Part 2 — Mitosis in Detail, Cyclins/CDKs, Checkpoints, Clinical/Experimental Aspects (Q26–50)

Q26.

Which is the longest stage of mitosis?
A. Prophase ✅
B. Metaphase
C. Anaphase
D. Telophase

Explanation:

A. Prophase (Correct): Chromosomes condense, spindle fibers form; it takes longest time.
**B. Metaphase: chromosomes align, shorter than prophase.
**C. Anaphase: shortest stage, rapid separation.
**D. Telophase: nuclear reformation, not the longest.

Q27.

During prometaphase, the following occurs:
A. DNA replication
B. Nuclear envelope breakdown ✅
C. Chromosome condensation begins
D. Chromatids separate

Explanation:

**A. DNA replication: happens in S phase.
**B. Correct — nuclear envelope fragments, kinetochores attach to microtubules.
**C. Chromosome condensation begins in prophase.
**D. Separation occurs in anaphase.

Q28.

At metaphase, chromosomes are aligned:
A. At spindle poles
B. At the metaphase plate ✅
C. Randomly within cytoplasm
D. At nuclear envelope

Explanation:

**A. Poles: anaphase destination.
**B. Correct — chromosomes align midway, spindle tension ensures proper attachment.
C/D. Incorrect positions.

Q29.

The shortest phase of mitosis is:
A. Prophase
B. Metaphase
C. Anaphase ✅
D. Telophase

Explanation:

**A. Prophase: longest.
**B. Metaphase: longer than anaphase.
C. Anaphase (Correct): Cohesin cleaved, chromatids pulled apart rapidly.
**D. Telophase: reformation, not shortest.

Q30.

Cytokinesis in animal cells occurs through:
A. Cell plate formation
B. Cleavage furrow ✅
C. Vesicle fusion
D. Spindle remnant

Explanation:

**A. Cell plate: in plant cells.
**B. Correct — actin-myosin contractile ring forms cleavage furrow in animal cells.
**C. Vesicle fusion forms plant cell plate.
**D. Spindle remnant does not directly divide cytoplasm.

Q31.

Cyclin levels fluctuate during cell cycle because:
A. Cyclins are permanently bound to DNA
B. Cyclins are synthesized and degraded in each cycle ✅
C. Cyclins are enzymes themselves
D. Cyclins are stored in lysosomes

Explanation:

**A. No DNA binding.
**B. Correct — periodic synthesis/degradation ensures stage-specific CDK activation.
**C. Cyclins are regulatory proteins, not enzymes.
**D. Not degraded in lysosomes but by proteasome (ubiquitination).

Q32.

Which checkpoint prevents cells with damaged DNA from entering S phase?
A. G1/S checkpoint ✅
B. G2/M checkpoint
C. Metaphase checkpoint
D. Cytokinesis checkpoint

Explanation:

**A. Correct — G1/S checks for DNA damage, cell size, growth signals before DNA replication.
**B. G2/M: checks replication completion & DNA damage.
**C. Metaphase: ensures spindle attachment.
**D. Cytokinesis checkpoint: not standard.

Q33.

p53 tumor suppressor protein primarily functions at:
A. G1/S checkpoint ✅
B. G2/M checkpoint
C. Metaphase checkpoint
D. Cytokinesis

Explanation:

**A. Correct — p53 halts cycle at G1/S upon DNA damage, induces repair or apoptosis.
**B. G2/M: regulated by other kinases.
C/D. Not p53 specific.

Q34.

Maturation-promoting factor (MPF) is composed of:
A. Cyclin D + CDK4
B. Cyclin B + CDK1 ✅
C. Cyclin A + CDK2
D. Cyclin E + CDK2

Explanation:

**A. Cyclin D/CDK4: G1 progression.
**B. Correct — Cyclin B/CDK1 = MPF, triggers mitotic entry.
**C. Cyclin A/CDK2: S-phase progression.
**D. Cyclin E/CDK2: G1/S transition.

Q35.

The spindle assembly checkpoint ensures:
A. DNA replication completed
B. Chromosomes aligned correctly before anaphase ✅
C. Cell has adequate nutrients
D. Cell grows in size

Explanation:

**A. Replication checked earlier.
**B. Correct — ensures kinetochores attached to spindle; prevents premature anaphase.
C/D. G1 checkpoint issues.

Q36.

The drug colchicine disrupts mitosis by:
A. Blocking DNA replication
B. Preventing spindle microtubule polymerization ✅
C. Breaking centromeres
D. Destroying kinetochores

Explanation:

**A. Colchicine doesn’t inhibit replication.
**B. Correct — binds tubulin, prevents spindle formation → metaphase arrest.
C/D. Not directly affected.

Q37.

Failure of spindle checkpoint may result in:
A. Aneuploidy ✅
B. Crossing over
C. Genetic recombination
D. DNA repair

Explanation:

**A. Correct — missegregation leads to abnormal chromosome number (aneuploidy).
B/C. Crossing over/recombination: meiosis.
**D. DNA repair: different pathways.

Q38.

Which experiment first proved that cell cycle progression depends on cytoplasmic factors (cyclins/CDKs)?
A. Hershey-Chase experiment
B. Cell fusion experiments ✅
C. Meselson-Stahl experiment
D. Griffith’s experiment

Explanation:

**A. Hershey-Chase: DNA as genetic material.
**B. Correct — fused cells in different phases → cytoplasmic regulators drive cycle transitions.
**C. Meselson-Stahl: semiconservative replication.
**D. Griffith: bacterial transformation.

Q39.

Inhibition of p53 leads to:
A. Cell cycle arrest
B. Increased apoptosis
C. Uncontrolled cell division (cancer risk) ✅
D. DNA repair activation

Explanation:

**A. p53 normally halts cycle, inhibition removes control.
**B. Apoptosis is reduced when p53 is inhibited.
**C. Correct — loss of p53 → unchecked cell division, cancer predisposition.
**D. p53 normally promotes repair, not its inhibition.

Q40.

During cytokinesis in plants, the new cell wall is formed from:
A. Contractile actin ring
B. Phragmoplast & vesicles from Golgi ✅
C. Lysosomal vesicles
D. Centrosomes

Explanation:

**A. Contractile ring: animal cells.
**B. Correct — vesicles coalesce at equator forming cell plate via phragmoplast.
C/D. Incorrect organelles.

Q41.

Which mitotic phase shows the maximum condensation of chromosomes for karyotyping?
A. Prophase
B. Metaphase ✅
C. Anaphase
D. Telophase

Explanation:

**A. Prophase: condensation begins but not maximum.
B. Metaphase (Correct): Fully condensed, aligned chromosomes best for karyotyping.
C/D. Already separated or decondensed.

Q42.

Which cyclin/CDK complex is critical for G1 to S transition?
A. Cyclin D/CDK4
B. Cyclin E/CDK2 ✅
C. Cyclin A/CDK1
D. Cyclin B/CDK1

Explanation:

**A. Cyclin D/CDK4: progression in G1.
**B. Correct — Cyclin E/CDK2 ensures transition into S phase.
**C. Cyclin A/CDK1: not S entry.
**D. Cyclin B/CDK1: mitotic entry.

Q43.

The protein retinoblastoma (Rb) regulates cell cycle by:
A. Promoting DNA replication
B. Binding and inhibiting E2F transcription factor ✅
C. Activating cyclins directly
D. Repairing DNA

Explanation:

**A. Rb suppresses, doesn’t promote replication directly.
**B. Correct — Rb prevents transcription of S-phase genes; phosphorylation releases E2F → cell cycle proceeds.
**C. Not direct activator.
**D. DNA repair role belongs to p53, not Rb.

Q44.

Overexpression of cyclin D can lead to:
A. Tumorigenesis ✅
B. Apoptosis
C. Cell cycle arrest
D. DNA repair

Explanation:

**A. Correct — excess cyclin D/CDK4 hyper-phosphorylates Rb → uncontrolled S-phase entry → cancer.
**B. Apoptosis is reduced.
**C. Arrest happens with underexpression, not over.
**D. Repair is not cyclin D’s function.

Q45.

The spindle fibers attach to which structure on the chromosome?
A. Telomere
B. Kinetochore ✅
C. Centromere DNA only
D. Histones

Explanation:

**A. Telomere: protective ends.
B. Kinetochore (Correct): Protein complex on centromere that attaches to spindle.
**C. DNA itself doesn’t bind spindles; kinetochore does.
**D. Histones: chromatin packaging, not spindle binding.

Q46.

Which clinical drug inhibits DNA synthesis during S phase, often used in chemotherapy?
A. Taxol
B. 5-Fluorouracil ✅
C. Colchicine
D. Vincristine

Explanation:

**A. Taxol: stabilizes microtubules.
**B. Correct — 5-FU blocks thymidylate synthase → inhibits DNA synthesis.
C/D. Microtubule inhibitors, not DNA synthesis.

Q47.

Which checkpoint ensures DNA replication is complete before entering mitosis?
A. G1/S
B. G2/M ✅
C. Spindle checkpoint
D. Cytokinesis

Explanation:

**A. G1/S: readiness for replication.
B. G2/M (Correct): Ensures replication finished, DNA intact before mitosis.
**C. Spindle: checks chromosome attachment.
**D. Cytokinesis: not a defined checkpoint.

Q48.

During which mitotic stage do spindle fibers shorten, pulling chromatids apart?
A. Metaphase
B. Anaphase ✅
C. Prophase
D. Telophase

Explanation:

**A. Metaphase: chromosomes align, no pulling.
B. Anaphase (Correct): Spindle microtubules shorten, chromatids move to poles.
C/D. Not separation stages.

Q49.

Which enzyme removes cohesin from sister chromatids at anaphase?
A. Ligase
B. Separase ✅
C. Helicase
D. Topoisomerase

Explanation:

**A. Ligase: seals DNA nicks.
B. Separase (Correct): Cleaves cohesin, triggering chromatid separation.
**C. Helicase: unwinds DNA.
**D. Topoisomerase: relieves supercoiling.

Q50.

Failure of cytokinesis after mitosis results in:
A. Apoptosis
B. Polyploidy ✅
C. Aneuploidy
D. Senescence

Explanation:

**A. Apoptosis: programmed cell death.
**B. Correct — cell contains multiple sets of chromosomes → polyploid.
**C. Aneuploidy: wrong chromosome number due to segregation errors, not cytokinesis failure.
**D. Senescence: growth arrest, not polyploidy.

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Part 2 — Mitosis in Detail, Cyclins/CDKs, Checkpoints, Clinical/Experimental Aspects (Q26–50)