Packaging of DNA into nucleosomes
Nucleosomes form the fundamental repeating units of eukaryotic chromatin, which is used to pack the large eukaryotic genomes into the nucleus while still ensuring appropriate access to it. Nucleosomes are folded through a series of successively higher order structures to eventually form a chromosome.
The packaging of DNA into nucleosomes shortens the fiber length about sevenfold. In other words, a piece of DNA that is 1 meter long will become a "string-of-beads" chromatin fiber just 14 centimeters (about 6 inches) long.
Despite this shortening, a half-foot of chromatin is still much too long to fit into the nucleus, which is typically only 10 to 20 microns in diameter.
Therefore, chromatin is further coiled into an even shorter, thicker fiber, termed the "30-nanometer fiber," because it is approximately 30 nanometers in diameter.
Thus, the 30-nanometer fiber may be highly irregular and not quite the uniform structure depicted in textbooks. Interestingly, histone H1 is very important in stabilizing chromatin higher-order structures, and 30-nanometer fibers form most readily when H1 is present.
Processes such as transcription and replication require the two strands of DNA to come apart temporarily, thus allowing polymerases access to the DNA template. However, the presence of nucleosomes and the folding of chromatin into 30-nanometer fibers pose barriers to the enzymes that unwind and copy DNA.
It is therefore important for cells to have means of opening up chromatin fibers and/or removing histones transiently to permit transcription and replication to proceed.
Generally speaking, there are two major mechanisms by which chromatin is made more accessible: Histones can be enzymatically modified by the addition of acetyl, methyl, or phosphate groups or can be displaced by chromatin remodeling complexes, thereby exposing underlying DNA sequences to polymerases and other enzymes.
It is important to remember that these processes are reversible, so modified or remodeled chromatin can be returned to its compact state after transcription and/or replication are complete.