Download Chapter 3 Divide and Conquer: Cellular Mitosis

Chapter 3
Divide and Conquer: Cellular Mitosis
In This Chapter
䊳 Following the steps of cell division
䊳 Understanding the results of errors in mitosis
E
ver had so many places to be that you wished you could just divide yourself in two?
Your cells already do that. Cell division is how one “mother” cell becomes two identical
twin “daughter” cells. Cell division takes place for several reasons:
⻬ G r owth: Multicellular organisms, humans included, each start out as a single cell —
the fertilized egg. That one cell divides (and divides and divides), eventually becoming
an entire complex being.
⻬ Injury repair: Uninjured cells in the areas surrounding damaged tissue divide to
replace those that have been destroyed.
⻬ Replacement: Cells eventually wear out and cease to function. Their younger, more
functional neighbors divide to take up the slack.
⻬ Asexual reproduction: No, human cells don’t do this. Only single-celled organisms do.
Cell division occurs over the course of two processes: mitosis, which is when the chromosomes within the cell’s nucleus duplicate to form two daughter nuclei; and cytokinesis, which
takes place when the cell’s cytoplasm divides to surround the two newly formed nuclei.
Although cell division breaks down into several stages, there are no pauses from one step to
another. Cell division as a whole is called mitosis because most of the changes occur during
that process. Cytokinesis doesn’t start until later. But mitosis and cytokinesis do end together.
Keep in mind: Cells are living things, so they mature, reproduce, and die. In this chapter, we
review the cell cycle (as mitosis also is known), and you get plenty of practice figuring out
what happens when and why.
The Mitotic Process
It may look like cells are living out their useful lives simply doing whatever specialized jobs
they do best, but in truth mitosis is a continuous process. When the cell isn’t actively splitting itself in two, it’s actively preparing to do so. DNA and centrioles are being replicated,
and the cell is bulking up on cytoplasm to make sure there’s enough for both daughter cells.
Mitosis may look like a waiting game, but there’s plenty going on behind the scenes.
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Part I: Building Blocks of the Body
Waiting for action: Interphase
Interphase is the period when the cell isn’t dividing. It begins when the new cells
are done forming and ends when the cell prepares to divide. Although it’s also called
a “resting stage,” there’s constant activity in the cell during interphase.
Interphase is divided into subphases, each of which lasts anywhere from a few hours
for those cells that divide frequently to days or years for those cells that divide less
frequently (nerve cells, for example, can spend decades in interphase). The subphases are as follows:
⻬ G1, which stands for “gap” or “growth.” During G1, the cell creates its organelles,
begins metabolism, grows, and synthesizes proteins.
⻬ S, which stands for “synthesis.” DNA synthesis or replication occurs during this
subphase. The single double-helix DNA molecule inside the cell’s nucleus
becomes two new “sister” chromatids, and the centrosome is duplicated.
⻬ G2, which stands for “gap.” Enzymes and proteins needed for cell division are
produced during this subphase.
Sorting out the parts: Prophase
As the first active phase of mitosis, prophase is when structures in the cell’s nucleus
begin to disappear, including the nuclear membrane (or envelope), nucleoplasm, and
nucleoli. The two centrioles that have formed from the centrosome push apart to
opposite ends of the nucleus. Using protein filaments, they form poles and a mitotic
spindle between them as well as asters (or astral rays) which radiate from the poles
into the cytoplasm. At the same time, the chromatin threads (or chromonemata)
shorten and coil, forming visible chromosomes. The chromosomes divide into chromatids that remain attached at an area called the centromere, which produces microtubules called kinetochore fibers. These interact with the spindle to assure that each
daughter cell ultimately has a full set of chromosomes. The chromatids start to
migrate toward the equatorial plane, an imaginary line between the poles.
Dividing at the equator: Metaphase
After the chromosomes are lined up and attached along the cell’s newly formed equator, metaphase officially debuts. The nucleus itself is gone. The chromatids line up
exactly along the center line of the cell (or the equatorial plane), attaching to the
mitotic spindle by the centromere. The centromere also is attached by microtubules to
opposite poles of the cell.
Packing up to move out: Anaphase
In anaphase, the centromeres split, separating the duplicate chromatids and forming
two chromosomes. The spindles attached to the divided centromeres shorten, pulling
the chromosomes toward the opposite poles. The cell begins to elongate. In late
anaphase, as the chromosomes approach the poles, a slight furrow develops in the
cytoplasm, showing where cytokinesis will eventually take place.
Chapter 3: Divide and Conquer: Cellular Mitosis
Pinching off: Telophase
Telophase occurs as the cell nears the end of division. The spindles and asters of early
mitosis disappear, and each newly forming cell begins to synthesize its own structure.
New nuclear membranes enclose the separated chromosomes. The coiled chromosomes unwind, becoming chromonemata once again. There’s a more pronounced
pinching, or furrowing, of the cytoplasm into two separate bodies, but there continues
to be only one cell.
Splitting up: Cytokinesis
Cytokinesis means it’s time for the big break-up. The furrow intended to divide the
newly formed sister nuclei at last gets to finish the job. It migrates inward until it
cleaves the single, altered cell into two new cells. Each new cell is smaller and contains
less cytoplasm than the mother cell, but the daughter cells are genetically identical to
each other and to the original mother cell.
Try this warm-up question on cell division:
Q.
Cell division takes place to
a. Repair injuries
b. Replace nonfunctioning cells
c. Grow the organism
d. All of the above
1.
Cells are dormant during interphase.
a. True
b. False
2.
The G1 subphase of interphase is
a. The period of DNA synthesis
b. The most active phase
c. The phase between S and G2
d. Part of cell division
3.
DNA is duplicated during which subphase?
a. S
b. G2
c. B
d. G1
A.
The correct answer is all of the
above. In addition, single-cell
organisms use cell division for
asexual reproduction.
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Part I: Building Blocks of the Body
4.
The nuclear membrane, or envelope, disappears during
a. Telophase
b. Metaphase
c. Prophase
d. Interphase
5.
Which of the following happens in prophase?
a. The chromatids align on the equatorial plane.
b. The chromosomes divide into chromatids.
c. The nucleus reappears.
d. The chromosomes move to opposite poles.
6.
Which of the following is true for metaphase?
a. The nuclear membrane appears.
b. The chromosomes move to the poles.
c. The chromatids align on the equatorial plane.
d. It’s composed of subphases G1, S, and G2.
7.
During metaphase, each chromosome consists of two duplicate chromatids.
a. True
b. False
8.
Identify an event that does not happen during anaphase.
a. Early cytokinesis occurs with slight furrowing.
b. The cell goes through subphase G1.
c. Spindles shorten.
d. The centromeres split.
9.
Genetically identical chromosomes are pulled toward opposite poles during
a. Telophase
b. Metaphase
c. Anaphase
d. Interphase
10.
Which event does not occur during telophase?
a. The chromosomes uncoil.
b. The chromosomes reach the poles.
c. The chromosomes become more distinct.
d. The nuclear membrane reforms.
11.
What structures disappear during telophase?
a. Spindles and asters
b. Nuclear membranes
c. Nucleolei
d. Chromonemata
Chapter 3: Divide and Conquer: Cellular Mitosis
12.
Which is the correct order of mitosis?
a. Prophase, interphase, metaphase, telophase, anaphase
b. Interphase, prophase, metaphase, anaphase, telophase
c. Metaphase, anaphase, telophase, interphase, prophase
d. Anaphase, metaphase, telophase, interphase, prophase
13.–24.
Use the terms that follow to identify the stages and cell structures shown in Figure 3-1.
13 ____
Nuclear pore
Nucleus
Nucleolus
19 ____
14 ____
15 ____
20 ____
Mitosis
Spindles
Fragments of
nuclear envelope
16 ____
17 ____
21 ____
18 ____
22 ____
Figure 3-1:
Cell structures and
changes
that make
up the
stages of
mitosis.
23 ____
24 ____
a. Anaphase
b. Centromere
c. Daughter cells
d. Chromatin
e. Cytokinesis
f. Telophase
g. Interphase
h. Chromosomes aligned at equator
i. Metaphase
j. Centrioles
k. Prophase
l. Chromatids
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Part I: Building Blocks of the Body
25.
The two newly formed daughter cells are
a. The same size as the mother cell
b. Not genetically identical to each other
c. Unequal in size
d. Genetically identical to the mother cell
26.
Cytokinesis can be described as
a. The period of preparation for cell division
b. The dividing of the cytoplasm to surround the two newly formed nuclei
c. The stage of alignment of the chromatids on the equatorial plane
d. The initiation of cell division
27.
Cytokinesis occurs during
a. Telophase
b. Interphase
c. Prophase
d. Metaphase
What Can Go Wrong
With the millions upon millions of cell divisions that happen in the human body, it’s
not surprising that sometimes things go wrong. An error during mitosis is called a
mutation. One kind of mutation is nondisjunction, or a failure to separate. In this mutation, newly formed chromosomes don’t quite divide, leaving one daughter cell with
one more chromosome than normal and the other daughter cell one chromosome shy
of a full complement. Down’s syndrome is an example of what happens when nondisjunction occurs. A normal human cell has 46 chromosomes, but that of a Down’s sufferer has 47.
Mitosis can also end up on fast-forward. Accelerated mitosis can lead to the formation
of a tumor, also called a neoplasm. The rate of division usually restricts itself to replacing worn out or injured cells, but with accelerated mitosis, the cells don’t know when
to stop dividing.
28.
Any change in a cell’s genetic information is known as
a. Phagocytosis
b. Mutation
c. Pinocytosis
d. Neurotransmission
Chapter 3: Divide and Conquer: Cellular Mitosis
29.–33.
Match the mitotic cell division stage with the appropriate activity.
29. _____ Chromatids line up along the equatorial plane
a. Anaphase
30. _____ Chromosomes contract and divide into chromatids
b. Prophase
31. _____ Nondividing nucleus
c. Metaphase
32. _____ Chromosomes enclosed again in nuclear
membrane at each pole
d. Interphase
e. Telophase
33. _____ Chromosomes attached to spindles, moving
to opposite ends of a molecule
34.
Use the space provided to draw a basic illustration of each of the six stages inside a cell
during mitosis.
Late interphase (1)
Prophase (2)
Metaphase (3)
Anaphase (4)
Telophase (5)
Cytokinesis (6)
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