Where Do the Microtubules of the Spindle Originate During Mitosis in Animal Cells?
Mitosis is a cellular process in which daughter cells have the same number of chromosomes as their parents. During mitosis, the spindle is responsible for pulling sister chromatids apart and aligning them. A microscope view of a mitotic cell will reveal the chromosomes, and the structure of the spindle can tell us how many chromatids there are on each chromosome.
The spindle fibers are continuously being formed and broken down, which causes them to be “pulled” along the chromosomes. During the G2 phase, these fibers contain proteins called kinetochores. They then connect to the poles of the chromosomes and depolymerize to disappear. As a result, the daughter cells are able to move into G1 and G2.
During mitosis, animal cells have three types of spindle microtubules. Astral microtubules radiate in all directions from the centrosome, contributing to forces that separate the chromosome poles. Kinetochore microtubules are formed at the centromere of each duplicated chromosome. Overlapping microtubules interdigitate along the equator of the spindle, which is responsible for the bipolar shape of the spindle.
The two forms of anaphase are dependent on the presence of motor proteins at the central spindle and poles. The central spindle is a bundle of antiparallel-overlap microtubules that push the poles apart. Initially, these two types of spindle were distinguished by different drug sensitivity, reflecting differences in microtubule motor proteins.
Two models exist for how microtubules form the spindle. The first model involves microtubules originating at the centrosomes, contacting chromosomes by chance, and stabilizing at the kinetochores to form the spindle. The second model, a self-assembly mechanism, involves microtubules nucleating in a condensed chromosome. In both models, microtubules are organized by motor proteins among the condensed chromosomes.
The spindle originates from the centrosome, which is also the organization center of the cytoskeleton. In addition, the centrosome determines the shape of the cell and initiates polymerization of microtubules to form the spindle. Microtubules are composed of alpha and beta tubulin dimers.