Several levels of DNA packing make up the compact chromosome seen in a metaphase cell. Even when a cell is not dividing, some of the DNA is folded tightly. The degree of DNA packing makes certain genes available or unavailable for transcription. Chromosomes consist of loops of supercoils, which are made up of tight helical fibers. These fibers are composed of nucleosomes, which consist of DNA wrapped around protein molecules. In the fiber, the DNA is unavailable for transcription. Here we can observe the unpacking of DNA which makes it accessible for transcription. Each nucleosome consists of a DNA segment wound around a protein core of eight histone molecules. Histones are involved in switching genes on and off. Chemical modifications of DNA and histones may affect chromatin structure and access of RNA polymerase to specific genes.
