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Modern Genetic Analysis Chapter 4. The Inheritance of Genes Cell Division
Genetics In Process 4-2: Mitosis

Mitosis (M) is usually the shortest segment of the cell cycle, lasting for approximately 5 to 10 percent of the cycle. DNA synthesis takes place during the S period. G1 and G2 are gaps between S and M. Together, G1, S, and G2 constitute interphase, the time between mitoses. (Interphase used to be called "resting period"; however, cells are active in many ways during interphase, not the least of which, of course, is DNA replication.) The chromosomes cannot be seen during interphase (part a of the figure), mainly because they are in an extended state and are intertwined with one another like a tangle of yarn.

For the sake of study, biologists divide mitosis into four stages called prophase, metaphase, anaphase, and telophase. It must be stressed, however, that any nuclear division is a dynamic process on which we impose such arbitrary stages only for our own convenience.

Prophase.

The onset of mitosis is heralded by the chromosomes' becoming distinct for the first time (part b). They get progressively shorter through a process of contraction, or condensation, into a series of spirals or coils; the coiling produces structures that are more easily moved around. As the chromosomes become visible, they appear double-stranded, each chromosome being composed of two longitudinal halves, the sister chromatids (part c). The two chromatids formed by one chromosome each contain one of the replicated DNA molecules. Because of semiconservative replication these replicate DNA molecules are each "half old and half new"; that is, in each double helix one of the nucleotide strands is newly polymerized. Nevertheless, despite being half old and half new, they are chemically identical. These sister chromatids are joined at a region called the centromere. At this stage the centromere has already divided into a pair of sister centromeres. The nucleolithe large intranuclear spherical structures that contain ribosomal componentsdisappear at this stage. The nuclear membrane begins to break down, and the nucleoplasm and cytoplasm become one.

Metaphase.

At this stage, the nuclear spindle becomes prominent. The spindle is a birdcage-like structure that forms in the nuclear area; it consists of a series of parallel proteinaceous fibers that point to each of two cell poles. These spindle fibers are polymers of a protein called tubulin. The chromosomes move to the equatorial plane of the cell, where one sister centromere becomes attached to a spindle fiber from one pole; the other sister centromere, to the other pole (part d).

Anaphase.

This stage begins when the pairs of sister chromatids separate, one of a pair moving to each pole (part e). The centromeres, which now clearly appear to have divided, separate first. As each chromatid moves, its two arms appear to trail its centromere; a set of V-shaped structures results, with the points of the V's directed at the poles.

Telophase.

Now a nuclear membrane re-forms around each daughter nucleus, the chromosomes uncoil, and the nucleoli reappearall of this effectively re-forming the interphase nuclei (part f). By the end of telophase, the spindle has dispersed, and the cytoplasm has been divided into two by a new cell membrane.

In each of the resultant daughter cells, the chromo-some complement is identical with that of the original cell. Of course, what were referred to as chromatids now take on the role of full-fledged chromosomes in their own right.

© 1999 by W. H. Freeman and Company. All rights reserved.