Cell biology — mitosis and cell cycle overview During eukaryotic cell division, the mitotic spindle (composed of microtubules and associated proteins that segregate chromosomes) first forms in which phase of the cell cycle?

Introduction / Context:
The mitotic spindle is a dynamic microtubule-based apparatus that separates duplicated chromosomes into two daughter nuclei. Knowing precisely when this structure assembles is essential to understanding how cells maintain genetic stability during division.

Given Data / Assumptions:

• Eukaryotic cell cycle phases: G1, S, G2 (collectively interphase) and M phase (mitosis + cytokinesis).
• Duplicated chromosomes must be captured and aligned before separation.
• Spindle assembly depends on centrosomes/spindle pole bodies and microtubules.

Concept / Approach:

The spindle apparatus assembles at the entry into M phase, specifically as cells progress through prophase and prometaphase. During these stages, centrosomes move apart, microtubules nucleate and form a bipolar array, and the nuclear envelope breaks down (in most metazoan cells), allowing microtubules to attach to kinetochores.

Step-by-Step Solution:

Verification / Alternative check:

Microscopy of synchronized cells shows that spindle microtubules reorganize dramatically at prophase/prometaphase, confirming formation during M phase and not in G1, S, or G2 (though centrosomes duplicate in S and mature in G2).

Why Other Options Are Wrong:

G1: growth and biosynthesis; no spindle. S: DNA replication and centrosome duplication, but no spindle. G2: preparation and checkpoint control, not full spindle assembly. Interphase collectively lacks the spindle structure that appears in mitosis.

Common Pitfalls:

Confusing centrosome duplication (S phase) or maturation (G2) with full spindle assembly, which occurs only in M phase.

Final Answer:

M phase