Chromatin structure
DNA and its associated proteins, the chromatin, are organized into a complex structure with varying degrees of condensation. In its most available, open state, the DNA strands are unwound as transcription occurs.
At other times, the fundamental basic structural units of chromatin are the nucleosomes. These highly repetitive units are made of clusters or cores of eight histone proteins oriented in repeating fashion along the DNA strand. If the DNA strand is envisioned as a ribbon, the histone clusters can be imagined as large Velcro-covered beads attached to the ribbon. Now imagine the ribbon with attached beads being wound into a repeating coil. This highly ordered structure allows a physical mechanism for the very long linear arrays of DNA to be compacted inside the nucleus but at the same time maintain critical orderliness. In addition to the physical aspects, the histones are also important regulators of gene expression. For example, changes in the methylation or phosphorylation of the histones bound to the DNA modify their capacity to sequester or bind the DNA. If the histones in a particular nucleosome become dissociated with the DNA, this would increase the opportunity for the DNA in that region to be available for transcription. Responses of some target cells to hormone stimulation are known to cause the synthesis of new proteins. Corresponding with this, many of these hormones also alter rates of methylation of nuclear proteins. This suggests that gene activation must ultimately depend on regulatory molecules that modify interactions between the histones and other nuclear proteins that function to control chromatin structure. The organization is illustrated in Figure 2.14.
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