Closed and Open Mitosis

Mitosis can proceed in two structurally different ways depending on what happens to the nuclear envelope. In closed mitosis, the nuclear envelope stays intact throughout division; the spindle forms inside the nucleus itself and moves the chromosomes to opposite poles while they remain enclosed the whole time (Figure 7.3). Closed mitosis is characteristic of many single-celled eukaryotes, including yeasts and slime moulds, for which keeping the nuclear contents separate from the cytoplasm even during division may be advantageous. In open mitosis, by contrast, the nuclear envelope breaks down before the spindle segregates the chromosomes, and it only re-forms afterwards, separately, around each of the two newly separated sets of chromosomes. Open mitosis is the pattern seen in most plants and animals, and it is the type described in detail in the rest of this chapter. One practical consequence of open mitosis worth noting is that some animals are able to regenerate whole body parts using it, since a fresh nuclear envelope can reliably re-form around each new cell's chromosome set as tissue is rebuilt. …

Prophase is the longest of the four phases of mitosis. At its start the chromosomes, which were dispersed and invisible during interphase, become visible as long, thin, thread-like structures that progressively condense into compact mitotic chromosomes. In plant cells, spindle-fibre initiation begins during prophase, the nucleolus disappears, and the nuclear envelope breaks down; the Golgi apparatus and endoplasmic reticulum are not visible as distinct organelles once mitosis is underway, since the cell's membrane systems are reorganised for division. In animal cells, the two centrioles that duplicated back in S phase move apart and each extends a radial array of microtubules outward towards the plasma membrane as they migrate to opposite poles of the cell; this star-shaped array of microtubules around each centriole pair is called an aster. Plant cells, which generally lack centrioles, do not form asters even thoug …

By metaphase, each chromosome - now consisting of two sister chromatids joined at the centromere - has become attached to spindle fibres via the kinetochore, a disc-like, trilaminar DNA-protein complex built on the centromeric DNA where the spindle microtubules physically dock (Figure 7.4 shows the inner and outer kinetochore layers, the fibrous corona, and the attached microtubule). The spindle fibres themselves are made of the protein tubulin. As every chromosome's kinetochores become correctly attached, the chromosomes are drawn into a tight, orderly band at the equator of the cell, called the metaphase plate; because the chromosomes are maximally condensed and aligned at this point, metaphase is the stage at which chromosome morphology (size, shape, centromere position) is easiest to study under the microscope, which is why karyotyping is normally done on metaphase chromosomes. The cell does not proceed blindly from here: the spindle assembly checkpoint monitors whether every chromosome's kinetochores are properly attached to spindle fibres from both poles, and only once this is sa …

At the start of anaphase, every chromosome splits simultaneously: each centromere divides longitudinally into two, freeing the two sister chromatids of every chromosome from one another, and the two now-independent daughter chromatids of each chromosome begin migrating towards opposite poles of the cell. This poleward movement is driven by a combination of shortening spindle fibres and the longitudinal splitting of the centromere, which together generate the pulling force that separates the two halves; each pole ends up receiving one full set of chromatids. The moment the sister chromatids separate marks the point at which the replicated genome has been fully and physically partitioned into two equal shares. The molecular trigger for this event is a ubiquitin ligase enzyme complex called the anaphase-promoting complex/cyclosome (APC/C, shown schematically in Figure 7.5): it targets key regulatory proteins - including the cohesin proteins that had been holding sister chromatids glued together - for degradation exactly at the metaphase-to-anaphase transition. By breaking down cohesin, APC/C releases the physical link between sister chr …

In telophase, the two sets of daughter chromosomes finally arrive at opposite poles of the cell and the mitotic spindle disappears, since its job of moving the chromosomes is now done. This marks the completion of karyokinesis, the division of the genetic material; cytokinesis, the division of the cytoplasm, is completed around the same time, and the nucleolus and nuclear membranes reform. A new nuclear envelope forms around each pole's set of chromatids, which - now that each has its own centromere and is once again enclosed in its own nucleus - are properly called chromosomes rather than chromatids from this point on; the chromosomes then decondense back into their dispersed, interphase-like state. In plant cells specifically, a structure called the phragmoplast forms between the two developing daughter cells, and a cell plate is laid down between them, along which a new cell wall is progressively reconstructed. Finally, the two daughter cells are physically separated as organ …