NEET Biology Cell Cycle and Cell Division MCQs Set 06

Answer: (d) 25

Explanation:
1. 2n = 50, which form 25 homologous pairs (bivalents).
2. All 25 bivalents lie at the equator in metaphase I.

Teacher's Note:
1. Number of bivalents equals n.
2. Chromosomes are 50 and chromatids 100.

Answer: (a) 625

Explanation:
1. 500 zygotes need 500 sperm and 500 eggs.
2. 500 sperm need 125 meioses (4 per meiosis) and 500 eggs need 500 meioses.
3. Total = 125 + 500 = 625.

Teacher's Note:
1. Males give four gametes, females one.
2. Add the two separately.

Answer: (b) one in mitosis, two in meiosis I and one in meiosis II

Explanation:
1. In mitosis and meiosis II the centromere splits, so each chromosome has one chromatid.
2. In anaphase I the centromere does not split and each chromosome has two chromatids.

Teacher's Note:
1. Mitotic anaphase and anaphase II are alike.
2. Anaphase I is the exception.

Answer: (a) 2n

Explanation:
1. DNA replication produces sister chromatids but they stay joined at the centromere.
2. The chromosome number therefore remains 2n.

Teacher's Note:
1. Count chromosomes by centromeres.
2. Only DNA content becomes 4C.

Answer: (d) two, one, one

Explanation:
1. The nucleus divides twice.
2. DNA replicates once (before meiosis I) and centromeres divide once (anaphase II).

Teacher's Note:
1. Two divisions, one replication, one centromere division.
2. Same fact as beginner questions on meiosis.

Answer: (d) A is mitosis, B is meiosis

Explanation:
1. In mitosis DNA doubles in S and returns to the starting level after one division, as in graph A.
2. In meiosis DNA doubles once and falls in two steps (meiosis I, then II), as in graph B.

Teacher's Note:
1. One rise and one fall means mitosis.
2. One rise and two falls means meiosis.

Answer: (d) Both Assertion and Reason are false.

Explanation:
1. G0 cells are metabolically active but not dividing, so the assertion is false.
2. The G0 stage is entered from G1, not G2, so the reason is also false.

Teacher's Note:
1. G0 cells are quiet but not dead.
2. They leave the cycle from G1.

Answer: (a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Explanation:
1. The spindle forms in the cytoplasm and must reach the chromosomes, so the envelope must break down.
2. This is why both statements are true, and the reason explains the assertion.

Teacher's Note:
1. Spindle outside, chromosomes inside: the envelope must go.
2. Exceptions occur in some protists.

Answer: (b) Both Assertion and Reason are true but the Reason is not a correct explanation of the Assertion.

Explanation:
1. Chromosomes are most condensed and easily visible in metaphase, so the assertion is true.
2. The reason is also true, but it does not explain why morphology is best studied then.

Teacher's Note:
1. Both statements are facts but unrelated causally.
2. Condensation is the real reason.

Answer: (a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Explanation:
1. Without kinetochores spindle fibres cannot attach to chromosomes.
2. So chromosomes cannot be moved and division fails.

Teacher's Note:
1. Kinetochore equals attachment site.
2. Both statements are true and linked.

Answer: (a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Explanation:
1. Centromere splitting converts each chromosome into two single-chromatid chromosomes.
2. So the number doubles, and the reason explains this.

Teacher's Note:
1. Count of centromeres equals count of chromosomes.
2. Doubling is temporary.

Answer: (a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Explanation:
1. As the cell grows, its nuclear volume relative to cytoplasm falls (karyoplasmic index decreases).
2. The nucleus can no longer control the cytoplasm, and this triggers division.

Teacher's Note:
1. Growth upsets the nucleus-cytoplasm balance.
2. Division restores it.

Answer: (b) Both Assertion and Reason are true but the Reason is not a correct explanation of the Assertion.

Explanation:
1. Meiosis I has crossing over and independent assortment, which create variation for evolution.
2. The reason is true but concerns chromosome number constancy, not variation.

Teacher's Note:
1. Evolution depends on variation.
2. Reduction maintains number, not variation.

Answer: (c) Assertion is true but the Reason is false.

Explanation:
1. The synaptonemal complex begins to form in zygotene, so the assertion is true.
2. It helps synapsis of homologous (not non-homologous) chromosomes, so the reason is false.

Teacher's Note:
1. Synapsis is between homologues only.
2. Watch for homologous and non-homologous swaps.

Answer: (d) Both Assertion and Reason are false.

Explanation:
1. Spindle fibres are not attached during S phase, so the assertion is false.
2. Bivalents are present in metaphase I and separate into single chromosomes in anaphase I, so the reason is false.

Teacher's Note:
1. Spindle attachment is a metaphase feature.
2. Bivalents separate in anaphase I.

Answer: (b) Both Assertion and Reason are true but the Reason is not a correct explanation of the Assertion.

Explanation:
1. Interkinesis has no S phase, so the assertion is true.
2. The centromere does divide in meiosis II, so the reason is true, but it is not the reason for no replication.

Teacher's Note:
1. Both are true but unrelated.
2. The reason concerns anaphase II.

Answer: (a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Explanation:
1. Meiosis halves the chromosome number from 2n to n.
2. That is precisely why it is called reduction division.

Teacher's Note:
1. The name comes from the function.
2. Meiosis I is the reductional step.

Answer: (b) Both Assertion and Reason are true but the Reason is not a correct explanation of the Assertion.

Explanation:
1. Crossing over occurs in pachytene, and the reason correctly defines crossing over.
2. The definition does not explain why it occurs specifically in pachytene.

Teacher's Note:
1. The reason defines the process but does not explain the timing.
2. Pachytene has fully paired homologues.

Answer: (b) Both Assertion and Reason are true but the Reason is not a correct explanation of the Assertion.

Explanation:
1. Both statements are true.
2. The second is a general statement that does not specifically explain why yeast takes 90 minutes.

Teacher's Note:
1. Human cells take about 24 hours.
2. Cycle duration varies with organism and cell type.

Answer: (a) Both Assertion and Reason are true and the Reason is a correct explanation of the Assertion.

Explanation:
1. The nucleus, cytoskeleton and organelles are all rearranged in M phase.
2. This reorganisation is why it is called dramatic.

Teacher's Note:
1. Dramatic equals major reorganisation.
2. It lasts only about an hour in a human cell.

Answer: (d) Both Assertion and Reason are false.

Explanation:
1. Small cells have a high karyoplasmic index and the nucleus controls their cytoplasm efficiently, so they are not less active.
2. Both statements are false.

Teacher's Note:
1. Small cell means high KI.
2. Large cells have low KI and divide.