IMPORTANT PROCESSES IN SINGLE DNA MOLECULES have been observed for the first time by using the atomic force microscope (AFM), in which the deflections of a tiny stylus over the contours of a surface can be turned into molecular-scale images. At the APS Meeting this week in Kansas City, Carlos Bustamante of the University of Oregon (541-346-1537) and his colleagues presented movies showing the first stages of DNA replication, in which a protein is seen to slide on DNA like a bead on a string to find the exact site where it could attach and start the replication process. Binding DNA and RNA polymerase (the protein that mediates the transcription of DNA into RNA) to a mica surface, Neil Thomson of UC-Santa Barbara (805-893-4544) and his colleagues produced 5-nm-resolution movies of the transcription process, in which RNA polymerase pins down the middle of a single DNA strand and then pulls the strand through as it starts transcribing the DNA into RNA using RNA-building-blocks called NTPs (Biochemistry, 21 Jan. 1997). Using an AFM, Gil Lee of the Naval Research Laboratory (202- 763-5383) found that a force of about 600 piconewtons was required to tear apart two complementary strands of DNA, namely a 20-base-pair-long strand of polycytosine (a form of single-strand DNA) from single strands of polyinosine averaging 160 base-pairs long.