The mitotic spindle is a bipolar microtubule-based machine that organizes replicated chromosomes for accurate segregation during mitosis.

Two spindle poles establish opposite sides of the division axis. Dynamic microtubules extend from these poles and interact with chromosomes and with one another.

Three functional microtubule populations are useful for understanding the spindle:

• kinetochore microtubules attach to chromosome kinetochores;
• interpolar microtubules overlap with microtubules from the opposite pole and help organize spindle length;
• astral microtubules extend toward the cell cortex and help position the spindle in many animal cells.

Dynamic microtubules search cellular space

Because microtubules repeatedly grow and shrink, their plus ends explore different regions of the cell. A growing microtubule that encounters a kinetochore can become stabilized and form a load-bearing attachment.

A replicated chromosome has two sister kinetochores. For accurate mitosis, the stable configuration is biorientation: one sister kinetochore is attached predominantly to microtubules from one spindle pole and the other sister kinetochore to microtubules from the opposite pole.

pole A ← microtubules — [sister 1 | sister 2] — microtubules → pole B

Tension helps distinguish correct from incorrect attachments

Because sister chromatids are held together, pulling them toward opposite poles generates tension across the pair. Attachments that fail to create proper opposing geometry are more likely to be destabilized and retried.

This error-correction process is essential because initial microtubule capture is stochastic and can produce incorrect arrangements.

Metaphase alignment emerges from attachment geometry

Bioriented chromosomes commonly accumulate near the spindle equator, producing the metaphase plate. Alignment is visually useful, but the crucial requirement is correct mechanical attachment to opposite poles.

Only after chromosomes have established suitable spindle attachments should sister-chromatid cohesion be released. A separate cell-cycle checkpoint monitors this readiness before the irreversible segregation step.

The spindle is therefore best understood as a dynamic error-correcting attachment system: microtubules explore, kinetochores capture them, incorrect configurations are destabilized, and stable biorientation prepares chromosomes for accurate segregation.