Q.17

Which of the following represents the CORRECT order of events in the eukaryotic cell

cycle?

(A) G1  S  G2  M

(B) S  G1  M  G2

(C) G1  S  M  G2

(D) S  G1  G2  M

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Understanding the Eukaryotic Cell Cycle Order: G1, S, G2, and M Phases

The eukaryotic cell cycle drives cell growth, DNA replication, and division in organisms from yeast to humans. Mastering the correct order of events in the eukaryotic cell cycle—G1 → S → G2 → M—is essential for molecular biology, genetics, and biochemistry research. This sequence ensures precise control over genome duplication and segregation, preventing errors like aneuploidy.

In multiple-choice questions (MCQs), options often test this knowledge. Let’s break down Q.17: Which of the following represents the CORRECT order of events in the eukaryotic cell cycle? with options (A) G1 → S → G2 → M, (B) S → G1 → M → G2, (C) G1 → S → M → G2, and (D) S → G1 → G2 → M.

The Correct Answer: Option (A) G1 → S → G2 → M

Option (A) is correct. This reflects the standard eukaryotic cell cycle order, divided into interphase (G1, S, G2) and mitosis (M).

• G1 phase (Gap 1): Cells grow, synthesize proteins, and organelles. Checkpoints assess nutrient availability and DNA integrity. Duration varies (hours to days).

• S phase (Synthesis): DNA replicates semiconservatively, doubling from 2C to 4C content. Histones and centromeres duplicate too.

• G2 phase (Gap 2): Cells prepare for division, repairing replication errors and growing further. Key checkpoint verifies DNA completeness.

• M phase (Mitosis): Chromosome segregation via prophase, metaphase, anaphase, telophase, followed by cytokinesis. Ends with two daughter cells entering G1 or G0 (quiescent).

Cyclins and CDKs regulate transitions, ensuring unidirectional progression. This order minimizes mutation risks, critical in cancer research.

Why Other Options Are Incorrect: Detailed Breakdown

Common distractors shuffle phases to test recall. Here’s why they fail:

Option (B) S → G1 → M → G2: Starts with DNA Synthesis—Wrong Timing

DNA replication can’t precede growth. G1 must build cellular machinery first; starting with S skips preparation, leading to incomplete replication forks.

Option (C) G1 → S → M → G2: Mitosis Before Final Checks—Dangerous Skip

Post-S, G2 verifies replication fidelity. Jumping to M risks dividing damaged DNA, causing genomic instability—think checkpoint defects in tumors.

Option (D) S → G1 → G2 → M: Reversed Early Phases—Illogical Flow

S before G1 defies logic: cells need G1 growth for replication enzymes. This backward order ignores the cycle’s preparatory logic.

These errors highlight why G1 → S → G2 → M is the correct order of events in the eukaryotic cell cycle.

Key Checkpoints and Regulation in the Cell Cycle Sequence

Visualizing this table reinforces the eukaryotic cell cycle phases order for exams or lab notes.

Applications in Research and Exams

In enzymology and genetics, disrupting this order models diseases: cyclin overexpression accelerates S entry, mimicking oncogenes. For NEET, CSIR NET, or grad school MCQs, always pick (A).