HUMAN BIOLOGY

356 Chapter 18

n    Mitosis occurs during the “lifetime” of a somatic cell, called

the cell cycle. Meiosis occurs solely in germ cells.

n    Links to Cell nucleus 3.6, Cytoskeleton 3.9

The cell cycle is a series of events

during which a cell grows and then

divides. The cycle starts when a new

cell is produced, and it ends when

the cell completes its own division.

Usually, the longest phase of the

cell cycle is interphase, as sketched

in Figure 18.3. This phase has three

parts in which a cell grows larger,

more or less doubles the number of

components in its cytoplasm, then

copies its DNA. This copying process duplicates the chro-

mosomes. Then comes mitosis (my-toe-sis), the division of

the cell nucleus into two nuclei, each with a full diploid set

of chromosomes (Figure 18.4). The parts of the cell cycle are:

G1 The part of interphase when the cell grows

S The part of interphase when a cell’s DNA is copied

and its chromosomes are duplicated

G2 The part of interphase after chromosomes are

duplicated, when other events prepare the cell

to divide

M Mitosis, when chromosomes (copied DNA) are

sorted into two sets and the cytoplasm divides

Overview of the Cell Cycle and Cell Division

Figure 18.3 Animated! The cell cycle has two main stages, interphase (orange

arrow) and mitosis (brown arrow). The duration of each interval differs among cells.

A An unduplicated pair of

chromosomes in a cell in G1.

B By G2, each chromosome

has been duplicated.

C Mitosis and cyto-

plasmic division pack-

age one copy of each

chromosome into each

of two new cells.

Figure 18.4 Mitosis maintains the chromosome number.

After mitosis is complete, the parent cell’s cytoplasm divides.

Section 18.4 explains this final step in cell division.

18.2

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interphase

prophase

metaphase

anaphase

telophase

mitosis

G1

S

G2G2

S

G1

1

2

3

4

(^5) mmmm
6
© Cengage Learning
(^4) G2 is the interval
after DNA replication and
before mitosis. The cell
prepares to divide during
this stage.
(^1) A cell spends most of
its life in interphase, which
includes three stages: G1,
S, and G2.
(^2) G1 is the interval of
growth before DNA repli-
cation. The cell’s chromo-
somes are unduplicated.
(^3) S is the time of synthe-
sis, during which the cell
copies its DNA.
(^5) The nucleus divides
during mitosis, the
four stages of which
are detailed in Section
18.3. After mitosis, the
cytoplasm may divide.
The cycle begins anew,
in interphase, for each
descendant cell.
(^6) Built-in checkpoints
stop the cycle from pro-
ceeding until certain con-
ditions are met.
cell cycle Series of events
during which a newly
formed cell grows and then
divides.
mitosis Process that
divides the nucleus of a
dividing somatic cell in a
way that maintains the chro-
mosome number in daugh-
ter cells.
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