UNBROKEN LOOP

There’s a lot of focus right now on microorganisms, so it was timely to feature a story in the November issue on bacteria and how they are dealt with. It is especially important to highlight the dangers of the overuse of antibiotics, which can lead to “superbugs” and result in a shortage of drugs to effectively combat diseases. One thing I wondered about was the circular chromosome in the bacterium diagram—it doesn’t look circular? James Degarmo, Coral Springs, FL

There are two kinds of bacterial DNA: One occurs in the form of plasmids, small extrachromosomal circular DNA strands in the cytoplasm that can replicate independently of the main chromosome and that are clearly round, and the other is the main DNA strand of the chromosome. Although the main strand doesn’t appear to be circular in the diagram, circular in this case just means a closed loop—no free ends, not linear— and due to the strand’s length it can still have folds and bends.