What Are the Phases of Mitosis (Cell Division) ? – Flashcards

Prophase is the first stage in mitosis, occurring after the conclusion of the G2 portion of interphase. During prophase, the parent cell chromosomes — which were duplicated during S phase — condense and become thousands of times more compact than they were during interphase. Because each duplicated chromosome consists of two identical sister chromatids joined at a point called the centromere, these structures now appear as X-shaped bodies when viewed under a microscope. Several DNA binding proteins catalyze the condensation process, including cohesin and condensin. Cohesin forms rings that hold the sister chromatids together, whereas condensin forms rings that coil the chromosomes into highly compact forms.
The mitotic spindle also begins to develop during prophase. As the cells two centrosomes move toward opposite poles, microtubules gradually assemble between them, forming the network that will later pull the duplicated chromosomes apart.

Prophase is the first stage in mitosis, occurring after the conclusion of the G2 portion of interphase. During prophase, the parent cell chromosomes — which were duplicated during S phase — condense and become thousands of times more compact than they were during interphase. Because each duplicated chromosome consists of two identical sister chromatids joined at a point called the centromere, these structures now appear as X-shaped bodies when viewed under a microscope. Several DNA binding proteins catalyze the condensation process, including cohesin and condensin. Cohesin forms rings that hold the sister chromatids together, whereas condensin forms rings that coil the chromosomes into highly compact forms.
The mitotic spindle also begins to develop during prophase. As the cells two centrosomes move toward opposite poles, microtubules gradually assemble between them, forming the network that will later pull the duplicated chromosomes apart.

When prophase is complete, the cell enters prometaphase — the second stage of mitosis. During prometaphase, phosphorylation of nuclear lamins by M-CDK causes the nuclear membrane to break down into numerous small vesicles. As a result, the spindle microtubules now have direct access to the genetic material of the cell.
Each microtubule is highly dynamic, growing outward from the centrosome and collapsing backward as it tries to locate a chromosome. Eventually, the microtubules find their targets and connect to each chromosome at its kinetochore, a complex of proteins positioned at the centromere. The actual number of microtubules that attach to a kinetochore varies between species, but at least one microtubule from each pole attaches to the kinetochore of each chromosome. A tug-of-war then ensues as the chromosomes move back and forth toward the two poles.

When prophase is complete, the cell enters prometaphase — the second stage of mitosis. During prometaphase, phosphorylation of nuclear lamins by M-CDK causes the nuclear membrane to break down into numerous small vesicles. As a result, the spindle microtubules now have direct access to the genetic material of the cell.
Each microtubule is highly dynamic, growing outward from the centrosome and collapsing backward as it tries to locate a chromosome. Eventually, the microtubules find their targets and connect to each chromosome at its kinetochore, a complex of proteins positioned at the centromere. The actual number of microtubules that attach to a kinetochore varies between species, but at least one microtubule from each pole attaches to the kinetochore of each chromosome. A tug-of-war then ensues as the chromosomes move back and forth toward the two poles.

As prometaphase ends and metaphase begins, the chromosomes align along the cell equator. Every chromosome has at least two microtubules extending from its kinetochore — with at least one microtubule connected to each pole. At this point, the tension within the cell becomes balanced, and the chromosomes no longer move back and forth. In addition, the spindle is now complete, and three groups of spindle microtubules are apparent. Kinetochore microtubules attach the chromosomes to the spindle pole; interpolar microtubules extend from the spindle pole across the equator, almost to the opposite spindle pole; and astral microtubules extend from the spindle pole to the cell membrane.
Metaphase leads to anaphase, during which each chromosomes sister chromatids separate and move to opposite poles of the cell. Enzymatic breakdown of cohesin — which linked the sister chromatids together during prophase — causes this separation to occur. Upon separation, every chromatid becomes an independent chromosome. Meanwhile, changes in microtubule length provide the mechanism for chromosome movement. More specifically, in the first part of anaphase — sometimes called anaphase A — the kinetochore microtubules shorten and draw the chromosomes toward the spindle poles. Then, in the second part of anaphase — sometimes called anaphase B — the astral microtubules that are anchored to the cell membrane pull the poles further apart and the interpolar microtubules slide past each other, exerting additional pull on the chromosomes

During telophase, the chromosomes arrive at the cell poles, the mitotic spindle disassembles, and the vesicles that contain fragments of the original nuclear membrane assemble around the two sets of chromosomes. Phosphatases then dephosphorylate the lamins at each end of the cell. This dephosphorylation results in the formation of a new nuclear membrane around each group of chromosomes.

Cytokinesis is the physical process that finally splits the parent cell into two identical daughter cells. During cytokinesis, the cell membrane pinches in at the cell equator, forming a cleft called the cleavage furrow. The position of the furrow depends on the position of the astral and interpolar microtubules during anaphase.
The cleavage furrow forms because of the action of a contractile ring of overlapping actin and myosin filaments. As the actin and myosin filaments move past each other, the contractile ring becomes smaller, akin to pulling a drawstring at the top of a purse. When the ring reaches its smallest point, the cleavage furrow completely bisects the cell at its center, resulting in two separate daughter cells of equal size
Figure 3: Mitosis: Overview of major phases
The major stages of mitosis are prophase (top row), metaphase and anaphase (middle row), and telophase (bottom row).
Figure 3 Details:
At the top of the diagram, a single circular cell is shown in prophase. The cell has a single round nucleus at its center, and within the nucleus are four chromosomes, each with two sister chromatids. Two of the chromosomes are green, and the other two are orange. Each chromatid is depicted as a thick line, and because the chromatids are connected at their middles, each chromosome looks like an "X". Outside the nucleus, two centrosomes are shown migrating to opposite sides of the cell. Two rectangular, white centrioles, which are oriented at right angles to each other, can be seen at the center of each centrosome, and thin, white, curved lines, which are microtubules, radiate outward from the centrosomes. In the middle left portion of the diagram is a cell in metaphase. The cells nuclear envelope has broken down and is no longer visible. The centrosomes are now at opposite sides of the cell; one centrosome is on the left, and the other is on the right. Some of the microtubules that originate at the centrosomes extend to the middle of the cell and form the mitotic spindle. The four chromosomes are lined up vertically in the middle of the cell and are attached to the spindle microtubules. From top to bottom, there is a green chromosome, an orange chromosome, a green chromosome, and an orange chromosome. Attachment to the spindle microtubules occurs at the kinetochore, which is located where the two sister chromatids are connected to each other. The sister chromatid on the left is attached via a single microtubule to the centrosome on the left, and the sister chromatid on the right is attached via a single microtubule to the centrosome on the right. The middle right portion of the diagram shows a cell in anaphase. The cell is very similar to the metaphase cell, except the two sister chromatids that made up each original chromosome have separated from each other and started to migrate toward the pole to which their microtubule is attached. The spindle microtubules that are attached to the newly separated chromosomes have shortened. The sister chromatids, which are now considered individual chromosomes, are "V" shaped as they are moving toward the centrosomes because they are being pulled by their kinetochores. At the bottom of the diagram is a telophase cell. Two nuclei have formed around each of the two sets of four separated chromosomes. Each nucleus contains two green chromosomes and two orange chromosomes. There is no longer a spindle, but the centrosomes still lie at opposite sides of the cell. The middle of the cell is slightly indented; it is pinching together because the cell is beginning cytokinesis.