: (b) either haploid or diploid and with duplicated chromosomes

Explanation:
1. Mitosis can occur in haploid cells (for example in plants) as well as diploid cells.
2. Any cell entering M has already completed the S phase, so its chromosomes are duplicated.

Teacher's Note:
1. Ploidy does not decide whether a cell can undergo mitosis.
2. Duplication always precedes entry into M.

Answer: (a) 2

Explanation:
1. Chromosomes were replicated in the S phase.
2. So every chromosome seen in prophase consists of two sister chromatids.

Teacher's Note:
1. Chromosomes are double-stranded from G2 until anaphase.
2. After anaphase each chromatid is called a chromosome.

Answer: (a) zygotene to pachytene

Explanation:
1. The complex starts forming in zygotene when homologues pair.
2. It is fully formed in pachytene and dissolves in diplotene.

Teacher's Note:
1. It holds paired homologues together during crossing over.
2. It is a feature of prophase I only.

Answer: (b) Diplotene - chiasmata appear

Explanation:
1. Chiasmata become visible in diplotene when the homologues begin to separate.
2. Bivalents form in zygotene, and terminalisation happens in diakinesis.

Teacher's Note:
1. Learn each prophase I sub-stage with its landmark event.
2. Leptotene, zygotene, pachytene, diplotene, diakinesis.

Answer: (b) Leptotene

Explanation:
1. Leptotene means thin thread.
2. Chromosomes begin to condense and appear as long slender threads.

Teacher's Note:
1. The name leptotene itself means thin threads.
2. Compaction increases through the later sub-stages.

Answer: (b) two nuclear divisions and one chromosome division

Explanation:
1. The nucleus divides twice (meiosis I and II).
2. Chromosomes (DNA and centromeres) are divided only once, at anaphase II.

Teacher's Note:
1. Two nuclear divisions give four haploid nuclei.
2. Replication happens only once, before meiosis I.

Answer: (a) anaphase of meiosis-II

Explanation:
1. In anaphase I whole chromosomes (with two chromatids) move apart.
2. The centromere splits only in anaphase II, separating the sister chromatids.

Teacher's Note:
1. Centromere division is absent in anaphase I.
2. This is the key difference from mitotic anaphase.

Answer: (d) Splitting of the centromere

Explanation:
1. Anaphase I separates homologous chromosomes and the centromeres stay intact.
2. In anaphase II the centromeres split and sister chromatids separate.

Teacher's Note:
1. Poleward movement happens in both anaphases.
2. Only centromere splitting is exclusive to anaphase II.

Answer: (b) pachytene

Explanation:
1. Homologues are fully synapsed in pachytene.
2. Recombination nodules mediate exchange between non-sister chromatids at this stage.

Teacher's Note:
1. Crossing over needs complete synapsis.
2. Its result (chiasmata) becomes visible in diplotene.

Answer: (a) animal cells

Explanation:
1. Animal cells divide by a furrow made by a contractile ring of actin and myosin.
2. The narrow bridge left at the end is the mid body.

Teacher's Note:
1. Plants form a cell plate instead of a furrow.
2. The rigid wall of plant cells prevents furrowing.

Answer: (b) centrifugally

Explanation:
1. The cell plate starts forming at the centre from vesicles of Golgi.
2. It grows outward toward the cell wall, so cytokinesis is centrifugal.

Teacher's Note:
1. Animal furrow is centripetal (outside inward).
2. Plant cell plate is centrifugal (inside outward).

Answer: (b) Meiosis and fertilisation

Explanation:
1. Meiosis halves the chromosome number to form gametes.
2. Fertilisation restores the diploid number by fusing two gametes.

Teacher's Note:
1. Meiosis alone would keep reducing the number.
2. The two events balance each other every generation.

Answer: (c) Anther

Explanation:
1. Meiosis occurs in spore and gamete mother cells.
2. Young anthers contain many pollen mother cells dividing by meiosis, so they are used for squashes.

Teacher's Note:
1. Root and shoot apices show only mitosis.
2. The anther is the classic material for meiosis.

Answer: (d) spindle fibres

Explanation:
1. Kinetochore microtubules of the spindle pull chromosomes toward the poles.
2. Astral rays only anchor the poles, and NOR is a nucleolar organiser region.

Teacher's Note:
1. Microtubules of the spindle do the pulling.
2. Remember kinetochore fibres attach at the centromere.

Answer: (a) terminalisation

Explanation:
1. Chiasmata move from their original position to the chromosome ends.
2. This process, terminalisation, occurs in diakinesis.

Teacher's Note:
1. Terminalisation is the process, diakinesis is the stage.
2. Congression is movement of chromosomes to the equator.

Answer: (a) Decondensation of chromatin

Explanation:
1. Chromatin condenses into visible chromosomes in prophase.
2. Decondensation is a telophase event.

Teacher's Note:
1. Prophase builds up, telophase breaks down.
2. Condensation and decondensation are opposite events.

Answer: (b) Metaphase

Explanation:
1. Chromosomes are most condensed and lie in one plane at metaphase.
2. This makes counting and study of morphology easiest.

Teacher's Note:
1. Karyotypes are prepared from metaphase cells.
2. Colchicine is used to arrest cells at this stage.

Answer: (d) 40

Explanation:
1. The egg is haploid, so n = 10 and the diploid number 2n = 20.
2. At anaphase the chromatids separate, doubling the chromosome number to 40.

Teacher's Note:
1. Anaphase temporarily doubles the number.
2. It returns to 2n after cytokinesis.

Answer: (b) Metaphase

Explanation:
1. Condensation increases through prophase and is maximum at metaphase.
2. That is why metaphase chromosomes are best seen.

Teacher's Note:
1. Maximum coiling, maximum visibility.
2. Decondensation starts in telophase.

Answer: (a) Prophase and metaphase

Explanation:
1. Chromosomes remain double until the centromere splits at the start of anaphase.
2. So prophase and metaphase chromosomes have two chromatids.

Teacher's Note:
1. Anaphase and telophase chromosomes are single chromatids.
2. The centromere split is the dividing line.

Answer: (a) G1 phase

Explanation:
1. After M phase the cell enters G1 before the S phase begins.
2. The G1 phase is the pre-DNA-synthesis gap.

Teacher's Note:
1. G2 comes after S, not before it.
2. Interkinesis is a meiotic gap with no DNA replication.