AP Syllabus focus:
‘During prophase, sister chromatids condense, the mitotic spindle forms, and centrosomes move; during metaphase, spindle fibers align chromosomes along the cell equator.’
Mitosis partitions duplicated chromosomes into two genetically identical nuclei. Prophase and metaphase establish the physical framework for chromosome movement by reorganising chromatin and building attachments between chromosomes and the spindle apparatus.
Multi-stage diagram of mitosis showing how chromosomes condense as the spindle forms and how chromosomes later align at the metaphase plate. The sequence helps connect prophase events (spindle establishment and chromosome condensation) to metaphase outcomes (bipolar attachment and equatorial alignment). Source
Prophase: organising chromosomes for movement
Chromatin condensation into sister chromatids
By prophase, each chromosome consists of two sister chromatids (identical DNA copies) joined at the centromere.
Condensation packs long DNA molecules into compact, visible structures, reducing tangling and enabling accurate movement.
Condensed chromosomes become distinct enough to track as individual units for later separation.
Mitotic spindle formation
The cell builds a dynamic fibre system that will position and move chromosomes.
Mitotic spindle: A microtubule-based structure that forms during mitosis to attach to chromosomes and organise their movement using microtubules and associated motor proteins.
Key points to know:
Microtubules (tubulin polymers) assemble from microtubule-organising centres and constantly grow/shrink, allowing “search and capture” of chromosomes.
Spindle microtubules are organised into functional groups:
Diagram of a mitotic spindle highlighting how microtubules are organized around spindle poles and attach to chromosomes at kinetochores. Use it to visually distinguish attachment microtubules (kinetochore microtubules) from other spindle microtubules involved in spindle structure and positioning. Source
Kinetochore microtubules attach to chromosomes at kinetochores (protein complexes on the centromere region).
Non-kinetochore microtubules overlap with microtubules from the opposite side, helping push poles apart.
Astral microtubules extend toward the cell cortex, helping position the spindle.
Centrosomes move to opposite poles
Centrosomes (microtubule-organising centres in animal cells) begin separating and moving toward opposite ends of the cell.
This poleward movement helps establish the spindle’s bipolar geometry, a requirement for aligning chromosomes and ensuring each future daughter nucleus receives one chromatid from each pair.
Metaphase: aligning chromosomes at the cell equator
Establishing chromosome attachments to spindle fibres
Spindle fibres (especially kinetochore microtubules) attach to kinetochores on sister chromatids.
Correct attachment typically means bi-orientation:
One sister chromatid’s kinetochore is connected to microtubules from one pole.
The other sister chromatid’s kinetochore is connected to microtubules from the opposite pole.
These opposing connections create tension, which stabilises attachments and promotes accurate positioning.
Chromosomes align at the metaphase plate
During metaphase, chromosomes are positioned along the cell equator (the metaphase plate).
Alignment occurs because microtubules continually adjust length while motor proteins and microtubule dynamics “fine-tune” chromosome position:
If a chromosome is too close to one pole, microtubule dynamics and forces tend to shift it toward the centre.
Balanced pulling forces from both poles hold chromosomes in line at the equator.
Why metaphase alignment matters
Proper alignment ensures that, when separation occurs later, each new nucleus is equally likely to receive one chromatid from every chromosome pair.
Misalignment or faulty attachment increases the risk of nondisjunction (incorrect chromosome distribution), which can alter chromosome number in daughter cells.
FAQ
Many plant cells lack centrosomes with centrioles.
They nucleate microtubules from dispersed organising sites, and spindle self-organisation emerges from microtubule dynamics plus motor proteins that sort microtubules into a bipolar array.
Kinetochores contain multiprotein couplers that maintain attachment to the microtubule plus-end.
As tubulin subunits are lost, these couplers convert microtubule depolymerisation into movement while reducing detachment probability.
Monotelic attachment lacks balanced tension.
Cells often correct it by destabilising the improper connection, allowing reattachment until both kinetochores are engaged by opposite poles.
Condensation depends on regulated chromatin-binding proteins (for example, condensin complexes) and phosphorylation-driven changes to chromatin architecture.
When regulatory signals change, these interactions weaken, allowing chromatin to decondense later.
Astral microtubules interact with the cortex, and cortical cues bias spindle position and orientation.
This helps place the metaphase plate in a location that supports accurate division geometry for that particular cell type.
Practice Questions
State two events that occur in prophase of mitosis. (2 marks)
Sister chromatids/chromosomes condense/become visible (1)
Mitotic spindle forms / centrosomes move to opposite poles (1)
Explain how spindle fibres align chromosomes at the cell equator during metaphase. (5 marks)
Spindle microtubules attach to chromosomes at kinetochores/centromeres (1)
Each sister chromatid attaches to microtubules from opposite poles (bi-orientation) (1)
Microtubules dynamically grow and shrink to adjust chromosome position (1)
Opposing forces/tension stabilise correct attachments (1)
Balanced pulling positions chromosomes at the metaphase plate/equator (1)
