The cell cycle is the ordered sequence of events a cell undergoes from one division to the next. NEET repeatedly tests whether you can place the right event in the right phase — and the trap is mixing up what happens in G1 versus S versus G2.
The four phases, in order:
G1 (Gap 1) — The cell grows in size, synthesises proteins and organelles, and carries out its normal metabolic functions. This is the longest and most variable phase. The cell decides here whether to proceed to division or exit into G0 (quiescent phase). NCERT Class 11 Biology Chapter 10, page 121) notes that cells that do not divide further (e.g., neurons) exit from G1 into G0.
S phase (Synthesis) — DNA replication occurs. Each chromosome is duplicated to form two sister chromatids joined at the centromere. The DNA content doubles from 2C to 4C, but the chromosome number stays the same. Centriole duplication also begins in S phase in animal cells.
G2 (Gap 2) — The cell continues to grow, synthesises proteins needed for mitosis (like tubulin for spindle fibres), and checks for DNA replication errors. G2 is shorter than G1.
M phase (Mitosis) — The actual division. This is the shortest phase of the cell cycle. It includes nuclear division (karyokinesis) followed by cytoplasmic division (cytokinesis).
The high-frequency trap: confusing where DNA replication occurs. DNA replication is S phase only — not G1, not G2, not M. A second common confusion: treating G0 as a separate phase of the cell cycle rather than an exit from G1.
Interphase = G1 + S + G2. Interphase is NOT a resting phase — it accounts for roughly 95% of the cell cycle duration, and the cell is metabolically active throughout.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
During which phase of the cell cycle does DNA replication occur?
Answer: D. DNA replication occurs exclusively during the S (Synthesis) phase of interphase. NCERT Class 11 Biology Chapter 10 confirms this as the defining event of S phase.
Why A is wrong: A is wrong because G1 is a growth phase where the cell synthesises RNA and proteins, but DNA replication has not yet begun.
Why B is wrong: B is wrong because M phase is when the already-replicated chromosomes are separated and distributed to daughter cells — no new DNA synthesis occurs here.
Why C is wrong: C is wrong because G2 occurs after DNA replication is complete; the cell is preparing for mitosis by synthesising tubulin and other mitotic proteins.
Which of the following correctly represents the sequence of phases in the cell cycle?
Answer: C. The cell cycle proceeds G1 → S → G2 → M, where G1, S, and G2 together constitute interphase. NCERT Class 11 Biology Chapter 10, pages 121–122.
Why A is wrong: A is wrong because it places G2 before S phase. DNA replication (S) must occur before the second gap phase (G2).
Why B is wrong: B is wrong because S phase does not initiate the cycle. After M phase, the daughter cell enters G1 first.
Why D is wrong: D is wrong because this sequence starts with M phase. A newly formed daughter cell enters G1 (or G0), not G2 directly.
A cell in G0 phase has exited the cell cycle from which phase?
Answer: B. Cells that no longer divide (e.g., neurons, mature RBCs in some contexts) exit the cell cycle from G1 into the quiescent G0 phase. NCERT Class 11 Biology Chapter 10, page 122.
Why A is wrong: A is wrong because S phase is committed to DNA replication. Cells do not exit into quiescence once they have entered S phase.
Why C is wrong: C is wrong because G2 follows completed DNA replication. The exit checkpoint for quiescence is in G1, not G2.
Why D is wrong: D is wrong because cells in M phase are already dividing. The decision to stop dividing is made at G1, before S phase commitment.
A cell has a DNA content of 2C at the beginning of interphase. What will its DNA content be at the end of S phase?
Answer: A. During S phase, each DNA molecule is replicated, so the DNA content doubles from 2C to 4C. The chromosome number remains unchanged — the replicated chromosomes exist as sister chromatids joined at the centromere.
Why B is wrong: B is wrong because 2C is the pre-replication content. DNA replication in S phase doubles the DNA content.
Why C is wrong: C is wrong because 1C would represent a reduction, which occurs after meiosis I, not after DNA replication in S phase.
Why D is wrong: D is wrong because 8C would require two rounds of replication. Only one round occurs in S phase per cell cycle.
Which of the following is synthesised during G2 phase in preparation for mitosis?
Answer: C. G2 phase involves synthesis of proteins required for mitosis, notably tubulin (the subunit of microtubules that form the mitotic spindle). NCERT Class 11 Biology Chapter 10.
Why A is wrong: A is wrong because DNA polymerase is needed during S phase for replication. By G2, DNA replication is already complete.
Why B is wrong: B is wrong because histone synthesis is associated with S phase, where newly replicated DNA needs to be packaged into chromatin.
Why D is wrong: D is wrong because general cellular growth and lipid synthesis are characteristic of G1 phase, not G2.
Interphase is sometimes incorrectly called the "resting phase." Which of the following facts directly contradicts this label?
Answer: B. B is correct. "Resting" is a claim about ACTIVITY, so only a fact about activity can contradict it. During interphase the cell doubles its DNA in S phase and carries out intense protein and organelle synthesis through G₁ and G₂ — it is the most metabolically demanding stretch of the whole cycle. Note that its long duration, roughly 95% of the cycle, is NOT the refutation: a phase can be long and still be quiescent, so length alone settles nothing. NCERT Class 11 Biology Chapter 10.
Why A is wrong: A is wrong because the absence of visible chromosome condensation is precisely what made early microscopists assume the cell was 'resting' — it supports the misconception, not contradicts it.
Why C is wrong: C is wrong because the absence of cytokinesis during interphase does not address whether the cell is metabolically active or resting.
Why D is wrong: D is wrong because an intact nuclear envelope is a structural observation that does not speak to the metabolic activity level of the cell.
A researcher observes that a particular cell type never enters S phase and remains metabolically active for the organism's lifetime. This cell is most likely in:
Answer: A. Cells that are metabolically active but permanently withdrawn from the cell cycle (never entering S phase) are in G0, the quiescent phase. Neurons are a classic example. G0 is an exit from G1, not a part of the active cycle. NCERT Class 11 Biology Chapter 10.
Why B is wrong: B is wrong because a cell in prolonged G1 is still within the active cell cycle and retains the potential to enter S phase. The question states the cell never enters S phase, indicating a permanent exit.
Why C is wrong: C is wrong because a cell in G2 has already completed S phase (DNA replication). The question states S phase is never entered.
Why D is wrong: D is wrong because prophase is part of M phase, which occurs after G2 and S phase. A cell that never enters S cannot be in M phase.
In a cell culture, the average cell cycle duration is 24 hours. If M phase takes 1 hour and S phase takes 8 hours, what is the combined duration of G1 and G2?
Answer: D. Total cell cycle = G1 + S + G2 + M. Therefore G1 + G2 = 24 − 8 − 1 = 15 hours. This is a two-step problem: identify the formula, then subtract the known phase durations.
Why A is wrong: A is wrong because 9 hours is simply M + S (1 + 8). The question asks for the remaining time, which is the total minus M and S.
Why B is wrong: B is wrong because 23 = 24 − 1, which subtracts only M phase and forgets to subtract S phase (8 hours).
Why C is wrong: C is wrong because 16 = 24 − 8, which subtracts only S phase and forgets to subtract M phase (1 hour).
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Given
A cell population has an average cell cycle of 20 hours. S phase = 10 hours, G2 = 4 hours, M phase = 0.5 hours.
Required
Find the duration of G1 phase.
Concept
The cell cycle consists of four sequential phases: G1, S, G2, and M. Their durations must sum to the total cell cycle time. Interphase = G1 + S + G2.
Formula
Total cell cycle = G1 + S + G2 + M
∴ G1 = Total − S − G2 − M
Substitution
G1 = 20 − 10 − 4 − 0.5
Calculation
G1 = 20 − 14.5 = 5.5 hours
Final answer
G1 = 5.5 hours. This confirms G1 is often the most variable phase — it can range from a few hours (rapidly dividing cells) to indefinite (cells entering G0).
Common trap
Forgetting that interphase includes three sub-phases (G1, S, G2). Some students subtract only S from the total and report the result as G1, ignoring G2. Others forget to subtract M phase entirely (treating total minus M as interphase, then failing to separate G1 from it correctly).
Similar NEET-style question
"In a cell with a 16-hour cell cycle, G1 = 5 hours, S = 7 hours, and G2 = 3 hours. What is the duration of M phase?" (Answer: 16 − 5 − 7 − 3 = 1 hour.)
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Interphase (~95%): G1 (cell growth), S (DNA replication, 2C → 4C), G2 (preparation for M). M phase: prophase, metaphase, anaphase, telophase, then cytokinesis. G0: quiescent / non-dividing.
-- NCERT Class 11 Biology, Ch. 10, p. 121More in Cell: Structure and Function: 14 exam traps and mistakes · 1 formula · 1 question pattern from its other lessons.
6 questions from NEET 2020, 2021, 2023, 2024, 2026. Answers verified against NTA official keys.
The correct sequence of adult cell cycle phases is ______.
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