Over the years, there has been a great deal of speculation concerning the manner in which nucleosomes are folded into 30-nanometer fibers (Woodcock, 2005). Therefore, chromatin is further coiled into an even shorter, thicker fiber, termed the "30-nanometer fiber," because it is approximately 30 nanometers in diameter (Figure 4). 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. 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. The packaging of DNA into nucleosomes shortens the fiber length about sevenfold. Note that only eukaryotes (i.e., organisms with a nucleus and Proteins are shown in blue (H3), green (H4), yellow (H2A), and red Phosphodiester backbones of theĭNA double helix are shown in brown and turquoise, while histone The model of the nucleosome that crystallographers constructedįrom their data is shown in Figure 3. The subunits were later named nucleosomes (Oudet et al., 1975) and were eventually crystallized (Luger et al.,ġ997). The "subunit theory" of chromatin (Kornberg, 1974 Van Holde et al., 1974) was adopted.
When this was considered together withĭata from electron microscopy and chemical cross-linking of histones, The simplestĮxplanation for this observation is that chromatin possesses aįundamental repeating structure. That is a multiple of mononucleosomal DNA (Noll, 1974). Separated on a gel, a number of bands will appear, each having a length (Hewish and Burgoyne, 1973).If DNA from MNase-treated chromatin is then Point, the treated chromatin will consist of mononucleosomes,ĭinucleosomes (connected by linker DNA), trinucleosomes, and so forth Stop the reaction before every linker DNA has been cleaved. Of cutting that occurs after application of MNase, it is possible to Preferentially cutting the linker DNA between nucleosomes well before itĬuts the DNA that is wrapped around octamers. One suchĮnzyme, micrococcal nuclease (MNase), has the important property of With an enzyme that cuts DNA (such enzymes are called DNases). The amount of DNA per nucleosome was determined by treating chromatin Each chromosome is thus a long chain of nucleosomes, which gives the appearance of a string of beads when viewed using an electron microscope (Figure 2 Olins & Olins, 1974, 2003). This joining DNA is referred to as linker DNA. Therefore, every chromosome contains hundreds of thousands of nucleosomes, and these nucleosomes are joined by the DNA that runs between them (an average of about 20 base pairs).
The resulting 166 base pairs is not very long, considering that each chromosome contains over 100 million base pairs of DNA on average.
#Scaffold protein h1 full#
The addition of one H1 protein wraps another 20 base pairs, resulting in two full turns around the octamer, and forming a structure called a chromatosome (Box 4 in Figure 1). Today, researchers know that nucleosomes are structured as follows: Two each of the histones H2A, H2B, H3, and H4 come together to form a histone octamer, which binds and wraps approximately 1.7 turns of DNA, or about 146 base pairs. This experiment demonstrated that H2A, H2B, H3, and H4 form a discrete protein octamer, which is fully consistent with the presence of a repeating histone-containing unit in the chromatin fiber. Another clue came from chemically cross-linking (i.e., joining) histones in chromatin (Thomas & Kornberg, 1975). The observation by electron microscopists that chromatin appeared similar to beads on a string provided an early clue that nucleosomes exist (Olins and Olins, 1974 Woodcock et al., 1976). The basic repeating structural (and functional) unit of chromatin is the nucleosome, which contains eight histone proteins and about 146 base pairs of DNA (Van Holde, 1988 Wolffe, 1999).
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