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© Rafael Tormo Molina

Karyokinesis and Cytokinesis - Mitosis In 1888, the longitudinal structures that could be observed with the microscope during mitosis were named chromosomes. The state at the beginning of mitosis, when they become visible was termed prophase. After the prophase the chromosomes arrange at the equatorial plane, a state called metaphase. Subsequently the halves of the chromosomes, the chromatids, are torn to the two opposing cell poles. This phase is called anaphase. Finally, during telophase, the two daughter nuclei form. While the chromosomes condense, the spindle fibres are assembled within the plasma. They form a structure called the mitotic spindle that provides the means for the separation of the chromatides. It consists of two different types of fibres, the polar microtubules that stretch from pole to pole and the kinetochore microtubules, that connect pole and a certain area of the chromosome, the kinetochore. As the name says the fibres are composed of microtubules. Their subunit is tubuline.

Normally cytokinesis and karyokinesis (divisions of cell and nucleus) are coupled. Exceptions do exist, but they will be discussed elsewhere. The process of the division of the nucleus in plant cells was elucidated by E. STRASBURGER and published in his fundamental work "Zellbildung und Zellteilung" (Cell Formation And Cell Division) in 1875. Already in 1884 the results had been part of his 'Kleines botanisches Praktikum' (A Short Course on Botany) and since 1894 they were an established part of his "Lehrbuch der Botanik" (A Textbook on Botany). In the 4th edition the division of the nucleus is described as follows:

" Except for special and very limited cases plant cell nuclei propagate by the so-called mitotic or indirect division. The process is also called karyokinesis (today usually mitosis). It is a rather complicated process that seems to be necessary to distribute the substance of the mother nucleus evenly on both daughter nuclei."

At first STRASBURGER did work with material that had been fixed with alcohol. But in 1879 A. LUNDSTRÖM was able to investigate the mitosis of a living specimen, the anthers of Tradescantia. Modern microscopic techniques, especially phase contrast and interference contrast microscopy allow today to make the process of the division of the nucleus visible in a lot of cell types.

Impressing educational films for schools on this process exist, so that every student of biology should actually be familiar with it even before the beginning of his study.

STRASBURGER's possibilities were much humbler. Even though he did notice that the nuclei stretch before or at the beginning of mitosis and take on a spindly shape. Longitudinal structures become visible. He saw states, where they shortened and finally appeared as compact little rods. In 1888 they were termed chromosomes (Greek: chroma=colour; soma=body) by WALDEYER, because they were particularly well stained with a certain nuclear dye.

The state at the beginning of mitosis, where the chromosomes become visible was termed prophase after a suggestion of STRASBURGER. The German anatomist W. FLEMMING recognized in 1880 that the chromosomes of the prophase are characterized by a longitudinal gap.

After the prophase the chromosomes are arranged at the plane of cell division forming the equatorial plane. This state is called the metaphase. Both chromosome halves (the chromatids) are clearly recognizable. Consequently they separate in opposite directions to form the two daughter nuclei. The phase of separation is termed anaphase, the formation of the daughter nuclei telophase.

Other processes are also involved. While the chromatin becomes shorter, disentangles and separates into single chromosomes, fibrous looking aggregates form in the plasma that strech from pole to pole. They are called spindle fibres and the whole structure has the name mitotic spindle. The spindle is composed of undisrupted fibres (they are bundles or aggregates of overlapping microtubules) that stretch from pole to pole (polar microtubules) and of other fibres that connect the pole with a chromosome (kinetochore microtubules). The separation of the chromatids is caused by the contraction of the kinetochore microtubules. The kinetochore microtubule is attached to a certain area of the chromosome, the kinetochore, that each chromosome develops during late prophase . Depending on its location the typical V-, L-, U- or I-shaped structures of the anaphase form.

Fluorescence-tagged antibodies and the application of the Confocal Laser Scan Microscope allowed the impressive depiction of the centromeres. The chromosomes - stained by the fluorochrome propidium jodidine - can be seen in the left picture. Sample Lilium longiflorum. Photos by: T. SUZUKI, N. IDE, I. TANAKA, 1997

It is known today that the fibres contain tubuline, the subunit of microtubules. It can be stained selectively with fluorescence-tagged antibodies. The spindle fibres are in fact bundles of microtubules. During late anaphase the new cell wall becomes visible in the equatorial plane. The wall formation is completed during telophase.

In contrast animal cells divide by constriction of the mother cell. The decisive importance of mitosis in both cases is the qualitatively and quantitatively exact distribution of the hereditary material, whereby the longitudinal splaying represents the most important point. Each daughter cell does accordingly inherit one half of a chromosome, a chromatid. The number of chromosomes is usually specific for a certain species. This discovery was also made by E. STRASBURGER and the zoologist RABL from Prague showed that it applies to animal cells, too. Exceptions are caused, when single chromosomes stick together at their ends, whereby the number of chromosomes changes. Such changes together with the simultaneous genetic changes proved finally that the theory of chromosomes was right.