The Biology of Isolated Chromatin: Chromosomes, biologically active in the test tube, provide a powerful tool for the study of gene action.

The isolated chromatin of higher organisms possesses several properties characteristic of the same chromatin in life. These include the presence of histone bound to DNA, the state of repression of the genetic material, and the ability to serve as template for the readout of the derepressed portion of the genome by RNA polymerase. The important respect in which isolated chromatin differs from the material in vivo, fragmentation of DNA into pieces shorter (5 x 10(6) to 20 x 10(6) molecular weight) than the original, does not appear to importantly alter such transcription. The study of isolated chromatin has already revealed the material basis of the restriction of template activity; it is the formation of a complex between histone and DNA. Chromatin isolated by the methods now available, together with the basis provided by our present knowledge of chromatin biochemistry and biophysics, should make possible and indeed assure rapid increase in our knowledge of chromosomal structure and of all aspects of the control of gene activity and hence of developmental processes.