HUMAN BIOLOGY

360 Chapter 18

What is cytokinesis?

• Cytokinesis completes cell division following mitosis. It divides
  the parent cell’s cytoplasm in a way that allots cytoplasm, one
  of the two newly formed nuclei, and plasma membrane to each
  daughter cell.


• In a diploid cell the paired set of sex chromosomes may be
  either two Xs or an X and a Y. Females have an XX pair. Males
  have an XY pair.

taKe-hoMe Message

Gender testing of elite female athletes once was

common due to revelations that some athletes

posing as females actually had the male (XY) set of

sex chromosomes. Due to scientific objections, most

international sports organizations no longer require

gender testing. Alone or with classmates, investigate

this controversial practice. What objections did the

scientific experts raise? Does gender testing still

occur in some sports settings?

ThiNk OuTsiDE ThE bOOk

cleavage furrow The area
of a dividing cell’s plasma
membrane where microfila-
ments pull the cell surface
inward until the cell is
pinched in two.

cytokinesis Division of the
cytoplasm of a dividing cell.

how the Cytoplasm divides

n    After mitosis produces two new cell nuclei, each with a set

of the parent cell’s chromosomes, the parent cell’s cytoplasm

also must be divided between the two daughter cells.

Division of the cytoplasm, or cytokinesis, usually begins

toward the end of anaphase. By this time, the two sister

chromatids of each chromosome have been separated and

are independent chromosomes. In an animal cell, about

midway between the cell’s two poles, a patch of plasma

membrane sinks inward, forming a

cleavage furrow. Microfilaments

made of the contractile protein actin

steadily pull the plasma membrane

inward all around the cell, until the

cell is pinched in two (Figure 18.7).

Now there are two new cells, each

with a nucleus, cytoplasm, and a

plasma membrane.

This concludes our tour of mitosis. Now we turn to mei-

osis, the nucleus-dividing mechanism that forms gametes.

It is difficult not to be in awe of the astonishing precision

with which both mitosis and meiosis take place.

Figure 18.7 Animated! Cytokinesis gives each new cell a share of the parent cell’s cytoplasm. A Steps of cytokinesis in an animal

cell. B The beltlike contracting ring of microfilaments inside a dividing animal cell that is undergoing cytokinesis. (© Cengage Learning)

Contractions continue;

the cell is pinched in two.

At the former spindle equator,

a ring of microfilaments attached to

the plasma membrane contracts.

2 As the microfilament ring

shrinks in diameter, it pulls the

cell surface inward.

Mitosis is over, and the 3

spindle is disassembling.

1 4

ring of microfilaments

Jennifer C. Waters/Science Source

David M. Phillips/Science Source

A

B

18.4

Copyright 2016 Cengage Learning. All Rights Reserved. May not be copied, scanned, or duplicated, in whole or in part. Due to electronic rights, some third party content may be suppressed from the eBook and/or eChapter(s).