Question 1

What are the main forces acting on chromosomes during mitosis?

Accepted Answer

Three primary forces act on chromosomes during mitosis: poleward forces pull chromosomes toward spindle poles through kinetochore-microtubule depolymerization and microtubule flux; polar ejection forces push chromosomes toward the cell equator via kinesin-4 and kinesin-10 motor proteins; and cohesive forces exerted by cohesin hold sister chromatids together until metaphase ends. These opposing forces balance to align chromosomes at the metaphase plate.

Question 2

How does kinetochore-microtubule attachment generate poleward force?

Accepted Answer

Ndc80 protein complexes at the kinetochore link chromosomes to microtubules through multiple low-affinity attachments. During microtubule plus-end depolymerization, these attachments break and reform at new sites, gradually pulling the chromosome toward the spindle pole as the microtubule shortens. This mechanism creates a sustained poleward force that moves chromosomes during mitosis.

Question 3

What role do motor proteins play in chromosome movement?

Accepted Answer

Kinesin-4 and kinesin-10 motor proteins generate polar ejection forces by linking chromosomal arms to interpolar microtubules. These plus-end directed motor proteins move chromosomes toward the spindle equator, opposing the poleward forces. The balanced interplay between motor protein-driven polar ejection and poleward forces enables precise chromosome alignment during metaphase.

Question 4

How does microtubule flux contribute to chromosome positioning?

Accepted Answer

Microtubule flux results from minus-end depolymerization that generates movement toward spindle poles, while compensatory plus-end polymerization maintains microtubule length. This flux creates a second poleward force that pulls chromosomes toward the poles. Combined with kinetochore-microtubule depolymerization, microtubule flux dynein kinesin and anaphase movements coordinate chromosome segregation during cell division.

Question 5

Why do chromosomes oscillate during prometaphase?

Accepted Answer

Chromosomes oscillate during prometaphase because poleward and polar ejection forces are unequal and vary with chromosome position. These opposing forces cause chromosomes to move back and forth as they congress toward the metaphase plate. Once bioriented, sister chromatids experience equal but opposing forces that stabilize their position and silence the spindle assembly checkpoint.

Question 6

What is the relationship between chromosome tension and checkpoint control?

Accepted Answer

During metaphase, bioriented sister chromatids experience equal but opposing poleward and polar ejection forces, creating tension across the kinetochore. This tension is sufficient to silence the spindle assembly checkpoint pathway, allowing cells to proceed into anaphase. The checkpoint ensures that chromosomes are properly attached and aligned before segregation occurs.

Question 7

How do cohesins and condensins affect chromosome structure during mitosis?

Accepted Answer

Cohesin proteins exert cohesive forces that hold sister chromatids together until metaphase ends, maintaining chromosome integrity during force-driven movements. Condensins generate resolving forces that allow chromosomes to form distinct rod-shaped structures, facilitating proper separation during anaphase. Together, these proteins ensure chromosomes maintain structural integrity while responding to spindle forces.

Forze che agiscono sui cromosomi

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