Cell cycle order: Which sequence correctly describes the progression of eukaryotic cell cycle phases?

Introduction / Context:
The eukaryotic cell cycle is a tightly regulated series of phases ensuring accurate DNA replication and segregation. Mastering the correct order is fundamental for understanding checkpoints, DNA damage responses, and proliferative control in development and disease.

Given Data / Assumptions:

• We consider a typical cycling somatic cell.
• Mitosis refers to nuclear division (prophase to telophase).
• Cytokinesis is the physical division of the cytoplasm.

Concept / Approach:
The canonical order is G1 (growth/prep) → S (DNA synthesis) → G2 (further growth/checks) → M phase (mitosis) → cytokinesis. Many cells then re-enter G1; some may exit to G0, a quiescent state, but G0 is not a mandatory phase of each cycle. Mitosis precedes cytokinesis; swapping them is incorrect.

Step-by-Step Solution:

Verification / Alternative check:
Fluorescence-based cell cycle assays (e.g., EdU for S phase, cyclin levels) align with this order; microscopy confirms mitosis before cleavage furrow ingression and final abscission in cytokinesis.

Why Other Options Are Wrong:

• Option b: starts mid-cycle; though cycles are continuous, the complete ordered sequence begins at G1.
• Option c: places cytokinesis before mitosis.
• Option d: flips cytokinesis and mitosis, which is biologically reversed.
• Option e: G0 is optional and not part of every proliferative cycle.

Common Pitfalls:
Conflating mitosis with cytokinesis; assuming G0 is a mandatory step.

Final Answer:
→ G1 → S → G2 → mitosis → cytokinesis →