Dynamics of Chromosome Movements During Cell Division
Events involving chromosome movement
Chromosomes are involved in a series of directed movements during both mitosis and meiosis. During mitosis, where chromosome movements have been studied in some detail, following events have been observed, which are shown in Figure 8.9. In prometaphase, chromosomes move (.05 to 1.0 μm sec-1) and attach to spindle microtubules both terminally and laterally, and atleast in some cases, can be seen to gather at the spindle poles. (Microtubules are unbranched tube like structures, present in the cell, and mainly consist of a protein called tubulin; they are involved in the structure of cilia, flagella and spindle and help in movements of cilia, flagella and chromosomes). Microtubules provide the framework for the mitotic spindle, and are oriented with their so called minus (-) ends at the spindle poles and their plus (+) ends overlapping in the centre of the spindle. As prometaphase progresses, the speed of chromosomes slows down (.01 to .05 μm sec-1). Subsequently, after a slow bi-directional shuffle,
the chromosomes migrate towards the mid-zone of the spindle through a process called congression and form the metaphase plate. After this, in a co-ordinated manner, the chromosomes split and proceed towards the spindle poles with slow and uniform motion. This stage is anaphase, which has two parts, anaphase A and anaphase B. Anaphase A (earlier part of anaphase) involves specific shortening of microtubules associated with kinetochore (discontinuous microtubules), while anaphase B (later part) involves elongation of the entire spindle (continuous microtubules).