Q.At what stage of cell cycle does DNA synthesis occur?
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Interphase Subphases
Think of a cell division like a relay race. The actual handoff — mitosis — is dramatic and fast. But the real work happens in the long stretch between handoffs, when the runner builds speed, checks their grip, and prepares for the next exchange. That long stretch is interphase.
Interphase is not a "resting" period in the sense of doing nothing. It is the most active phase of the cell cycle in terms of metabolism, growth, and molecular work. The cell spends roughly 90% of its time here, and it is divided into three distinct subphases: G1, S, and G2.
G1 Phase (First Gap)
The cell has just finished dividing. It is now smaller, with half the normal amount of cytoplasm and organelles. The first job is to grow back to full size. The cell synthesises new proteins, builds more mitochondria or chloroplasts (depending on the organism), and produces all the enzymes needed for DNA replication later.
This is also a decision point. If conditions are unfavourable — not enough nutrients, or the cell is too crowded — the cell may enter a quiescent state called G0 instead of continuing. Many mature nerve cells and muscle cells stay in G0 permanently.
G1 is the most variable in length. Some cells finish it in hours; others take days or years.
S Phase (Synthesis)
The name says it all: this is when DNA is synthesised. Each chromosome, which was a single chromatid at the end of G1, is duplicated to form two identical sister chromatids held together at the centromere.
The amount of DNA in the nucleus doubles. If a human cell starts G1 with 2C (two copies) of DNA, it ends S phase with 4C. But the chromosome number (n) does not change — you still have 46 chromosomes in a human cell, just each one is now a double structure.
Histone proteins are also synthesised here to package the new DNA. The cell is now committed to division; it will not turn back.
A common mistake: students think the chromosome number doubles in S phase. It does not. The DNA content doubles. The chromosome count stays the same until anaphase of mitosis.
G2 Phase (Second Gap)
The cell checks its work. DNA is inspected for damage or incomplete replication. Any errors are repaired before the cell dares to enter mitosis. The cell also continues to grow, synthesising tubulin for spindle fibres and other proteins needed for division. …
The cell cycle is divided into interphase (G1, S, G2) and the M phase; DNA replication occurs in one specific sub-phase of interphase. …
DNA is synthesised (replicated) during the S phase of interphase.
The cell cycle consists of two main phases - interphase and the M phase (mitosis). Interphase is further divided into three sub-phases: G1, S and G2.
- G1 phase - the cell grows and is metabolically active, but no DNA replication occurs.
- S (Synthesis) phase - DNA replication (synthesis) takes place here. The DNA content of the cell doubles (from 2C to 4C), but the number of chromosomes does not change because the two copies remain joined as sister chromatids. …
- CBSE 2026Set ANNUAL1 markMCQQ.In which stage of the cell cycle does DNA replication occur ?(a) G1 phase(b) G2 phase(c) M phase(d) S phase
›Reveal solutionSolution
DNA replication occurs in the S phase, so the answer is (D).
Interphase has three sub-phases:
- G1 phase — the cell grows and carries out its functions; DNA amount is unchanged.
- S (synthesis) phase — DNA replicates so that each chromosome now has two sister chromatids; the DNA content doubles. …
- CBSE 2025Set ANNUAL1 markMCQQ.In cell cycle, DNA replication takes place in :(a) G1-phase(b) G2-phase(c) S-phase(d) Anaphase
›Reveal solutionSolution
DNA replication happens during the S (Synthesis) phase of interphase, between G1 and G2.
The cell cycle is divided into Interphase (G1, S, G2) and M-phase (mitosis/meiosis). Interphase, often called the "resting phase," is actually the busiest metabolic phase:
- G1 phase: the cell grows and prepares metabolically for DNA synthesis; DNA is NOT yet replicated.
- S phase: DNA replication occurs — the amount of DNA per cell doubles (from 2C to 4C), though chromosome number stays the same, since each chromosome now consists of two sister chromatids. In animal cells, the centriole also duplicates in the S phase. …
- CBSE 2023Set ANNUAL1 markMCQQ.Which of the following is not a part of the interphase?(a) G2 Phase(b) G1 Phase(c) S Phase(d) Anaphase
›Reveal solutionSolution
Anaphase belongs to M phase, not interphase.
Interphase comprises G1 (first gap), S (DNA synthesis) and G2 (second gap). Anaphase is a phase of mitotic division (M phase) during which sister chromatids move to opposite poles, so it is NOT pa …
- CBSE 2022Set TERM11 markMCQQ.The number of chromosomes in G1 phase is 36; the number of chromosomes in S phase is(a) 18(b) 36(c) 22(d) 38
›Reveal solutionSolution
Chromosome number does not change during S phase -- only DNA content per chromosome doubles.
The cell cycle is divided into interphase (G1, S, G2) and M phase (mitosis). During G1, the cell has its normal diploid chromosome number with each chromosome consisting of a single DNA molecule (one chromatid). During S (synthesis) phase, DNA replication occurs: each chromosome's DNA is copied, so each chromosome now consists of two sister chromatids joined at the centromere. Crucially, this does NOT create new, separate chromosomes -- the chromosome NUMBER stays exactly the same (36 here); only the DNA content doubles (and the chrom …
- CBSE 2022Set TERM11 markMCQQ.DNA replication occurs in(a) S-phase(b) G1-phase(c) G2-phase(d) M-phase
›Reveal solutionSolution
DNA is replicated only during the S phase of the cell cycle.
Interphase, the 'resting' phase between two successive M phases, is itself divided into three phases: G1 (Gap 1, cell grows and prepares for DNA synthesis), S (Synthesis, DNA replication occurs -- each chromosome's DNA content doubles, though chromosome NUMBER stays the same, now as two sister chromatids per chromosome), and G2 (Gap 2, the cell continues to grow and prepares for mitosis, synthesising proteins needed for cell division). DNA replication is strictly conf …
- CBSE 2021Set ANN1 markQ.Fill in the blank. DNA synthesis take place in the _____ phase of cell cycle.
›Reveal solutionSolution
DNA replication occurs during the S (Synthesis) phase of interphase.
The cell cycle is divided into Interphase (G1, S, G2) and M phase (mitosis/meiosis). During the S phase, the amount of DNA per cell doubles as each chromosome replicates to form two sister chromatids, although the chromosome number itself does not change at this stage. T …
- CBSE 2021Set ANNUAL1 markQ.What does happen in Interphase?
›Reveal solutionSolution
Interphase (G1, S, G2) is when the cell grows, duplicates organelles, and precisely replicates its DNA in preparation for the next division.
Interphase is the interval between the end of one mitotic/meiotic division and the beginning of the next. Although it is sometimes loosely called the 'resting phase', the cell is in fact extremely metabolically active during this time. Interphase is divided into three sub-phases: G1 phase (Gap 1) — the cell grows in size and synthesizes RNA and proteins needed for DNA synthesis; S phase (Synthesis) — the amount of DNA per cell doubles as DNA replication takes place (the centriole also duplicates in animal cells); a …
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