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.

NANOTUBES, stiffer than steel, only a nanometer wide but many microns long, are essentially rolled-up sheets of carbon hexagons. The following are highlights from a dozen APS sessions on the subject: Richard Smalley (713-527-4845) of Rice University, winner of the 1996 Nobel Prize in chemistry for his discovery of Buckyballs in 1985, reported that his lab currently produces nanotubes ("more precious than platinum") at a rate of grams/day but that within 5 to 10 years this could be increased (on a commercial basis) to tons/day. Smalley showed pictures of nanotubes that swallow their own tails, forming closed Bucky toruses (see also Nature, 27 Feb.), and diagrams of bundles of nanotubes in which one tube stuck out further than its neighbors. Nested stages of such bundles, he said, could be used to fashion pointers---macroscopic at one end but tapering down to a single carbon cell at the other end---with which one could (like an artist dipping a paintbrush into a palette of colors) "write" patterns of molecules on a substrate. Electrically, the versatile nanotubes can be insulators, semiconductors, or conductors and are expected to exhibit magnetoresistance qualities . The pointy nanotubes have been used as field emitters in flat panel displays. Attaching a semiconducting nanotube to a metallic nanotube one gets a nano-diode. If, furthermore, the joint is angled, the composite tube can actually conduct better in a bent state than straight up; this property makes the structure into a possible nanoswitch or strain gauge. Cees Dekker of Delft University is able to study the electron transport properties of single nanotubes by draping them across a pair of electrodes; the current-versus- voltage plot is a series of steps, indicative of a "quantum wire." In general one would expect this behavior when the movement of electrons through a conductor is restricted to one dimension. Steven Louie of LBL (510-642-1709) studies nanotubes made of boron and nitrogen, or of carbon mixed with B and N. In one configuration, a ribbon of conducting carbon hexagons forms a corkscrew pattern up the length of the tube. The current flow through such a tube is therefore helical; in effect this nanotube is the world's smallest solenoid magnet.