Part 1 — Cell Cycle Basics, Interphase & Overview of Cell Division (Q1–25)

Q1.

Which of the following phases of the cell cycle is primarily dedicated to DNA replication?
A. G1 phase
B. S phase ✅
C. G2 phase
D. M phase

Explanation:

A. G1 phase: Gap 1 is a period of growth, metabolic activity and preparation for DNA synthesis; DNA replication does not occur here.
B. S phase (Correct): S (synthesis) phase is when the entire genome is duplicated — DNA replication occurs here.
C. G2 phase: Gap 2 is a period of preparation for mitosis after DNA synthesis is complete.
D. M phase: M phase is mitosis/cell division, not DNA replication.

Q2.

Cells that permanently exit the cell cycle and do not divide further enter which stage?
A. G0 phase ✅
B. S phase
C. G2 phase
D. Metaphase

Explanation:

A. G0 phase (Correct): Quiescent state where cells (e.g., neurons, some muscle cells) exit the cycle and typically do not re-enter division.
B. S phase: Phase where DNA replication occurs; not an exit state.
C. G2 phase: Preparation for mitosis.
D. Metaphase: A stage of mitosis, not an exit.

Q3.

Which checkpoint monitors DNA damage and completeness of DNA replication before mitosis begins?
A. G1/S checkpoint
B. G2/M checkpoint ✅
C. Spindle (metaphase) checkpoint
D. Cytokinesis checkpoint

Explanation:

A. G1/S checkpoint: Assesses whether the cell is ready to enter S phase (nutrients, growth signals, DNA integrity).
B. G2/M checkpoint (Correct): Verifies that DNA replication is complete and checks for DNA damage before entry into mitosis.
C. Spindle checkpoint: Ensures chromosomes are correctly attached to the spindle before anaphase.
D. Cytokinesis checkpoint: There is no widely recognized dedicated checkpoint named this; cytokinesis is a process after mitosis.

Q4.

Which proteins form the core of cell-cycle regulation by phosphorylating target proteins?
A. Cyclins only
B. Cyclin-dependent kinases (CDKs) ✅
C. DNA polymerases
D. Histones

Explanation:

A. Cyclins only: Cyclins regulate CDKs but do not phosphorylate directly; they bind and activate CDKs.
B. CDKs (Correct): CDKs are serine/threonine kinases that phosphorylate substrates when bound to appropriate cyclins, driving cell cycle transitions.
C. DNA polymerases: Involved in DNA replication, not general cell-cycle phosphorylation.
D. Histones: DNA-packaging proteins; not kinases.

Q5.

During which stage of interphase does the cell grow and synthesize proteins and organelles in preparation for DNA replication?
A. G1 phase ✅
B. S phase
C. G2 phase
D. M phase

Explanation:

A. G1 phase (Correct): Cell increases in size, produces RNA and proteins necessary for DNA replication and cell function.
B. S phase: DNA replication occurs.
C. G2 phase: Further growth and preparation for mitosis after DNA replication.
D. M phase: Division stage.

Q6.

Which of the following statements about the mitotic spindle is TRUE?
A. It is composed of actin microfilaments
B. It is assembled from microtubules and associated proteins ✅
C. It is formed from intermediate filaments
D. It is absent in animal cells

Explanation:

A. Actin filaments: Actin is involved in cytokinesis (contractile ring), not spindle formation.
B. Microtubules (Correct): Spindle fibres are microtubule-based structures organized by MTOCs (centrosomes) in animal cells.
C. Intermediate filaments: Provide structural support, not spindle function.
D. Absent in animals: False — animal cells use centrosome-based spindles.

Q7.

The point where sister chromatids are held together is called the:
A. Telomere
B. Centromere ✅
C. Origin of replication
D. Kinetochore

Explanation:

A. Telomere: Chromosome end sequences; protect chromosome tips.
B. Centromere (Correct): Constricted chromosome region where sister chromatids remain attached until anaphase.
C. Origin of replication: DNA replication initiation sites.
D. Kinetochore: Protein complex assembled on the centromere that attaches to spindle microtubules — distinct from the centromere region itself.

Q8.

Which enzyme complex targets securin for degradation, thereby allowing sister chromatid separation?
A. Cyclin-CDK complex
B. Anaphase-promoting complex/cyclosome (APC/C) ✅
C. DNA helicase
D. Cohesin

Explanation:

A. Cyclin-CDK: Regulates cell cycle progression, but APC/C performs ubiquitination in anaphase entry.
B. APC/C (Correct): Ubiquitin ligase that marks securin for degradation, freeing separase to cleave cohesins and permit anaphase.
C. DNA helicase: Unwinds DNA during replication.
D. Cohesin: Protein complex that holds sister chromatids together; not responsible for targeting securin.

Q9.

During which mitotic phase do sister chromatids separate and move to opposite poles?
A. Prophase
B. Metaphase
C. Anaphase ✅
D. Telophase

Explanation:

A. Prophase: Chromosomes condense and spindle forms.
B. Metaphase: Chromosomes align at the metaphase plate.
C. Anaphase (Correct): Cohesin is cleaved; sister chromatids (now daughter chromosomes) are pulled apart toward poles.
D. Telophase: Chromosomes de-condense and nuclear membranes re-form.

Q10.

Which of the following is a correct functional significance of mitosis?
A. Production of genetically variable gametes
B. Growth, tissue repair and asexual reproduction ✅
C. Reduction of chromosome number by half
D. Crossing over of homologous chromosomes

Explanation:

**A. Genetically variable gametes are produced by meiosis, not mitosis.
**B. Correct — mitosis produces genetically identical daughter cells important in growth, repair, and some forms of asexual reproduction.
**C. Reduction division is a feature of meiosis I.
**D. Crossing over occurs during meiosis prophase I, not mitosis.

Q11.

Which mitotic stage is characterized by reformation of nuclei and decondensation of chromosomes?
A. Metaphase
B. Anaphase
C. Telophase ✅
D. Prophase

Explanation:

A. Metaphase: Alignment of chromosomes.
B. Anaphase: Chromatid separation.
C. Telophase (Correct): Nuclear envelopes re-form around separated chromosome sets; chromosomes de-condense.
D. Prophase: Condensation and spindle assembly.

Q12.

Centrosomes play a pivotal role in mitosis because they:
A. Contain DNA that directs mitosis
B. Serve as microtubule-organizing centers (MTOCs) to nucleate spindle microtubules ✅
C. Break down the nuclear envelope
D. Hold sister chromatids together

Explanation:

**A. Centrosomes do not contain DNA.
**B. Correct — centrosomes organize spindle microtubules and help form bipolar spindle poles in animal cells.
**C. Nuclear envelope breakdown involves phosphorylation of nuclear lamins, not centrosome action directly.
**D. Cohesin holds chromatids together, not centrosomes.

Q13.

Which protein complex holds sister chromatids together until anaphase?
A. Cohesin ✅
B. Condensin
C. Separase
D. Topoisomerase

Explanation:

A. Cohesin (Correct): Ring-shaped complex that encircles sister chromatids and maintains cohesion until separase cleaves cohesin.
B. Condensin: Helps condense chromosomes during mitosis.
C. Separase: Protease that cleaves cohesin at anaphase onset.
D. Topoisomerase: Relieves DNA supercoiling; not the cohesion protein.

Q14.

Which phase of mitosis immediately follows prometaphase?
A. Anaphase
B. Metaphase ✅
C. Telophase
D. Interphase

Explanation:

**A. Anaphase follows metaphase, not prometaphase.
B. Metaphase (Correct): After prometaphase (nuclear envelope gone, kinetochores attached), chromosomes align at the metaphase plate in metaphase.
**C. Telophase occurs after anaphase.
**D. Interphase is the non-mitotic phase.

Q15.

Which structure on the chromosome attaches to spindle microtubules?
A. Telomere
B. Centromere
C. Kinetochore ✅
D. Origin of replication

Explanation:

A. Telomere: Protects ends; not for spindle attachment.
B. Centromere: DNA region; kinetochore builds on centromere.
C. Kinetochore (Correct): Multi-protein complex assembled at centromere where spindle microtubules bind.
D. Origin of replication: For DNA replication initiation, not spindle attachment.

Q16.

During which substage of prophase I does homologous pairing (synapsis) begin?
A. Leptotene
B. Zygotene ✅
C. Pachytene
D. Diplotene

Explanation:

A. Leptotene: Chromosomes begin to condense but pairing is not yet established.
B. Zygotene (Correct): Synapsis begins; homologous chromosomes start pairing through synaptonemal complex formation.
C. Pachytene: Synapsis complete and crossing over (recombination) occurs.
D. Diplotene: Synaptonemal complex disassembles; chiasmata become visible.

Q17.

Crossing over (genetic recombination) primarily occurs during which substage of prophase I?
A. Leptotene
B. Zygotene
C. Pachytene ✅
D. Diakinesis

Explanation:

A. Leptotene: Chromosome condensation starts.
B. Zygotene: Synapsis starts but recombination initiates later.
C. Pachytene (Correct): Homologous chromosomes are fully synapsed and exchange genetic material at chiasmata.
D. Diakinesis: Final condensation before metaphase I; crossing over already occurred.

Q18.

Which process during meiosis contributes to independent assortment of alleles?
A. DNA replication in S phase
B. Random orientation of bivalents at metaphase I ✅
C. Cleavage furrow formation
D. DNA repair

Explanation:

**A. DNA replication duplicates chromosomes but does not shuffle allele combinations.
**B. Correct — random orientation of homologous chromosome pairs on the metaphase I plate leads to independent segregation of maternal and paternal chromosomes.
**C. Cleavage furrow is part of cytokinesis.
**D. DNA repair is unrelated to independent assortment.

Q19.

Which of the following best describes meiosis I?
A. Equational division producing identical sister chromatids
B. Reductional division reducing chromosome number by half ✅
C. Mitosis in germ cells
D. Fusion of gametes

Explanation:

**A. Equational division refers to meiosis II/mitosis (sister chromatids separate).
B. Reductional division (Correct): Meiosis I separates homologous chromosomes, reducing ploidy from diploid to haploid.
**C. Mitosis is different; meiosis I is specialized reduction division.
**D. Fusion of gametes describes fertilization, not meiosis.

Q20.

At the end of meiosis I, what is the chromosome composition of daughter cells?
A. Diploid, chromatids joined
B. Haploid, each chromosome still consists of two sister chromatids ✅
C. Haploid, single-chromatid chromosomes
D. Diploid, single-chromatid chromosomes

Explanation:

A. Diploid: Incorrect — meiosis I reduces ploidy.
**B. Correct — after meiosis I, cells are haploid (n), but each chromosome is composed of two sister chromatids (duplicated).
**C. Single-chromatid chromosomes occur after meiosis II.
**D. Diploid and single-chromatid is not produced by meiosis I.

Q21.

Non-disjunction during meiosis can lead to:
A. Point mutations
B. Aneuploidy (e.g., trisomy 21) ✅
C. Silent mutations
D. Increased DNA replication fidelity

Explanation:

**A. Point mutations are nucleotide substitutions, not chromosomal segregation errors.
**B. Correct — nondisjunction (failure of homologs or sister chromatids to separate) results in gametes with abnormal chromosome numbers leading to aneuploid zygotes like trisomy 21 (Down syndrome).
**C. Silent mutations are base changes that do not alter amino acids.
**D. Nondisjunction decreases fidelity of segregation, not increases DNA replication fidelity.

Q22.

Which of the following is a difference between meiosis II and mitosis?
A. Meiosis II involves homologous chromosome pairing
B. Meiosis II occurs without prior DNA replication ✅
C. Mitosis reduces chromosome number
D. Meiosis II produces genetically identical daughter cells

Explanation:

**A. Homolog pairing occurs in meiosis I, not meiosis II.
**B. Correct — meiosis II follows meiosis I without an intervening S phase, so there is no additional DNA replication before meiosis II.
**C. Mitosis is an equational division; meiosis (I) reduces chromosome number.
**D. Meiosis II produces genetically different gametes (due to crossing over/independent assortment earlier), so daughter cells are not identical.

Q23.

Which event ensures genetic diversity in sexually reproducing organisms?
A. DNA replication accuracy
B. Crossing over and independent assortment ✅
C. Mitosis producing somatic cells
D. Exact chromosome segregation only

Explanation:

**A. Replication accuracy preserves genome, not generate diversity.
**B. Correct — recombination (crossing over) and independent assortment during meiosis produce novel allele combinations, increasing genetic variation.
**C. Mitosis creates identical somatic cells, not diversity.
**D. Exact segregation maintains genome integrity; diversity arises from reshuffling events.

Q24.

Which structure protects the ends of eukaryotic chromosomes and is shortened progressively with successive divisions in somatic cells?
A. Centromere
B. Telomere ✅
C. Kinetochore
D. Origin of replication

Explanation:

A. Centromere: Constriction for kinetochore assembly; not the chromosome end.
B. Telomere (Correct): Repetitive sequences at chromosome ends; shorten with each somatic cell division (replicative senescence) unless telomerase acts.
C. Kinetochore: Binds microtubules at centromere, not chromosome ends.
**D. Origins are internal replication start sites.

Q25.

Which enzyme can extend telomeres in germ cells and many cancer cells, allowing continued division?
A. DNA polymerase I
B. Telomerase ✅
C. Helicase
D. Ligase

Explanation:

**A. DNA pol I (in bacteria) participates in replication/repair but not telomere extension in eukaryotes.
B. Telomerase (Correct): A reverse transcriptase enzyme that adds telomeric repeats using an RNA template, maintaining telomere length in germline and many cancer cells.
**C. Helicase: unwinds DNA strands.
**D. Ligase: seals nicks; not responsible for telomere addition.

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Part 1 — Cell Cycle Basics, Interphase & Overview of Cell Division (Q1–25)