Prime-time looping

摘要

When the replication machinery copies DNA, it must unwind the double helix in one direction while synthesis of one of the strands proceeds in the other. Making transient DNA loops may solve this directional dilemma.rnIf you are a cell about to divide, you will first need to use a multi-protein machine called a replisome to simultaneously make copies of both strands of your chromosomal DNA so that one strand can be passed to each daughter cell. Replisomes have long been thought to couple synthesis of both DNA strands by forming a 'trombone loop' of DNA that expands and relaxes as synthesis takes place discontinu-ously on one of the strands. Two papers, one by Pandey et al. on page 940 of this issue and another by Manosas et al. published in Nature Chemical Biology, show that a second type of loop, called the 'priming loop, is transiently produced in the replisome.