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The Cell Cycle
All reproduction begins at the cellular level. The pro-cess
by which a cell divides into two and duplicates its genetic-
material is called the cell cycle (Figure 4-10). The cell
cycle is divided into three main stages: interphase (the
stage in which great activity is occurring but this activity- is
not visible; thus, this stage used to be called a “resting
stage”), mitosis, and cytokinesis. Two of these three stages
have substages. We shall discuss all stages in detail. The
time to complete a cell cycle will vary greatly among
different organisms. Cells in a devel-oping embryo will
complete the cell cycle in less than 20 minutes. A dividing
mammalian cell will complete the cycle in approximately
24 hours. Other cells in our bodies rarely duplicate and
undergo the cell cycle, such as nerve cells and muscle cells.
Human liver cells will divide- only if damaged. They
usually have cell cycles lasting a full year.
Interphase
Refer to Figure 4-10 for illustrations of the stages of the
cell cycle discussed below. A cell spends most of its time in
the stage of the cell cycle known as interphase. This
phase, the longest and most dynamic part of a cell’s life, is
not part of cell division. In fact, interphase means between-
phases. Yet during this time the cell is growing,
metabolizing, and maintaining itself. During-
Chapter 4this time, the nucleus is seen as a distinct structure
surrounded- by its nuclear membrane. Inside the nu-
cleoplasm the unwound- strands of chromosomes are only
visible as dark threads called chromatin (KRO-mah-tin).
Interphase- has three subphases: growth one (G 1 ),
synthesis- (S), and growth two (G 2 ). Some authors called
the G phases gap one and gap two.
G 1 is the primary growth phase of the cell. It
occupies- the major portion of the life span of the cell.
The synthesis or S phase is when the strands of DNA
duplicate themselves. Each chromosome now consists of
two sister chromatids attached to each other at a central
region called the centromere but are not yet visible. Most
chromosomes consist of 60% protei n and 40% DNA.
The G 2 phase is the final phase for the preparation of
cell division. In animal cells, the centrioles begin move-
ment to the opposite poles of the cell. Mitochondria are
replicated as the chromosomes now condense and coil into
tightly compacted bodies. Tubulin is synthesized. This is
the protein material that forms the microtubules and
assembles at the spindle.Mitosis
Mitosis is the process of cellular reproduction that occurs in
the nucleus and forms two identical nuclei. Because of the
intricate movement of daughter chromosomes as they
separate, this phase of the cell cycle has received a great
deal of study by biologists. This phase can also be easily
observed with a light microscope. Although mi-(^) eco
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tosis is a continuous process, it is subdivided into four stages:
prophase , metaphase, anaphase, and telophase
(TELL-oh-faze). Refer to Figure 4-11 for an illustration of
the stages of mitosis. The cells resulting from mitosis are
exact duplicates or clones of the parent cell.
Media Link
Watch an animation on mitosis
on the Student- Companion
Website..
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Prophase II.
The coiled, duplicated chromosomes have shortened and
thickened and are now visible. Each chromosome con-sists
of two sister or daughter chromatids. The sister chro-matidsremain attached to one another at the -centromere^
Figure 4- 10 The stages of the cell
cycle.
(SIN-troh-meer). The centromere is a constricted or