I. Structure of chromatin. Introductory remarks

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Abstract
The DNA of a eukaryotic chromosome is probably one single molecule, amounting to several centimetres in length if laid out straight, and it must be highly folded to make the compact structure one can see in a chromosome. At the same time it is organized into separate genetic or functional units, and the manner in which this folding is achieved, genes organized and their expression controlled is the subject of this Discussion Meeting. The subject is a wide ranging one but there has been such rapid progress in the various branches that the organizers felt it timely to review the current position and to see what connections could be made between structural organization at various levels and functional control. The first session is devoted to the topic of chromatin, the name given to the chromosomal material when extracted. It consists mainly of DNA, tightly associated with an equal weight of a small set of rather basic proteins called histones. Other proteins and RNA are present in lesser amounts, and these non-histone proteins include polymerases which transcribe the DNA, other enzymes and putative regulatory proteins. For many years the histones themselves appeared rather complex, but by about 1970 (see, for example, DeLange & Smith 1973) it had become clear that there were only five types, the apparent proliferation of types or species arising from various post-synthetic chemical modifications such as acetylation or phosphorylation, which take place during various phases of the cell cycle.