Q.16 DNA synthesis in eukaryotes occurs during which phase of the mitotic cell cycle?
(A) M (B) G1 (C) S (D) G0
DNA synthesis in eukaryotes occurs during the S phase of the mitotic cell cycle. The correct answer is (C) S. This phase is dedicated to replicating the cell’s DNA to ensure each daughter cell receives an identical copy during division.
Option Analysis
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(A) M: This is the mitotic phase where the nucleus divides through prophase, metaphase, anaphase, and telophase, followed by cytokinesis. No DNA synthesis happens here; replicated DNA from earlier phases is segregated.
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(B) G1: The first gap phase after mitosis focuses on cell growth, protein synthesis, and checking conditions for DNA replication. DNA remains unreplicated at 2N content.
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(C) S: The synthesis phase replicates DNA precisely, doubling the genome from 2N to 4N. Enzymes like DNA polymerase ensure semi-conservative replication occurs.
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(D) G0: This quiescent phase involves non-dividing cells exiting the cycle for differentiation or rest, such as in neurons. No DNA synthesis takes place.
DNA synthesis in eukaryotes mitotic cell cycle happens exclusively in the S phase, a critical step ensuring genetic continuity before cell division. This process, part of interphase, prepares cells for mitosis by duplicating chromosomal DNA semi-conservatively.
Cell Cycle Overview
The mitotic cell cycle in eukaryotes includes interphase (G1, S, G2) and M phase. Interphase dominates (~90% of cycle time), with growth and replication; M phase handles division.
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G1: Cell enlarges, synthesizes proteins, assesses DNA integrity at G1 checkpoint.
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S: DNA synthesis doubles genetic material; centrosomes replicate.
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G2: Growth continues, proteins for mitosis form; G2 checkpoint verifies replication.
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M: Chromosome segregation and cytokinesis produce identical daughters.
Why S Phase for DNA Synthesis?
S phase timing prevents replication errors; checkpoints (e.g., at G1/S transition) ensure favorable conditions like nucleotide availability. Cyclin-dependent kinases (CDKs) trigger replication origins firing across the genome. Eukaryotic DNA, organized in chromatin, unwinds via helicases for polymerase action, contrasting prokaryotic cycles.
Exam Relevance
For IIT JAM or NEET, memorize: DNA synthesis = S phase only. Common traps include confusing G1 (preparation) with S or M (division). Practice prior questions reinforces this for competitive biology sections.
