Cell Division

CELL DIVISION

Cell division, or mitosis (Gr. mitos, a thread), can be observed with the light microscope. During this process, the parent cell divides, and each of the daughter cells receives a chromosomal set identical to that of the parent cell. Essentially, a longitudinal duplication of the chromosomes takes place, and these chromosomes are distributed to the daughter cells. The period between mitoses is called interphase, during which the DNA is replicated and the nucleus appears as it is most commonly seen in histological preparations. The process of mitosis is subdivided into four phases (Figures 3–14 and 3–15).

Figure 3–14.

Phases of mitosis.

Chromosomal changes during mitosis are easily seen and most commonly studied in large cultured cells or in the large cells in the very early embryos of invertebrates or primitive vertebrates after sectioning. Shown here are cells in sections of a fish blastodisc. a. During the relatively long prophase the centrosomes move to opposite poles, the nuclear envelope fragments, and chromosomes condense and become visible. Having undergone DNA replication, each chromosome consists of two chromatids joined at their centromere regions by a kinetochore protein complex. b. At the short metaphase the chromosomes have become aligned at the equatorial plate as a result of their attachments to the dynamic microtubules which run from the kinetochores to the centrosomes. c. During anaphase the kinetochores come apart and the chromatids (now called chromosomes themselves) are pulled on microtubules toward the two centrosomes. d. In telophase the cell pinches in two by constriction of bundled actin filaments in the cell cortex and the chromosomes decondense, transcription resumes, nucleoli reappear, and the nuclear lamina and nuclear envelopes reassemble. X600. H&E.

Figure 3–15.

Confocal immunofluorescent images of mitotic cells.

Images obtained with a confocal laser scanning microscope from cultured cells in various phases of mitosis. Chromosomes are stained orange and microtubules, green. (a): Prophase: The chromosomes have undergone DNA replication and each consists of two very close sister chromatids. Two microtubule-organizing centers, the centrosomes, have moved apart and each is associated with microtubules forming the mitotic spindle. (b): Prometaphase: Chromosomes attach to spindle microtubules at their kinetochores and begin to be moved. (c): Metaphase: Chromosomes have become aligned at the middle of the spindle, near the cell equator. Kinetochore microtubules attach to each sister chromatid and to opposite poles of the spindle. (d): Anaphase: Sister chromatids separate from each other to become individual chromosomes which are pulled toward the spindle poles. The poles move apart and the kinetochore microtubules get shorter. (e): Telophase: The two sets of daughter chromosomes arrive at the spindle poles. (f): Late telophase and cytokinesis: A contractile ring of myosin-associated actin filaments forms a cleavage furrow that pinches the cell into two daughter cells, each with one nucleus and a complete set of chromosomes ready to undergo another round of DNA replication. (With permission, from Julie C. Canman and Ted Salmon, Department of Biology, University of North Carolina at Chapel Hill.)

In prophase of mitosis the replicated chromatin condenses into discrete rod-shaped bodies, the chromosomes, each consisting of duplicate sister chromatids closely associated longitudinally. Outside the nucleus, the centrosomes with their centrioles separate and migrate to opposite poles of the cell. The duplication of the centrosomes and centrioles occurs during interphase. Simultaneously with the centrosome migration, the microtubules of the mitotic spindle appear between the two centrosomes and the nucleolus disappears as transcriptional activity there stops. Late in prophase, the nuclear envelope breaks down when proteins of

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