Chromatin Structure

Chromatin is the substance which becomes visible chromosomes during cell division.  Its basic unit is nucleosome, composed of 146 bp DNA and eight histone proteins.  The structure of chromatin is dynamically changing, at least in part, depending on the need of transcription (see Chapter 4 Section G).  In the metaphase of cell division, the chromatin is condensed into the visible chromosome.  At other times, the chromatin is less condensed, with some regions in a "Beads-On-a-String" conformation.

Figure 3-D-1.  The condensed structure of chromatin.
(a) The 30 nm chromatin fiber is associated with scaffold proteins (notably topoisomerase II) to form loops.  Each loop contains about 75 kb DNA.  Scaffold proteins are attached to DNA at specific regions called scaffold attachment regions (SARs), which are rich in adenine and thymine.
(b) The chromatin fiber and associated scaffold proteins coil into a helical structure which may be observed as a chromosome.  G bands are rich in A-T nucleotide pairs while R bands are rich in G-C nucleotide pairs.