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Cell Division: Eukaryotes
Nuclear Division
Metaphase

the events

The three main phases of a single cell cycle are: interphase, nuclear division and cytoplasmic division.

Nuclear Division

With the right techniques, the next stage in the cell cycle, mitosis (M), can be observed using a good light microscope. Early microscopists found it convenient to subdivide the nuclear division of cells into stages that were easily seen under the microscope using colored dyes that stained the chromosomes and some of the other participants.

Nuclear division or karyokinesis is a continuous process, however, and there are no artificial divisions in actively growing cells.

prophase - Metaphase

A dividing cell is generally considered to be in metaphase when the mitotic spindle is completely formed. When viewed using a light microscope, the "spindle" (named after a device used for spinning thread) looks like a hairy, elongated ball originating (in animal cells) from the asters around the centrioles, or from opposite sides of the plant cell.

Very little detail can be seen, but there is a clear attachment to the chromosomes.

Using more powerful techniques, it is now possible to show that the spindle consists of a series of microtubule threads, each about 25 nm in diameter, and consisting of proteins that strongly resemble those found in muscle.

Although it is probably an artificial distinction, these spindle microtubules can be divided into three types:

  • astral - those tubules that extend out from the region surrounding the centrioles, the microtubule-organizing center (MTOC) and point away from the chromosomes.

  • kinetochore - those tubules that attach to the chromosomes at special regions called kinetochores.

  • interpolar - those tubules that extend between the poles of the cell (and/or the centrioles), or about half way across the cell (very few of the tubules extend the full distance across the cell), without coming in contact with the chromosomes.

Microtubules

The microtubules that make up the spindle apparatus in a dividing cell are not static "ropes", but very dynamic structures that are constantly forming and breaking down.

Use of a special, polarizing microscope shows that spindle fibers are constantly being formed, and then broken down again. Protein subunits, present in the cytoplasm and made in G2 phase, suddenly come together to form the fibers that connect to the poles and to the chromosomes, and just as suddenly depolymerize and vanish.

It is thought that these fibers, which are important in moving the parts of the chromosomes around inside the cell, "pull" on their cargoes by adding new elements at one end of the fiber, while removing similar elements from the other end of the fiber.

In this way the spindle fiber appears to move, or "pull" the chromosome around. This process can be stopped by adding drugs such as colchicine, when the spindle appears to "freeze" in place.

Kinetochore

Some of the microtubules stretch from the MTOC region directly to the individual chromosomes. Each half of the chromosome, the sister chromatids, has a granule located somewhere near the centromere. This is the kinetochore, and is the place where the spindle microtubules attach.

The microtubules attached to the kinetochore and the MTOC shorten and pull on the chromosome. Since the chromosome is attached to two sets of fibers, pulling in opposite directions, the whole chromosome structure is tugged into the center of the cell, where all of the kinetochores of all the chromosomes line up.

The alignment of all the chromosomes in the center of the cell (the equatorial plane) is a very distinctive event in metaphase.

The kinetochores are not the same as the centromeres of the chromosome. While they are located in about the same position, the centromere is a constriction in the chromosome where the DNA is enriched in the A+T bases. This allows the proteins of the kinetochore to form, which in turn attach to the spindle fibers.

At the end of metaphase, all the kinetochores are lined up on the equatorial plane, facing in opposite directions, and attached to the opposite poles of the cell by means of spindle fibers. The arms of the sister chromatids crowd together in the center of the cell. All is now ready for the next phase.


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© 2001, Professor John Blamire