The MItotic Division of an Animal Cell

A nuclear envelope bounds the nucleus
The nucleus contains one or more nucleoli

Two centrosomes have formed by replication of a single centrosome
In animal cells, each centrosome features two centrioles

Chromosomes, duplicated during S phase, cannot be seen individually because they have not yet condensed

The chromatin fibers become more tightly coiled, condensing into discrete chromosomes observable with a light microscope.

the nucleoli disappear

Each duplicated chromosome appears as two identical sister chromatids joined together at their centromeres and all along their arms by cohesins (sister chromatid cohesion)

The mititic spindle begins to form. It is composed of the centrosomes and the microtubules that extend from them. The radial arrays of shorter microtubules that extend from the centrosomes are called asters ("starts")

the centrosomes move away from each other, apparently propelled by the lengthening microtubules between them

the nuclear envelope fragments

the microtubules extending from each centrosome can now invade the nuclear area

the chromosomes have become even more condensed

each of the two chromatids of each chromosome now has a kinetochore, a specialized protein structure located at the centromere

some of the microtubules attach to the kinetochores, becoming "kinetochore microtubules"; these jerk the chromosomes back and forth

Nonkinetochore microtubules inateract with those from the opposite pole of the spindle

Longest stage of mitosis, often lasting about twenty minutes

the centrosomes are now at opposite poles of the cell

the chromosomes convene on the metaphase plate, an imaginary plane that is equidistant between the spindle's two poles. The chromosomes' centromeres lie on the metaphase plate

for each chromosome, the kinetochores of the sister chromatids are attached to kinetochore microtubules coming from opposite poles

the shortest stage of mitosis, often lasting only a few minutes

begins when the cohesin proteins are cleaved. This allows the two sister chromatids of each pair to part suddenly. Each chromatid thus becomes a full-fledged chromosome

the two liberated daughter chromsomes begin moving twoard opposeite ends of the cell as their kinetochore microtubules shorten. Because these microtubules are attached at the centromere region, the chromosomes move centromere first

the cell elongates as the nonkinetochore microtubules lengthen

by the end of anaphase, the two ends of the cell have equivalent-and complete- collections of chromosomes

two daughter nuclei form in the cell

nuclear envelopes arise from the fragments of the parent cell's nuclear envelope and other portions of the endomembrane system

nucleoli reappear

the chromosomes become less condesnsed

Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete

the division of teh cytoplsam is usually well under way by late teloophase, so the two daughter cells appear shortly after the end of mitosis

in anima cells, it involves the formation of a cleavage furrow, which pinches the cell in two