Successive Levels of Chromosome Packaging
The importance of the organization of DNA into chromatin and of chromatin into mitotic chromosomes can be illustrated by considering that a human cell stores its genetic material in a nucleus about 5 to 10 µm in diameter.
In the overall transition from a fully extended DNA helix to the extremely condensed status of the mitotic chromosome, a packing ratio (the ratio of DNA length to the length of the structure containing it) of about 500 to 1 must be achieved.
First Level Packaging
During interphase, chromosomes exist as chromatin fibers composed of DNA and proteins (positively charged histones and less positively charged nonhistones). The chromatin fibers resemble beads (~100A0 diameter) on a string (~20A0 diameter).
The string is DNA while the bead is 147 bp length of DNA coiled 1.7 turns round a core composed of 8 histones (octamer) – two each of H2A, H2B, H3 and H4. The fifth type of histone, H1 lies outside the core.
Second Level Packaging
Further packaging of several (6?) 100A0-nucleosomes gives a solenoid of about 300A0. This further reduces the chromosomes length by 1/6.
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Third Level Packaging
In transition to mitosis, the 300A0 structure forms a series of looped domains of about 3,000A0 in diameter.
Fourth Level
The condensed fibers are further coiled into chromosome arms (of about 7000A0) that constitute a chromatid seen at mitotic metaphase.
At metaphase, the chromosomes are at their highest level of coiling and therefore appear more condensed, shorter and thicker than in any other stage.
This makes chromosomes most ideal for cytological study at metaphase because, they are most sharply defined at this stage.
Successive Levels of Chromosome Packaging
In summary, the enormous eukaryotic DNA should be well packaged in order to be contained in the nucleus. Thus, the highly condensed chromosome seen in metaphase is as a result of coiling and recoiling of chromatin which is made up of DNA and histones.
Eukaryotic chromatin is a nucleoprotein organized into repeating units called nucleosomes. Composed of 200 base pairs of DNA, an octamer of four types of histones, plus one linker histone, the nucleosome is important for condensing the extensive chromatin fiber within the interphase nucleus into the highly condensed chromosome seen in mitosis.
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