98
UNIT 1
Organization of the Body
3
details of meiosis in Chapter 27. Here we concentrate on mi-
totic cell division.
Mitosis
Mitosis is the series of events that parcels out the repli-
cated DNA of the mother cell to two daughter cells. Described as
four phases—
prophase
,
metaphase
,
anaphase
, and
telophase
mitosis is actually a continuous process, with one phase merging
smoothly into the next. Its duration varies according to cell type,
but in human cells it typically lasts about an hour or less.
Focus
on Mitosis
(Figure 3.33)
, pp. 100–101, describes the phases of
mitosis in detail.
Cytokinesis
Cytokinesis, or the division of the cytoplasm,
begins during late anaphase and is completed aFer mitosis
ends. A
contractile ring
made of actin filaments (±igure 3.33)
draws the plasma membrane inward to form a
cleavage fur-
row
over the center of the cell. Te furrow deepens until it
pinches the cytoplasmic mass into two parts, yielding two
daughter cells. Each is smaller and has less cytoplasm than
the mother cell, but is genetically identical to it. Te daughter
cells then enter the interphase portion of the life cycle until it
is their turn to divide.
Cell Division
Cell division is essential for body growth and tissue repair. Cells
that continually wear away, such as cells of the skin and intesti-
nal lining, reproduce themselves almost continuously. Others,
such as liver cells, divide more slowly (to maintain the size of the
organ they compose) but retain the ability to reproduce quickly
if the organ is damaged. Most cells of nervous tissue, skeletal
muscle, and heart muscle lose their ability to divide when they
are fully mature, and repairs are made with scar tissue (a fibrous
type of connective tissue).
Events of Cell Division
In most body cells, cell division, which
is called the
M (mitotic) phase
of the cell cycle, involves two
distinct events (±igure 3.31, yellow area):
Mitosis
(mi-to
9
sis;
mit
5
thread;
osis
5
process), the divi-
sion of the nucleus
Cytokinesis
(si-to-kĭ-ne
9
sis;
kines
5
movement), the divi-
sion of the cytoplasm
A different process of nuclear division called
meiosis
(mi-
o
9
sis) produces sex cells (ova and sperm) with only half the
number of genes found in other body cells. We discuss the
Adenine
Thymine
Cytosine
Guanine
Old (template) strand
Two new strands (leading and lagging)
synthesized in opposite directions
DNA polymerase
DNA polymerase
Lagging
strand
Leading
strand
Replication
bubble
Free nucleotides
Old (parental) strand acts as a template for
synthesis of new strand
Chromosome
Enzymes unwind
the double helix and
expose the bases
Old DNA
Replication
fork
Figure 3.32
Replication of DNA: summary.
Once the DNA helix is uncoiled, and the hydrogen
bonds between its base pairs are broken, each nucleotide strand of the DNA acts as a template for
constructing a complementary strand, as illustrated on the right-hand side of the diagram. (The step
in which RNA primers are formed to start the process at replication bubbles is not shown.) DNA
polymerases work in one direction only, so the two new strands (leading and lagging) are synthesized in
opposite directions. (The DNA ligase enzymes that join the DNA fragments on the lagging strand are
not illustrated.) Each DNA molecule formed consists of one old (template) strand and one newly
assembled strand and constitutes a chromatid of a chromosome.
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