The electron microscopy of nucleic acid molecules has become a routine tool in the analysis of genome structure and function. This is primarily due to the availability of simple reliable methods for the visualization of single- and double-stranded DNA and RNA molecules, of RNA-DNA hybrids, and of nucleic acid-protein complexes. Most of these methods were developed by using viral genomes because these nucleic acid molecules were of considerable physical homogeneity and could be produced in abundant quantities