HUMAN BIOLOGY

358 Chapter 18

What are the stages of mitosis?

• Mitosis occurs in consecutive stages, called prophase,
  metaphase, anaphase, and telophase.


• Before mitosis, each chromosome in a cell’s nucleus is
  duplicated, so that it consists of two sister chromatids.


• A spindle of microtubules moves the sister chromatids of
  each chromosome apart, to opposite spindle poles. A new
  nuclear envelope forms around the two chromosome clusters.
  Both daughter nuclei have the same number of chromosomes
  as the parent cell’s nucleus.

taKe-hoMe Message

the Four stages of Mitosis

Next comes metaphase

A lot happens between prophase and metaphase—so

much that this transitional period has its own name, “pro-

metaphase.” The nuclear envelope breaks apart, allow-

ing the chromosomes in the nucleus to interact with

microtubules extending toward them from the poles of

the forming spindle. Cell biologists have watched this

process occur. Microtubules from both poles harness each

chromosome and start pulling on it. The two-way pull-

ing tugs the chromosome’s two sister chromatids toward

opposite poles. Meanwhile, overlapping spindle micro-

tubules ratchet past each other and push the poles of the

spindle apart. Soon the chromosomes reach the middle of

the spindle.

When all the duplicated chromosomes are lined up

midway between the poles of a spindle, we say the cell is

in metaphase (meta- means “midway between”). This align-

ment sets the stage for anaphase, the next stage of mitosis.

Anaphase, then telophase follow

During anaphase, the sister chromatids of each chromo-

some separate from each other and move to opposite

spindle poles. Two mech anisms produce this movement.

First, the micro tubules attached to the centromeres pull

the chromosomes toward the poles. Second, the spindle

elongates as overlapping microtubules continue to ratchet

past each other and push the two spindle poles even farther

apart. Once each chromatid is separated from its sister, it is

an independent chromosome.

Telophase begins as soon as the two clusters of chromo-

somes each arrive at a spindle pole. The chromosomes are

no longer connected to microtubules, and they return to

threadlike form. Bit by bit, a new nuclear envelope forms

around each cluster, separating it from the cytoplasm. Now

there are two new nuclei. Each new nucleus has the same

chromosome number as the parent nucleus. Once two

nuclei form, telophase is over—and so is mitosis.

n When interphase ends, a cell has stopped making new parts

and its DNA has been replicated. Mitosis can begin.

n Links to Cell nucleus 3.6, Microtubules 3.9

The four stages of mitosis are prophase, metaphase,

anaphase, and telophase, as shown here and in Fig-

ure 18.6.

Mitosis begins with prophase

When prophase begins, a cell’s chromosomes are thread-

like. (“Mitosis” comes from the Greek mitos, for “thread.”)

During interphase each chromosome was duplicated,

forming two sister chromatids

joined at the centromere. The sis-

ter chromatids of each chromosome

twist and fold into a more compact

form. By the end of prophase, all

chromosomes will be condensed

into thick rod shapes.

Meanwhile, in the cytoplasm,

most microtubules of the cell’s

cytoskeleton are breaking apart

into their subunits (Section 3.9)

and new microtubules are form-

ing near the nucleus. The nuclear

envelope physically prevents them

from making contact with the chro-

mosomes inside the nucleus, but not

for long: The nuclear envelope starts

to break up as prophase ends.

In both mitosis and meiosis, chro-

mosomes move into new positions

with help of a spindle. This struc-

ture consists of microtubules that

extend from centrioles. Each barrel-shaped centriole was

duplicated in interphase, so there are two pairs of them in

prophase. In prophase, microtubules start moving each pair

of centrioles to the opposite ends, or “poles,” of the spindle.

18.3

interphase

prophase

metaphase

anaphase

telophase

mitosisiiisi

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G2

© Cengage Learning

anaphase Stage of mitosis
when sister chromatids of
each chromosome sepa-
rate and move to opposite
spindle poles.

metaphase Stage of
mitosis when duplicated
chromosomes are lined up
midway between the poles
of the spindle.

prophase The first stage of
mitosis, when a cell’s chro-
mosomes condense.

spindle Array of microtu-
bules that moves chromo-
somes during mitosis or
meiosis.

telophase The last stage
of mitosis, when a nuclear
envelope forms around each
of the two nuclei of forth-
coming daughter cells.

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