Kinetochore and spindle in chromosome movement

Physical Basis of Heredity 3.  Genetics, Biochemistry and Dynamics of Cell Division
Genetics of cell division cycle
Biochemistry of cell division 
Biochemistry of mitosis
Biochemistry of meiosis
Dynamics of chromosome movements during cell division
Events involving chromosome movement
Kinetochore and spindle in chromosome movement
Basic questions about kinetochore function
Kinetochore, which is coincident with the centromere and is a trilaminar plate situated at the primary constriction (or centromere) of the chromosome, plays an important role in chromosome movements. It is through these specialized structures called kinetochores, that the chromosomes get attached to microtubules (Fig. 8.9) and help in their movement during anaphase. Recent studies have also shown that kinetochores are sites for generation of force required for chromosome movement. Following generalizations (based on experiments) have been made regarding kinetochore, spindle apparatus, centrosome and interactions between them : (i) At the time of spindle formation, there is an increase in phosphorylation of several proteins, (ii) Kinetochore microtubules (MTs) can both add (polymerization) and lose (depolymerization) subunits at or near the kinetochores; similarly, subunits addition (polymerization) at the centrosome-distal ends of MTs leads to elongation of continuous fibres, (iii) Kinetochores are capable of both binding to and sliding on the microtubules. (iv) During formation of spindle at prometaphase, a chromosome is under tension from both ends of the spindle, so that a centrosome directed force is acting at each kinetochore.
(v) Kinetochore-spindle fibre interactions are unstable, until there is an opposing force which may resist chromosome movement; this may stabilize chromosome attachment, the stability being dependent on proper orientation of chromosome with respect to spindle. Thus improper chromosome orientations will not survive, (vi) A single centrosome pulls on a kinetochore and pushes on the rest of the chromosome. This duality of forces may be significant for prometaphase motion of chromosomes to metaphase plate, (vii) Once the spindle is established at metaphase, the centrosome becomes dispensable. (viii) Kinetochore-centrosome attractions result in chromosome movement, so that when two chromatids separate, they move automatically towards the poles, due to disassembly (depolymerization) of microtubules attached to kinetochores. (ix) The force acting on anaphase chromosomes is 7 x 10-5 dynes, (x) The discontinuous spindle fibres carrying the kinetochores contract and pull the chromosomes towards the poles, while the continuous fibres elongate and hold the poles apart.