Metaphase chromosomes condensation

In the previous sections, we have seen that a regulated Increase in MPE activity induces entry Into mitosis. Presumably, the entry into mitosis Is a consequence of the phosphorylation of specific proteins by the protein kinase activity of MPE. Although many of the critical substrates of MPE remain to be Identified, we now know of examples that show how regulation by MPF phosphorylation mediates many of the early events of mitosis leading to metaphase chromosome condensation, formation of the mitotic spindle, and disassembly of the nuclear envelope (see Figure 20-29). 

Figure 36-3. Shown is the extent of DNA packaging in metaphase chromosomes (fop) to noted dupiex DNA (bottom). Chromosomai DNA is packaged and organized at severai ieveis as shown (seeTabie 36-2), Each phase of condensation or compaction and organization (bottom to top) decreases overaii DNA accessibiiity to an extent that the DNA sequences in metaphase chromosomes are aimost totaiiy transcriptionaiiy inert, in toto, these five ieveis of DNA compaction resuit in neariy a 10 -foid iinear decrease in end-to-end DNA iength. Compiete condensation and decondensation of the iinear DNA in chromosomes occur in the space of hours during the normai repiicative ceii cycie (see Figure 36-20).

It is interesting and important to note that several species, such as Schizosac-charomyces pombe, lack histone HI (Wood et al 2002) (see also section 2.4). The nucleosome-repeat length is slightly shorter in S. pombe than in human (Godde and Widom, 1992). The contribution of histone HI to the mitotic chromosome condensation has been examined with the use of a cell-free system of Xenopus eggs, in which the condensed sperm nuclei can be transformed into metaphase chromosomes. Even when histone HI is removed from the extract, the metaphase chromosomes can still be formed (Ohsumi et al, 1993). In addition, an elimination of all HI genes in Tetrahymena exerts no phenotypic effect (Shen et al, 1995). 

Uemura T, Ohkura H, Adachi Y, Morino K, Shiozaki K, Yanagida M (1987) DNA topoisomerase 11 is required for condensation and separation of mitotic chromosomes in S. pombe. Cell 50 917-925 Ushiki T, Hoshi O, Iwai K, Kimura E, Shigeno M (2002) The structure of human metaphase chromosomes its histological perspective and new horizons by atomic force microscopy. Arch Histol Cytol 65 377-390... 

In higher eukaryotes, at the onset of S phase cyclin A accumulates which stimulates DNA synthesis. The amount of cyclin A continues to be high after the S phase because of its role in chromosome condensation. Cyclin A is degraded when cells enter prometaphase. The level of another cyclin called cyclin B rises during G2 phase, which helps to complete the chromosome condensation and spindle assembly, which allow transition to metaphase. Cyclin B is degraded by APC during metaphase. ... 

Metaphase Cyclin B Chromosome condensation and spindle assembly 349... 

During mitosis, aU the DNA is highly condensed to allow separation of the sister chromatids. This is the only time in the ceE cycle when the chromosome structure is visible. Chromosome abnormalities may be assessed on mitotic chromosomes by karyotype analysis (metaphase chromosomes) and by banding techniques (prophase or prometaphase), which identify aneu-ploidy, translocations, deletions, inversions, and duplications. 

Figure I l-l. Compaction of DNA into a metaphase chromosome. Each phase of condensation or compaction reduces availability of DNA for replication or transcription to the point where a metaphase chromosome is nearly inert for these processes. The overall, packaging reduces the length of the DNA I0 -fold.

Considering the fact that the ensuing condensation of the chromatin fibre into the metaphase chromosome is achieved by further winding of the molecule, it is fair to assume that this follows a similar mechanism, creating a self-similar sequence, a cascade of Bonnet transformations [8]. 

Pairs of four different histones (H2A, H2B, H3, and H4) combine to form an eight-protein bead around which DNA is wound this bead-like structure is called a nucleosome (Figure 24-10). A nucleosome has a diameter of 10 nm and contains approximately 200 base pairs. Each nucleosome is linked to an adjacent one by a short segment of DNA (linker) and another histone (HI). The DNA in nucleosomes is further condensed by the formation of thicker structures called chromatin fibers, and ultimately DNA must be condensed to fit into the metaphase chromosome that is observed at mitosis (Figure 24-11). 

Hypothetical stages in the condensation of DNA with chromatin to form a chromosome, (a) Double-stranded DNA. (b)-(e) Formation of nucleosome beads and fibers consisting of histones and condensed DNA to form (f) a metaphase chromosome. These proposed intermediates are derived from dissociation reactions with intact chromosomes. 

For most cell types, mitosis typically lasts for less than Ih, and can be divided into prophase (chromosome condensation, growth of mitotic spindle and breakdown of nuclear envelope), metaphase (alignment of chromosomes along the equator of the parent cell), anaphase (separation of chromosomes) and telophase (formation of new nuclear membrane around chromosomes). Finally, furrowing and division of the cell takes place during cytokinesis to yield two distinct daughter cells. 

These differences in transcriptional activity between the male and female pronuclei likely reflect the different origins of the male and female pronuclei that may lead to intrinsic underlying differences in nuclear/chromatin structure. Such differences may account for the observation that fusion of zygotic halves containing either a female or male pronucleus with a metaphase Il-arrested egg resulted in a more rapid premature chromosome condensation of the maternal chromatin, which also achieved a greater degree of condensation than its paternal counterpart (Ciemerych and Czolowska, 1993). 

Each eukaryotic chromosome contains a single DNA molecule packaged into nucleosomes and folded into a 30-nm chromatin fiber, which is attached to a protein scaffold at specific sites (see Figure 10-24). Additional folding of the scaffold further compacts the structure into the highly condensed form of metaphase chromosomes. 

When metaphase chromosomes decondense during in-terphase, certain regions, termed heterochromatin, remain much more condensed than the bulk of chromatin, called euchromatin. 

This cultured rat kidney cell in metaphase shows condensed chromosomes (blue), microtubules of the spindle apparatus (red), and the inner nuclear envelope protein POM121 (green). The POM121 staining demonstrates that the inner nuclear envelope proteins retract into the ER during mitosis. 

Fig. 1. Hierarchy of chromatin folding in the nucleus, during interphase, chromosomes are spread in a diffuse form and the nucleoli appear as dense structures. During metaphase chromosomes which consist of condensed sections of chromatin are formed. Chromatin in interphase nuclei and in the condensed sections of chromosomes contains looped domains which are formed by the 30 nm chromatin fibre. In vitro it is possible to further unfold the 30 nm fibre to a 3-D zig-zag structure revealing the nucleosomes that consist of histone HI, the core particle and the linker DNA...

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