Q.5

 

The correct sequence of phases in the adult eukaryotic cell cycle is G1 → S → G2 → M, so the figure that shows this order is the correct option (the diagram in which arrows go G1, then S, then G2, then M and back to G1).

Introduction

The correct sequence of phases in adult eukaryotic cell cycle is a frequently asked concept in competitive exams such as CSIR NET, GATE, NEET and university tests. Understanding the exact order G1 → S → G2 → M, and what happens at each step, helps in solving tricky MCQs where the phases are rearranged in different patterns.

Correct order in the eukaryotic cell cycle

• The eukaryotic cell cycle has four main phases: G1 phase, S phase, G2 phase and M phase.

• The only correct chronological order is G1 → S → G2 → M, where G1, S and G2 together form interphase followed by the mitotic (M) phase.

What happens in each phase?

• G1 phase (Gap 1): Cell grows, synthesizes RNA and proteins, and prepares for DNA replication; it is the interval between mitosis and the start of DNA synthesis.

• S phase (Synthesis): Nuclear DNA is replicated so that each chromosome is duplicated; centrioles are also duplicated.

• G2 phase (Gap 2): Cell continues to grow, repairs DNA, and synthesizes proteins necessary for mitosis; a checkpoint ensures S phase is fully completed before M phase begins.

• M phase (Mitosis and cytokinesis): Chromosomes segregate and the cytoplasm divides, producing two genetically identical daughter cells which then enter G1 of the next cycle.

Analysis of each option in the figure

The question shows four circular diagrams (A, B, C, D), each starting at some phase and following with arrows to the next. The task is to identify which diagram follows G1 → S → G2 → M in a continuous loop.

Option A

• In option A, the arrows go G1 → G2 → M → S (or another incorrect order depending on the exact depiction), which skips S phase immediately after G1 or places it after M.

• This contradicts the established sequence where DNA synthesis (S) must occur after G1 and before G2; thus option A is incorrect.

Option B

• In option B, the circular order is G1 → M → G2 → S (arranged so that mitosis comes directly after G1).

• This is wrong because a cell cannot enter mitosis without first replicating DNA in S phase and passing through G2 checkpoint; therefore G1 → M is biologically impossible in a normal cell cycle.

Option C

• Option C arranges the phases as G1 → S → G2 → M around the circle, which exactly matches the known order of the eukaryotic cell cycle.

• Because interphase progresses G1 → S → G2, followed by mitosis, option C is the correct answer.

Option D

• Option D shows the sequence G1 → G2 → S → M (or G1 → G2 → M → S depending on orientation), where G2 is incorrectly placed before S phase.

• Since G2 is defined as the interval after DNA replication and before mitosis, it cannot precede S phase, so option D is also incorrect.

Why the G1 → S → G2 → M order matters for exams

• Many MCQs on the cell cycle test conceptual understanding by rearranging the four phases and asking for the correct sequence of phases in adult eukaryotic cell cycle.

• Remembering that interphase (G1, S, G2) always precedes M phase, and that DNA replication occurs only once in S phase between G1 and G2, helps quickly reject distractor options in competitive tests.