HOW IS THE DNA IN EUKARYOTIC CHROMOSOMES ORGANIZED?

HOW IS THE DNA IN EUKARYOTIC CHROMOSOMES ORGANIZED?

1. The length of DNA in the nucleus is far greater than the size of the compartment in which it is contained. To fit into this compartment the DNA has to be condensed in some manner.

2. DNA is not packaged directly into final structure of chromatin. Instead, it contains several hierarchies of organization. The first level of packing is achieved by the winding of DNA around a protein core to produce a "bead-like" structure called a nucleosome.

   • This gives a packing ratio of about 6. This structure is invariant in both the euchromatin and heterochromatin of all chromosomes.

3. The second level of packing is the coiling of beads in a helical structure called the 30 nm fiber that is found in both interphase chromatin and mitotic chromosomes. This structure increases the packing ratio to about 40.

4. The final packaging occurs when the fiber is organized in loops, scaffolds and domains that give a final packing ratio of about 1000 in interphase chromosomes and about 10,000 in mitotic chromosomes.

5. Eukaryotic chromosomes consist of a DNA-protein complex that is organized in a compact manner which permits the large amount of DNA to be stored in the nucleus of the cell.

6. The subunit designation of the chromosome is chromatin. The fundamental unit of chromatin is the nucleosome.

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