Info

Microtubule Dynamics and Motor Proteins in Mitosis

Mitosis is the process that partitions newly replicated chromosomes equally into separate parts of a cell. The last step in the cell cycle, mitosis takes about 1 hour in an actively dividing animal cell (see Figure 1-17). In that period, the cell builds and then disassembles a specialized microtubule structure, the mitotic apparatus. Larger than the nucleus, the mitotic apparatus is designed to attach and capture chromosomes, align the chromosomes, and then separate them so that the genetic material is evenly partitioned to each daughter cell. Fifteen hours later, the whole process is repeated by the two daughter cells.

Figure 20-29 depicts the characteristic series of events that can be observed by light microscopy in mitosis in a eu-karyotic cell. Although the events unfold continuously, they are conventionally divided into four substages: prophase, metaphase, anaphase, and telophase. The beginning of mitosis is signaled by the appearance of condensing chromosomes, first visible as thin threads inside the nucleus. By late prophase, each chromosome appears as two identical filaments, the chromatids (often called sister chromatids), held together at a constricted region, the centromere. Each chro-matid contains one of the two new daughter DNA molecules produced in the preceding S phase of the cell cycle; thus each cell that enters mitosis has four copies of each chromosomal DNA, designated 4n.

1 Centrosomes

Sister chromatids

(c) Prometaphase (d) Metaphase

Spindle poles _/ Kinetochore

Sister chromatids

(c) Prometaphase (d) Metaphase

Spindle poles _/ Kinetochore

(e) Anaphase

(f) Telophase

(e) Anaphase

(f) Telophase

▲ FIGURE 20-29 The stages of mitosis and cytokinesis in an animal cell. For simplicity, only two sets of duplicated chromosomes, distinguished by color, are depicted. (a) Interphase (G2): DNA and centrosome replication. After DNA replication during the S phase, the chromosomes, each containing a sister chromatid, are decondensed and not visible as distinct structures. By G2 the centrioles have replicated to form daughter centrosomes.

(b) Prophase: centrosome migration. The centrosomes, each with a daughter centriole, begin moving toward opposite poles of the cell. The chromosomes begin to condense, appearing as long threads.

(c) Prometaphase: spindle formation. The nuclear envelope fragments into small vesicles and spindle microtubules enter the nuclear region. Chromosome condensation is completed; each visible chromosome is composed of two chromatids held together at their centromeres. Kinetochores at centromeres attach chromosomes to spindle microtubules. (d) Metaphase: chromosome alignment. The chromosomes move toward the equator of the cell, where they become aligned in the equatorial plane. (e) Anaphase: chromosome separation. The two sister chromatids separate into independent chromosomes. Each chromosome, attached to a kinetochore microtubule, moves toward one pole. Simultaneously, the poles move apart. (f) Telophase and cytokinesis. Nuclear membranes re-form around the daughter nuclei; the chromosomes decondense and become less distinct. The spindle disappears as the microtubules depolymerize, and cell cleavage proceeds. (g) Interphase (G1): Following cleavage, the daughter cells enter G1 of interphase.

Qijj

m c ai EsM

Your Heart and Nutrition

Your Heart and Nutrition

Prevention is better than a cure. Learn how to cherish your heart by taking the necessary means to keep it pumping healthily and steadily through your life.

Get My Free Ebook


Post a comment