Biology Now, 2e

Toxic Plastic ■ 103

Good Cells Gone Bad

Cell division is not always a good thing. Runaway

cell division can create a tumor (see “Cancer:

Uncontrolled Cell Division” on page 101). In a

developing organism, it can also cause an organ

such as the heart or liver to form incorrectly

and not function properly. It is little wonder,

then, that the cell cycle is carefully controlled

in healthy individuals. The decision to divide a

cell is made during the G 1 phase of the cell cycle

in response to internal and external signals.

In humans, external signals that influence the

commitment to divide include hormones and

proteins called growth factors. Some hormones

and growth factors act like the gas pedal in a car

and push a cell toward cell division; others act

like a brake and prevent cell division.

After a cell enters the cell cycle, special cell

cycle regulatory proteins are activated. These

proteins “throw the switch” that enables the

cell to pass through critical checkpoints and

proceed from one phase of the cell cycle to the

the equal and symmetrical partitioning of the

replicated genetic material. Normally, no daugh-

ter cell winds up short a chromosome, nor does it

acquire duplicates. Unless an error occurs, each

daughter cell inherits the same genetic informa-

tion that the parent cell possessed in the G 1 phase

of its life.

After the replicated DNA has been divided

in two, half to each end of the parent cell, the

cytoplasm is divided by a process called cytoki-

nesis (“cell movement”), like pulling apart a ball

of Silly Putty into two halves. Cytokinesis gives

rise to two self-contained daughter cells that are

clones of each other.

Mitotic division can serve both the eukaryotic

organism’s need to replace itself (to reproduce)

and its need to add new cells to its body. Many

multicellular eukaryotes use mitotic division to

reproduce asexually, including seaweeds, fungi,

and plants, and some animals, such as sponges

and flatworms. All multicellular organisms also

rely on mitotic division for the growth of tissues

and organs and the body as a whole, and for

repairing injured tissue and replacing worn-out

cells. Mitosis is why children grow taller and

why skin closes over a cut.

Overlapping with the end of

mitosis, the cell physically

divides into two daughter

cells through cytokinesis.

The new chromosomes reach

the poles. The nuclear envelopes

re-form, and the chromatin

decondenses.

The sister chromatids are

separated and become individual

chromosomes. These new

chromosomes are moved toward

the poles of the cell by the

shortening of the mitotic spindle.

New chromosomes

Chromosomes

decondensing

Nuclear

envelope

forming

Anaphase Telophase

Mitosis Cytokinesis

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