Book excerpt: Molecular Genetics, Part III: Chromosome Structure explores the structure and modification of DNA, chromatin, and higher order organization and possible subunits of chromosomes at the molecular level. It describes major changes in concepts of chromatin structure and packaging of DNA based on studies of nuclease digests and electron micrographs; the role of restriction endonucleases in molecular genetics; the involvement of DNA topoisomerases in concerted breaking and rejoining of DNA backbone bonds; enzymatic methylation of DNA; and transcriptional units in eukaryotic chromosomes. Organized into seven chapters, this volume begins with an overview of the general properties of type I and type II restriction enzymes, basic aspects of restriction enzyme technology, and applications of restriction enzyme technology to the study of chromosome structure and function. It then discusses recombinant DNA technology; possible biological roles of DNA topoisomerases; recognition and control sequences in nucleic acids; composition and substructure of nucleosomes; analysis of chromosome fibers by electron microscopy; organization of fibers into chromosomes; and functional aspects of organization of chromosome fibers. Molecular biologists, geneticists, scientists, and electron microscopists will find this book extremely helpful.