Stages Of Mitosis

Mitosis is the division of the nucleus, which occurs during cell division. Mitosis is a continuous process that allows for the organized distribution of a cell's copied DNA to offspring cells. The process of mitosis is usually divided into four phases for ease of understanding: prophase, metaphase, anaphase, and telophase.

Prophase

Prophase is the first phase of mitosis. Prophase, shown in step Q of Figure 8-6, begins with the shortening and tight coiling of DNA into rod-shaped chromosomes that can be seen with a light microscope. Recall that during the S phase, each chromosome is copied. The two copies of each chromosome—the chromatids—stay connected to one another by the centromere. At this time, the nucleo-lus and the nuclear membrane break down and disappear.

Two pairs of dark spots called centrosomes appear next to the disappearing nucleus. In animal cells, each centrosome contains a pair of small, cylindrical bodies called centrioles. The centrosomes of plant cells lack centrioles. In both animal and plant cells, the centrosomes move toward opposite poles of the cell during prophase.

As the centrosomes separate, spindle fibers made of micro-tubules radiate from the centrosomes in preparation for metaphase. This array of spindle fibers is called the mitotic spindle, which serves to equally divide the chromatids between the two offspring cells during cell division. Two types of spindle fibers make up the mitotic spindle: kinetochore fibers and polar fibers. Kinetochore fibers attach to a disk-shaped protein—called a kinetochore—that is found in the centromere region of each chromosome. Kinetochore fibers extend from the kinetochore of each chromatid to one of the centrosomes. Polar fibers extend across the dividing cell from centrosome to centrosome but do not attach to the chromosomes.

Spindle microtubules

Spindle microtubules

figure 8-7

This micrograph of the spindle apparatus during metaphase shows the kinetochore fibers moving the chromosomes to the center of the dividing cell. The wormlike structures in the center are the chromosomes. (LM 1,080x)

Metaphase

Metaphase, as shown in step © of Figure 8-6, is the second phase of mitosis. During metaphase, chromosomes are easier to identify by using a microscope than during other phases; thus, karyotypes are typically made from photomicrographs of chromosomes in metaphase. As shown in Figure 8-7 above, the kinetochore fibers move the chromosomes to the center of the dividing cell during metaphase. Once in the center of the cell, each chromosome is held in place by the kinetochore fibers.

Anaphase

During anaphase, shown in step © of Figure 8-6 on the previous page, the chromatids of each chromosome separate at the centromere and slowly move, centromere first, toward opposite poles of the dividing cell. After the chromatids separate, they are considered to be individual chromosomes.

Telophase

Telophase is shown in step © in Figure 8-6 on the previous page. After the chromosomes reach opposite ends of the cell, the spindle fibers disassemble, and the chromosomes return to a less tightly coiled chromatin state. A nuclear envelope forms around each set of chromosomes, and a nucleolus forms in each of the newly forming cells.

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