Download Objectives Key Terms The Mitosis Dance

Objectives
•
•
Summarize the major events that occur during each phase of mitosis.
Explain how cytokinesis differs in plant and animal cells.
Key Terms
•
•
•
•
•
•
•
spindle
centrosome
prophase
metaphase
anaphase
telophase
cell plate
You can think of mitosis as a lively "dance" of the chromosomes. Before the action
begins, the chromatin of each chromosome doubles during interphase. Then the
elaborately "choreographed" stages of the mitotic phase take place rapidly, distributing
the duplicate sets of chromosomes to two daughter nuclei. Finally, cytokinesis divides the
cytoplasm, producing two daughter cells. In this section you will read in more detail
about the processes that occur during mitosis.
The Mitosis Dance
During mitosis, the chromosomes' movements are guided by a football-shaped
framework of microtubules called the spindle. The spindle microtubules grow from two
centrosomes, regions of cytoplasmic material that in animal cells contain structures called
centrioles. The role of centrioles in cell division remains a mystery in spite of much
research into this question. Destroying them does not interfere with normal spindle
formation, and most plant cells lack them entirely.
Figure 9-8 reveals the events taking place within an animal cell during each phase of the
cell cycle. Although mitosis is a continual process, biologists divide the mitotic phase
into four main stages in order to study it: prophase, metaphase, anaphase, and telophase.
Try using different-colored toothpicks or pieces of yarn to represent chromosomes as you
follow the steps of the mitosis dance.
Figure 9-8
Mitosis begins after the chromosomes have duplicated in interphase and ends when
telophase is completed.
Interphase As you read in Concept 9.2, during interphase the cell is busy making new
molecules and organelles. The cell shown here is in late interphase (G2). By this time the
cell has duplicated its DNA. However, you can't see the individual chromosomes yet
because they are still loosely packed chromatin fibers. The presence of the nucleolus
indicates that the cell is still producing ribosomes.
Prophase In prophase, the first stage of mitosis, the chromosome "dancers" make their
appearance on the dance floor. In the nucleus, the chromatin fibers have condensed and
are thick enough to be seen with a light microscope. With high magnification, each
chromosome can be clearly seen now to consist of a pair of sister chromatids joined at the
centromere. The nucleolus disappears, and the cell stops making ribosomes. Late in
prophase, the nuclear envelope breaks down. Meanwhile, in the cytoplasm, a footballshaped structure called the mitotic spindle forms. The chromatids now attach to the
microtubules that make up the spindle. The spindle starts tugging the chromosomes
toward the center of the cell for the next step in the dance.
Metaphase During metaphase, the brief second stage, the chromosomes all gather in a
plane across the middle of the cell. The mitotic spindle is now fully formed. All the
chromosomes are attached to the spindle microtubules, with their centromeres lined up
about halfway between the two ends, or poles, of the spindle.
Anaphase Anaphase is the third stage of the mitosis dance. The sister chromatids
suddenly separate from their partners. Each chromatid is now considered a daughter
chromosome. Proteins at the centromeres help move the daughter chromosomes along the
spindle microtubules toward the poles. At the same time, these microtubules shorten,
bringing the chromosomes closer to the poles. However, spindle microtubules that are not
attached to centromeres do just the opposite—they grow longer, pushing the poles farther
apart.
Telophase and Cytokinesis The final stage of mitosis, telophase, begins when the
chromosomes reach the poles of the spindle. During this stage, the processes that
occurred in prophase are reversed. The spindle disappears, two nuclear envelopes reform
(one around each set of daughter chromosomes), the chromosomes uncoil and lengthen,
and the nucleoli reappear. Mitosis, the division of one nucleus into two genetically
identical daughter nuclei, is now finished.
Cytokinesis completes the cell division process by dividing the cytoplasm into two
daughter cells, each with a nucleus. Usually this process occurs along with telophase.
Cytokinesis in Animals and Plants
Cytokinesis, the actual division of the cytoplasm into two cells, typically occurs during
telophase. In animal cells, the first sign of cytokinesis is the appearance of an indentation
around the middle of the cell, as shown in Figure 9-8. This indentation is caused by a ring
of microfilaments in the cytoplasm just under the plasma membrane. The ring contracts
like the pulling of a drawstring, deepening the indentation and pinching the parent cell in
two. Because the two new nuclei are forming at the ends of the cell, cytokinesis results in
two new cells.
Cytokinesis in a plant cell occurs differently (Figure 9-9). A disk containing cell wall
material called a cell plate forms inside the cell and grows outward. Eventually this new
piece of cell wall divides the cell in two. The result is two daughter cells, each bounded
by its own continuous membrane and its own cell wall.
Figure 9-9
In a dividing plant cell the growing cell plate eventually fuses with the
plasma membrane of the parent cell, and the cell wall material joins the
existing cell wall. Two daughter cells result, each with its own plasma
membrane and cell wall.
Concept Check 9.3
1. Describe a significant event that occurs in each of the four stages of mitosis.
2. Compare and contrast cytokinesis in animal and plant cells.
3. In what sense may prophase and telophase in mitosis be characterized as opposites?
Copyright © 2006 by Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights reserved.