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NEL Cell Division 561

The Stages of Mitosis

Prophase

Prophase is the first phase of mitosis. The chromosomes in the nucleus become visible under

a microscope as they shorten and thicken (Figure 4). In animal cells, a small body in the

cytoplasm separates and its parts move to opposite poles of the cell as the chromosomes

become visible. These tiny structures, called centrioles, provide attachment for the

spindle fibres, which serve as guide wires for the attachment and movement of the chro-

mosomes during cell division. Collectively, the centrioles and spindle fibres make up the

spindle apparatus. Most plant cells do not have centrioles, but spindle fibres still form

and serve a similar purpose. The centromere joining the two chromatids helps anchor

the chromosomes to the spindle fibres. When viewed under a microscope during prophase,

the nuclear membrane appears to fade; in effect, it is dissolving to allow the separation of

chromosomes and cell organelles.

Section17.

Metaphase

Chromosomes line up

at the equatorial plate.

The nuclear membrane

completely dissolves.

Anaphase

The centromeres divide and

the resulting chromosomes,

formerly chromatids, move to

opposite poles of the cell. An

identical set of chromosomes

moves to each pole.

Telophase

Chromosomes lengthen again, the

spindle fibres dissolve, and a nuclear

membrane forms around the

chromosomes. In humans, each new

nucleus contains 46 unique

chromosomes.

centriolesmall protein body found

in the cytoplasm of animal cells that

provides attachment for spindle

fibres during cell division

spindle fibreprotein structure that

guides chromosomes during cell

division