Download Web Tutorial 2.1: Mitosis and the Cell Cycle

WEB TUTORIAL 2.1
Mitosis and
the Cell Cycle
OVERVIEW
Many cells undergo a continuous alternation between division and nondivision called the
cell cycle. During mitosis, a cell divides to produce two daughter cells that contain the
same number of chromosomes as the parent cell. This tutorial demonstrates how mitosis
takes place and how it fits within the context of the cell cycle.
TEXTBOOK REFERENCES
Mitosis Partitions Chromosomes into Dividing Cells (p. 23)
Interphase and the Cell Cycle (pp. 23-24)
Interphase and Mitosis
The cell cycle can be divided into mitosis, the period during which the cell undergoes division, and interphase, the interval between mitotic divisions.
In the following animated sequences, we will describe the cell cycle using illustrations of
animal cells and photomicrographs of plant cells. Plants and animals differ in a few
aspects of cell division (e.g., plants lack structures called centrioles). However, the critical
genetics issue is what happens to the chromosomes-and in both plants and animals, chromosomes behave in essentially the same way during the cell cycle.
Interphase
Interphase can be divided into the stages G1, S, and G2. The cell spends most of its time
in interphase, during which it carries out metabolic functions and performs DNA synthesis in preparation for cell division. Cells also differentiate into various cell types during
interphase. Slowly dividing cells, or cells that require a specific stimulus to divide, enter a
stage known as the G0 state and become nondividing.
The image at lower left shows a cell of Haemanthus in interphase. Move the mouse over the
lower right corner of this and subsequent micrographs (+) to display labels.
Mitosis
Once the G1, S, and G2 stages of interphase are complete, the cell enters mitosis. For most
cells, mitosis is a relatively short phase compared with interphase, but its brevity belies its
importance. During mitosis, parent cells pass on their genetic information to daughter
cells with great precision, resulting in daughter cells that are genetically identical to each
other and to the parent cell.
Let us examine the different stages that make up mitosis: prophase, prometaphase,
metaphase, anaphase, and telophase.
Mitosis: Prophase
The first stage of mitosis is called prophase. Often over half of mitosis is spent in this initial phase. An early event in prophase is the migration of two pairs of centrioles and surrounding spindle fibers toward opposite ends of the cell.
Note: Centrioles, which are found only in animal cells, divide during G2 of interphase. In
plants, the spindle-organizing area lacks centrioles.
Early in prophase, the chromosomes are thin and dispersed in the nucleus. Although the
chromosomes were duplicated in the previous interphase, this duplication does not
become apparent until later in prophase.
At this stage, the nuclear envelope begins to break up and eventually disappears. The
genetic material in the nucleus is also transforming. The diffuse chromatin begins to condense, thicken, and become clearly visible.
Late in prophase, it becomes apparent that each chromosome is actually a double structure, the first visible evidence of the chromosome doubling that took place in interphase.
As the chromosomes continue to condense into rodlike structures, one can see that each
doubled chromosome is joined at a single point called a centromere. The two parts of each
chromosome are called chromatids. The DNA contained in each chromatid results from
the duplication of a single chromosome, so the chromatids of a pair are genetically identical. They are called sister chromatids.
Mitosis: Prometaphase
The next stage of mitosis is called prometaphase. In prometaphase, the centrioles take up
polar positions, and the spindle fibers extend into the nuclear region. The chromosomes
attach to the spindle fibers and begin migrating to the equatorial plane (also called the
metaphase plate) defined by the spindles at the poles.
Mitosis: Metaphase
Prometaphase becomes metaphase when the sister chromatids have aligned in the equatorial plane, attached through their centromeres to the spindle fibers.
Spindle fibers attach to structures called kinetochores that are associated with the centromeres of the chromosomes. A kinetochore consists of multilayer plates that form on
opposite sides of each centromere.
Mitosis: Anaphase
The next stage of mitosis is called anaphase. In anaphase, the sister chromatids that were
aligned along the equatorial plane during metaphase separate as the spindle fibers pull
them to opposite poles of the cell.
Mitosis: Telophase
Telophase is the final stage of mitosis. In telophase, the two sets of chromosomes at opposite poles are partitioned into two new cells. The most important event in telophase is formation of the nuclear membrane around the chromosomes at each pole, creating two
genetically identical daughter cells.
Cytokinesis, the division of the cytoplasm, may occur simultaneously with the nuclear
events of telophase, as shown here, or may be delayed. Occasionally, nuclei may divide
without cytoplasmic separation, forming a multinucleate cell.
The daughter cells make the transition from mitosis back to interphase. Generally, a reversal of the steps of prophase occurs here. In both new cells, the chromosomes begin to
uncoil and become diffuse chromatin again, the nuclear envelope re-forms around them,
and the cells enter interphase.
Mitosis in plant cells is somewhat different from that in animal cells because of the plant
cell's structural rigidity. During telophase, a cell plate is laid down dividing the two sets
of chromosomes. This will become the middle lamella; a cell wall develops between the
two daughter cells along the middle lamella.
Many plants (and fungi and algae) do not have centrioles, although spindle fibers are still
present during mitosis. Thus, it seems that centrioles are not always responsible for organizing spindle fibers.
CONCLUSION
The cell cycle consists of two phases: interphase and mitosis. During interphase, a cell
grows and duplicates its chromosomes in preparation for cell division. Mitosis is the period of cell division and is characterized by five distinct stages: prophase, prometaphase,
metaphase, anaphase, and telophase. Each stage is associated with specific events that
together lead to the production of two daughter cells that are genetically identical to the
parent cell.
YOU
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SHOULD NOW BE ABLE TO
Explain the role of mitosis in the context of the cell cycle.
Describe the key events that occur during each stage of mitosis.
Identify the phases and events of interphase.
KEY TERMS
anaphase
cell cycle
cell plate
centriole
centromere
cytokinesis
G0 phase
G1 phase
G2 phase
interphase
kinetochore
metaphase
metaphase plate
prometaphase
prophase
S phase
spindle fibers
telophase