Part 7 — Critical & Complex NEET-style Integrative Questions (Q151–175)

Q151.

A drug inhibits Cyclin B/CDK1 activity. Which process will be blocked?
A. G1 → S transition
B. S-phase DNA replication
C. Entry into mitosis ✅
D. Chromatid separation

Explanation:

**A. G1 → S: Cyclin E/CDK2 controls.
**B. DNA replication: Cyclin A/CDK2.
**C. Correct — Cyclin B/CDK1 (MPF) triggers mitosis.
**D. Separation = APC/C–separase system.

Q152.

A patient has uncontrolled cell division due to mutation in p53. What checkpoint is lost?
A. G1/S checkpoint ✅
B. G2/M checkpoint
C. Spindle checkpoint
D. Cytokinesis checkpoint

Explanation:

**A. Correct — p53 halts cells at G1/S if DNA damaged.
**B. G2/M: controlled by other kinases.
**C. Spindle checkpoint: APC/C based.
**D. Cytokinesis: not relevant.

Q153.

If a cell undergoes mitosis without cytokinesis, what will result?
A. Aneuploidy
B. Multinucleated cell ✅
C. Haploid cells
D. Apoptosis

Explanation:

**A. Aneuploidy = abnormal chromosome numbers due to nondisjunction.
**B. Correct — nuclei divide but cytoplasm doesn’t → multinucleated.
**C. Haploid cells from meiosis.
**D. Not necessarily cell death.

Q154.

In which situation will a cell be arrested at metaphase?
A. DNA damage persists
B. Kinetochores not attached to spindle ✅
C. Telomeres shortened
D. Cyclin E degraded

Explanation:

**A. DNA damage: G1/S or G2/M.
**B. Correct — spindle assembly checkpoint halts anaphase until attachment complete.
**C. Telomeres: senescence, not metaphase arrest.
**D. Cyclin E regulates G1/S.

Q155.

A gamete has 22 autosomes + XX. After fertilization with a normal sperm, the zygote will be:
A. Turner’s syndrome
B. Down syndrome
C. Klinefelter’s syndrome ✅
D. Normal female

Explanation:

**A. Turner’s: XO.
**B. Down: trisomy 21.
**C. Correct — XX gamete + Y sperm → XXY male = Klinefelter.
**D. Normal female would be XX, not XXY.

Q156.

A drug prevents degradation of securin. Which process is inhibited?
A. DNA replication
B. Chromatid separation ✅
C. Telomere elongation
D. Crossing over

Explanation:

**A. Replication unaffected.
**B. Correct — securin inhibits separase; without degradation, cohesins remain → no separation.
**C. Telomerase unrelated.
**D. Crossing over occurs earlier in meiosis I.

Q157.

Which feature distinguishes meiosis I from mitosis?
A. Sister chromatid separation
B. Homologous recombination ✅
C. Cytokinesis
D. Chromosome condensation

Explanation:

**A. Sister chromatids separate in mitosis & meiosis II.
**B. Correct — crossing over unique to meiosis I.
**C. Both processes have cytokinesis.
**D. Condensation occurs in both.

Q158.

Loss of function of Rb protein results in:
A. Arrest at G1/S
B. Premature S-phase entry ✅
C. Chromatid mis-segregation
D. Apoptosis

Explanation:

**A. Active Rb = arrest; loss removes brake.
**B. Correct — unchecked E2F → uncontrolled replication.
**C. Mis-segregation relates to spindle defects.
**D. Apoptosis linked to p53.

Q159.

A person with genotype 47, XY + 21 suffers from:
A. Klinefelter’s syndrome
B. Turner’s syndrome
C. Down syndrome ✅
D. Edwards syndrome

Explanation:

**A. Klinefelter = XXY male.
**B. Turner = XO female.
**C. Correct — trisomy 21 → Down syndrome.
**D. Edwards = trisomy 18.

Q160.

If a gamete contains 24 chromosomes instead of 23, and fertilizes a normal gamete, the result is:
A. Monosomy
B. Trisomy ✅
C. Polyploidy
D. Haploidy

Explanation:

**A. Monosomy = missing one.
**B. Correct — extra chromosome → trisomy.
**C. Polyploidy = whole set.
**D. Haploid = n = 23.

Q161.

Which event ensures that meiosis, but not mitosis, generates genetic variation?
A. DNA replication
B. Random alignment of homologues in metaphase I ✅
C. Cytokinesis
D. Nuclear envelope breakdown

Explanation:

**A. Replication occurs in both.
**B. Correct — independent assortment = unique gametes.
C/D. Common to both.

Q162.

If meiosis II fails but meiosis I occurs normally, gametes will be:
A. Haploid
B. Diploid with duplicated chromatids ✅
C. Aneuploid
D. Normal gametes

Explanation:

**A. Haploid requires meiosis II.
**B. Correct — gametes remain diploid with chromatids unseparated.
**C. Aneuploid = irregular counts, not uniform diploidy.
**D. Not normal.

Q163.

In cancer, uncontrolled division may result from:
A. Overactive proto-oncogenes ✅
B. Loss of telomerase
C. Perfect checkpoint control
D. Reduced cyclin levels

Explanation:

**A. Correct — mutations turn proto-oncogenes → oncogenes (e.g., Ras).
**B. Loss of telomerase causes senescence.
**C. Perfect control prevents cancer.
**D. Reduced cyclins would stop division.

Q164.

Why are cancer cells often immortal in culture?
A. Apoptosis activation
B. High telomerase activity ✅
C. Lack of DNA replication
D. Spindle checkpoint failure

Explanation:

**A. Apoptosis reduces survival.
**B. Correct — telomerase maintains telomere length.
**C. Replication active, not absent.
**D. Checkpoint failure causes aneuploidy, not immortality.

Q165.

A defect in spindle fibers during meiosis II will lead to:
A. Polyploidy
B. Aneuploid gametes ✅
C. Normal gametes
D. Reduced crossing over

Explanation:

**A. Polyploidy requires cytokinesis failure.
**B. Correct — mis-segregation → aneuploid gametes.
**C. Not normal.
**D. Crossing over occurs in meiosis I.

Q166.

A karyotype of 45, XO indicates:
A. Down syndrome
B. Turner’s syndrome ✅
C. Edwards syndrome
D. Klinefelter’s syndrome

Explanation:

**A. Down = 47, +21.
**B. Correct — monosomy X = Turner’s.
**C. Edwards = trisomy 18.
**D. Klinefelter = XXY.

Q167.

A cancer therapy targets CDK inhibitors (CKIs). What will be the outcome?
A. Cell cycle arrest
B. Increased CDK activity ✅
C. Apoptosis directly
D. Reduced cyclin levels

Explanation:

**A. CKIs block CDKs, so inhibiting CKIs increases activity.
**B. Correct — more CDK activity → faster cycle.
**C. Not direct apoptosis.
**D. Cyclins unaffected.

Q168.

Which situation explains mosaicism in humans?
A. Mutation after fertilization ✅
B. Non-disjunction in meiosis I
C. Non-disjunction in meiosis II
D. Crossing over

Explanation:

**A. Correct — post-zygotic mutations → mosaicism.
B/C. Meiotic errors affect all cells.
**D. Crossing over = variation, not mosaicism.

Q169.

A spermatocyte undergoes nondisjunction of chromosome 21 in meiosis I. How many abnormal gametes will form?
A. 1
B. 2 ✅
C. 3
D. 4

Explanation:

**A. 1 abnormal = meiosis II nondisjunction.
**B. Correct — two gametes n+1, two gametes n–1.
C/D. Not correct counts.

Q170.

The G2/M checkpoint is bypassed. Which risk increases?
A. Unrepaired DNA entering mitosis ✅
B. Chromosomes not attaching to spindle
C. Crossing over failure
D. Cytokinesis not occurring

Explanation:

**A. Correct — damaged/unfinished DNA → mitosis.
**B. Spindle = metaphase checkpoint.
**C. Crossing over in meiosis.
**D. Cytokinesis is after mitosis.

Q171.

Why is meiosis essential for sexual reproduction?
A. Maintains diploid chromosome number across generations ✅
B. Produces identical gametes
C. Repairs damaged DNA
D. Ensures mitosis in zygote

Explanation:

**A. Correct — meiosis halves chromosome number, fertilization restores diploidy.
**B. Gametes differ.
**C. Recombination repairs some DNA, but not main role.
**D. Zygote undergoes mitosis, but meiosis makes gametes.

Q172.

If APC/C is inhibited, what happens?
A. Chromatids fail to separate ✅
B. Telomeres shorten
C. Cyclin D accumulates
D. DNA replication halts

Explanation:

**A. Correct — securin not degraded → no chromatid separation.
**B. Telomere length independent.
**C. Cyclin D not APC/C target.
**D. Replication unaffected.

Q173.

A 2n = 14 plant undergoes meiosis. After fertilization of two gametes, the zygote chromosome number will be:
A. 7
B. 14
C. 21
D. 14 (restored diploid) ✅

Explanation:

**A. Gametes only.
**B. Diploid = zygote.
**C. 21 = triploid.
**D. Correct — haploid (7) + haploid (7) → diploid (14).

Q174.

A patient with chronic myeloid leukemia has a Philadelphia chromosome due to:
A. BCR-ABL fusion ✅
B. Rb mutation
C. BRCA1 loss
D. Trisomy 21

Explanation:

**A. Correct — reciprocal translocation t(9;22).
**B. Rb mutation = retinoblastoma.
**C. BRCA1 = breast/ovarian cancer.
**D. Trisomy 21 = Down.

Q175.

Which event differentiates meiosis II from mitosis?
A. Ploidy of starting cell ✅
B. Chromatid separation
C. Cytokinesis
D. Spindle formation

Explanation:

**A. Correct — meiosis II starts with haploid cells, mitosis with diploid.
B–D. Common to both.

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Part 7 — Critical & Complex NEET-style Integrative Questions (Q151–175)