Question

Which motor proteins are used during anaphase A and anaphase B? Explain the mechanisms by which these motors are used to separate both the sister chromatids and the spindle poles.

Answer 1

In a dividing cell microtubules can be classified into four types

Kinetochore microtubules- These attach to the centromere of chromosome with the help of kinetochore (a protein complex is attached to centromere called as kinetochore)

Chromosomal microtubules- These connects the ends of chromosome via. chromokinesin.

Polar microtubules- These do not attach to chromosome and stabilized by overlapping with each other at the center of cell.

Astral microtubules- These extend outward and have freely exposed + ends.

Two types of microtubule motors are involved in this process plus end directed kinesins and minus end directed cytoplasmic dyneins (Note that plus ends extend outwards and minus ends of microtubules are embedded in cetrosome hence whenever movement is towards centrosome minus end directed dyneins are involved and vice versa.)

Process involved during Anaphase A(=Early Anaphase)- In anaphase A chromosomes move towards spinde poles along kinetochore mirotubules. This movement is driven by minus end directing motor protein called cytoplasmic dynein. This motor protein is associated with kinetochore.(Note that minus ends of microtubules are embedded in cetrosome and + ends extend outwards). In anaphase A shortening and disassembly of kinetochore and chromosome microtubules takes place and chromosomes are moved to poles.

Process involved in Anaphase B(=Late Anaphase)- In anaphase B separation of spindle poles takes place .Two types of movement takes place during this process

1)Overlapping polar microtubules slide over each other and push the poles apart. Plus end directed kinesins are involved in this movement.

2) A pulling force acts directly on poles to pull them apart in this movement minus end directed dynein is involved.