In eucaryotic cells virtually all of the DNA is complex with proteins to form a unit fiber approximately 100 A in diameter. Chromatin is formed by the higher order coiling of the unit fiber. In procaryotic cells, as exemplified by E. coli, the actual structure of the chromosome is less clear (218), but the discovery of the DNA gyrase raises the possibility that the DNA helix in the cell is maintained in an underwound state. It may be important to consider these structural features of DNA in future biochemical studies on replication, transcription, and recombination. The recent discoveries of the DNA swivelases, the DNA gyrase, and the DNA unwinding enzymes considerably increase our knowledge of DNA biochemistry. As more is learned about these enzymes and their interaction with DNA, the prospects for understanding the details of DNA transcription, DNA recombination, and particularly DNA replication appear to be good.