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GATE 2019 BT Q.6: solution and answer

GATE 2019 · BTQ.6 · 1 mark · MCQ

DNA synthesis in eukaryotes occurs during which phase of the mitotic cell cycle?

  1. (A)

    M

  2. (B)

    G_1

  3. (C)

    S

  4. (D)

    G_0

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Answer and solution

ANSWER

DNA synthesis in eukaryotes occurs during S phase (option C).

Official answer: CSource: Official answer key, GATE 2019 BT (IIT Madras)

  1. The eukaryotic mitotic cell cycle consists of Interphase (G1, S, G2) followed by M phase (mitosis).

  2. G1 (Gap 1) phase: the cell grows and synthesizes the enzymes and proteins required for replication, but DNA itself is not yet replicated.

  3. S (Synthesis) phase: this is, by definition, the phase in which the entire genome is replicated, so DNA content rises from 2C to 4C — this is where DNA synthesis occurs.

  4. G2 (Gap 2) phase: the cell grows further and checks for replication errors before mitosis; DNA synthesis is already complete by this stage.

  5. M (Mitotic) phase: replicated chromosomes condense and segregate into two daughter cells; no new DNA synthesis occurs here.

  6. G0 is a quiescent/resting state outside the active division cycle, not a phase of DNA replication.

  7. Hence DNA synthesis occurs specifically in S phase — option (C).

KEY IDEA

The eukaryotic mitotic cell cycle is Interphase (G1 → S → G2) followed by M phase; DNA replication (synthesis of new DNA, doubling content from 2C to 4C) is confined to S phase, giving it its name ("Synthesis").

WATCH OUT FOR

Confusing G1 and G2 (both non-synthetic growth phases) with S phase, or assuming G0 (quiescence) is simply "before M phase" and therefore synthetic.

From the GATE 2019 BT paper. Every question of this paper · Download the full paper · All 15 Cell biology questions, with notes

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