DNA covered with histones and twisted into a series of nucleosomes resembles a string of beads and is sometimes called the "beads on a string" form. However, the folding process continues. The chain of nucleosomes is wound into a giant helical structure with six nucleosomes per turn. It is now known as the 30 nanometer fiber (Fig. 4.26). In turn, these fibers are looped back and forth. The loops vary in size, averaging about 50 of the helical turns (i.e., about 300 nucleosomes) per loop. The ends of the loops are attached to a protein scaffold, or chromosome axis.
Further folding of chromosomes occurs in preparation for cell division. The precise nature of this is uncertain, but condensed mitotic chromosomes are 50,000 times shorter than fully extended DNA. Highly condensed chromatin is known as hete-rochromatin, appearing dense in the light and electron microscope. In this form it cannot be transcribed (see Ch. 10 for discussion). [Note that some regions of DNA (e.g., satellite DNA near the centromeres) are always found as heterochromatin whereas active regions of the genome condense into heterochromatin during cell division.] An overall summary of DNA folding is presented in Figure 4.27.
Between cell divisions, regions of heterochromatin persist around the centromere and at the ends of the chromosome. These regions include the satellite DNA discussed above and make up about 10 percent of the chromosome. The rest of the chromatin, the euchromatin, is in the more extended form shown as a string of beads in panels B and C of Figure 4.27. About 10 percent of this euchromatin is even less condensed and is either being transcribed or is accessible for transcription in the near future (see Ch 10 for details).This is the "active chromatin." During both replication and transcription, the histones are temporarily displaced from short regions of the DNA. After the synthetic enzymes have passed by, the histone cores reassemble on the DNA.
30 nanometer fiber Chain of nucleosomes that is arranged helically, approximately 30 nm in diameter euchromatin Normal chromatin, as opposed to heterochromatin heterochromatin A highly condensed form of chromatin that cannot be transcribed because it cannot be accessed by RNA polymerase
Eukaryotic DNA is so long that it needs several successive levels of folding to fit into the nucleus.
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