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562 Chapter 17 NEL

Metaphase

The second phase of mitosis is metaphase. Chromosomes composed of sister chromatids

move toward the centre of the cell. This centre area is called the equatorial plate, because,

like the equator of Earth, it is midway between the poles of the cell. The chromosomes

appear as dark, thick filamentous structures that are attached to the spindle fibres. Even

though they are most visible at this stage, it is still very difficult to count the number of

chromosomes in most cells because the chromosomes are entangled. Chromatids can

become intertwined during metaphase.

Anaphase

Anaphase is the third phase of mitosis. The centromeres divide and the sister chro-

matids, now referred to as chromosomes, move to opposite poles of the cell. If mitosis

proceeds correctly, the same number and type of chromosomes will be found at each

pole. Occasionally, segments of the chromatids will break apart, and may reattach, in

anaphase.

Telophase

The last phase of mitosis is telophase. The chromosomes reach the opposite poles of the

cell and begin to lengthen. The spindle fibres dissolve and a nuclear membrane forms

around each mass of chromatin. Telophase is followed by cytokinesis, the division of

the cytoplasm.

Cytokinesis

Once the chromosomes have moved to opposite poles, the cytoplasm begins to divide.

Cytokinesis appears to be quite distinct from nuclear division. In an animal cell, a

furrow develops, pinching off the cell into two parts. This is the end of cell division.

In plant cells, the separation is accomplished by a cell plate that forms between the two

chromatin masses. The cell plate will develop into a new cell wall, eventually sealing

off the contents of the new cells from each other.

Cells are grown in culture.

Total: 50 cell divisions

Cells are frozen

in liquid

nitrogen after

20 divisions.

Cells are frozen

in liquid

nitrogen after

40 divisions.

After cells thaw,

they divide 30

more times.

After cells thaw,

they divide 10

more times.

Figure 5
Cell division appears to be
controlled by a biological clock.

Situation A Situation B

Practice

1. List the stages of mitosis. Briefly describe what occurs in each stage. To help in your
   description, sketch the sequence of events that occurs in an animal cell. Include
   labels for different structures.
   2.A cell with 10 chromosomes undergoes mitosis. Indicate how many chromosomes
   would be expected in each of the daughter cells.

A Cell Clock

How old can cells become? If cells continue to undergo mitosis, could an organism

stay eternally young and live forever? Research on cultured cells (cells grown in a nutrient

medium) indicates that a biological clock may regulate the number of cell divisions

available to cells. When immature heart cells maintained in tissue culture were frozen,

they revealed an internal memory of the number of cell divisions they had undergone.

If a cell had undergone twenty divisions before freezing, the cell completed another

thirty divisions once it thawed, then died. When a cell was frozen after ten divisions, it

completed another forty divisions after thawing and then died. Cells always completed

a total of fifty divisions no matter how long the freezing or at what stage the cell divi-

sion was suspended (Figure 5).

Mitosis and Cell Division in

Plants and Animals

This Audio Clip highlights the

observable differences between

plant and animal cell mitosis and

cytokinesis.

+ EXTENSION

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