96
UNIT 1
Organization of the Body
3
Cell Growth and Reproduction
The Cell Cycle
List the phases of the cell cycle and describe the key events
of each phase.
Describe the process of DNA replication.
Te
cell cycle
is the series of changes a cell goes through from
the time it is formed until it reproduces. Te outer ring of
Figure 3.31
shows the two major periods of the cell cycle:
Interphase
(in green), in which the cell grows and carries on
its usual activities
Cell division
or the
mitotic phase
(in yellow), during which it
divides into two cells
Interphase
Interphase
is the period from cell formation to cell division.
Early cytologists, unaware of the constant molecular activity in
cells and impressed by the obvious movements of cell division,
called interphase the resting phase of the cell cycle. (Te term
interphase
reflects this idea of a stage
between
cell divisions.)
However, this image is misleading because during interphase a
cell is carrying out all its routine activities and is “resting” only
from dividing. Perhaps a more accurate name for this phase
would be
metabolic phase
or
growth phase
.
Subphases
In addition to carrying on its life-sustaining reac-
tions, an interphase cell prepares for the next cell division. In-
terphase is divided into G
1
, S, and G
2
subphases (the Gs stand
for
gaps
before and aFer the S phase, and S is for
synthetic
). In
all three subphases, the cell grows by producing proteins and or-
ganelles, but chromatin is reproduced only during the S subphase.
CELL PART*
STRUCTURE
FUNCTIONS
Nucleus (Figures 3.2, 3.29)
Nuclear envelope (Figure 3.29)
Double-membrane structure pierced by
pores. Outer membrane continuous with the
endoplasmic reticulum.
Separates the nucleoplasm from the cytoplasm
and regulates passage of substances to and
from the nucleus.
Nucleolus (Figure 3.29)
Dense spherical (non-membrane-bounded)
bodies, composed of ribosomal RNA and
proteins.
Site of ribosome subunit manufacture.
Chromatin (Figure 3.30)
Granular, threadlike material composed of
DNA and histone proteins.
DNA constitutes the genes.
Table 3.3
Parts of the Cell: Structure and Function
(continued)
may indicate that the cell is about to commit suicide. On the
other hand, addition of acetyl groups to histone exposes differ-
ent DNA segments, or genes, so that they can dictate the speci-
fications for synthesizing proteins or various RNA species. Such
active chromatin segments, referred to as
extended chromatin
,
are not usually visible under the light microscope. Te generally
inactive
condensed chromatin
segments are darker staining and
more easily detected. Understandably, the most active body cells
have much larger amounts of extended chromatin.
Interestingly, particular chromatin strands occupy discrete
regions in the nucleus called
chromosome territories
. Depend-
ing on the specific genes contained, and the cell and tissue type,
the chromosome territory patterns change during development.
At the simplest level, active and inactive genetic regions can be
separated from each other, which in turn would enhance or re-
press genetic expression.
When a cell is preparing to divide, the chromatin threads coil
and condense enormously to form short, barlike bodies called
chromosomes
(“colored bodies”) (±igure 3.30
5
and
6
). Chro-
mosome compactness prevents the delicate chromatin strands
from tangling and breaking during the movements that occur
during cell division. Next, we describe the functions of DNA and
the events of cell division.
Table 3.3
summarizes the parts of the cell beginning on p. 94.
Check Your Understanding
23.
If a cell ejects or loses its nucleus, what is its fate and why?
24.
What is the role of nucleoli?
25.
What is the importance of the histone proteins present in the
nucleus?
For answers, see Appendix H.
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