101
CHAPTER THREE
Cells
interphase, following DNA replication (discussed in
chapter 4, page 127), each chromosome consists of two
identical structures, called chromatids, temporarily
attached by a region on each called a
centromere.
The centrioles of the centrosome replicate just
before the onset of mitosis (F
g. 3.36
a
), and during
prophase, the two newly formed pairs of centrioles
move to opposite sides of the cell. Soon the nuclear
envelope and the nucleolus disperse and are no longer
visible. Microtubules are assembled from tubulin
proteins in the cytoplasm, and these structures associate
with the centrioles and chromosomes. A spindle-shaped
array of microtubules (spindle F
bers) forms between the
centrioles as they move apart (F
g. 3.36
b
).
2.
Metaphase.
Spindle F
bers attach to the centromeres
so that a F
ber accompanying one chromatid attaches
to one centromere and a F
ber accompanying the other
chromatid attaches to its centromere (F
g. 3.36
c
). The
chromosomes move along the spindle F
bers and are
aligned about midway between the centrioles as a result
of microtubule activity.
3.
Anaphase.
Soon the centromeres of the chromatids
separate, and these identical chromatids are now
considered individual chromosomes. The separated
chromosomes move in opposite directions, and once
again, the movement results from microtubule activity.
The spindle F
bers shorten and pull their attached
chromosomes toward the centrioles at opposite sides of
the cell (F
g. 3.36
d
).
4.
Telophase.
The F
nal stage of mitosis begins when
the chromosomes complete their migration toward
the centrioles. It is much like the reverse of prophase.
As the identical sets of chromosomes approach their
respective centrioles, they begin to elongate and unwind
from rodlike structures to threadlike structures. A
nuclear envelope forms around each chromosome set,
and nucleoli become visible within the newly formed
nuclei. ±inally, the microtubules disassemble into free
tubulin molecules (F
g. 3.36
e
).
Table 3.4
summarizes the stages of mitosis.
Cytoplasmic Division
Cytoplasmic division (cytokinesis) begins during anaphase
when the cell membrane starts to constrict around the middle,
which it continues to do through telophase. The musclelike
contraction of a ring of actin microF laments pinches off two
cells from one. The microfilaments assemble in the cyto-
plasm and attach to the inner surface of the cell membrane.
The contractile ring forms at right angles to the microtubules
that pulled the chromosomes to opposite ends of the cell dur-
ing mitosis. As the ring pinches, it separates the two newly
formed nuclei and apportions about half of the organelles into
each of the daughter cells. The newly formed cells may dif-
fer slightly in size and number of organelles and inclusions,
but they have identical chromosomes and thus contain
a sperm fertilizes an egg, the total number of 46 chromo-
somes is restored. Chapter 22 (pp. 831–833) considers meio-
sis in detail.
During mitosis, the nuclear contents divide in an event
called karyokinesis, which means “nucleus movement.”
Then the cytoplasm is apportioned into the two daugh-
ter cells in a process called cytokinesis, which means “cell
movement.” Mitosis must be very precise so that each new
cell receives a complete copy of the genetic information.
The chromosomes were duplicated in interphase, but it is in
mitosis that the chromosome sets evenly distribute between
the two forming cells.
Mitosis is a continuous process, but it is described in
stages that indicate the sequence of major events, as follows:
1.
Prophase.
One of the F
rst indications that a cell is going
to divide is the condensation of chromatin F
bers into
tightly coiled rods. These are the chromosomes. During
Restriction
checkpoint
G
1
phase:
cell growth
S phase:
genetic
material
replicates
Apoptosis
Remain
specialized
Proceed
to division
G
2
phase
Prophase
Metaphase
Anaphase
Telophase
M
i
t
o
s
i
s
I
n
t
e
r
p
h
a
s
e
Cytokinesis
FIGURE 3.35
The cell cycle is divided into interphase, when cellular
components duplicate, and cell division (mitosis and cytokinesis),
when the cell splits in two, distributing its contents into two daughter
cells. Interphase is divided into two gap phases (G
1
and G
2
), when
specif
c molecules and structures duplicate, and a synthesis phase (S),
when DNA replicates. Mitosis can be considered in stages—prophase,
metaphase, anaphase, and telophase.
Mitosis is sometimes called cellular reproduction, because it results
in two cells From one—the cell reproduces. This may be conFusing,
because meiosis is the prelude to human sexual reproduction. Both
mitosis and meiosis are Forms oF cell
division,
with similar steps but
di±
erent outcomes, and occurring in di±
erent types oF cells.
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