Helicases are ubiquitous molecular motors. By unwinding nucleic acids, they drive DNA replication, repair, transcription and recombination. In eukaryotes, the CDC45–MCM–GINS (CMG) helicase stands at the heart of the replisome, the machine that duplicates chromosomes. The replisome must coordinate DNA unwinding with DNA synthesis, which is asymmetric; it also handles nucleosome reassembly at the newly replicated DNA and ensures faithful transmission of epigenetic information. It is therefore essential to understand the architecture and orientation of replisome components at the replication fork.

Georgescu and colleagues from the Li and the O’Donnell laboratories were the first to resolve the orientation of the CMG helicase at the fork. Using cryo-electron microscopy, they showed that DNA enters the CMG at the N-terminal end of the ring-shaped MCM hexamer and exits at the C-terminal end. This orientation places the N-terminal end of MCM in front of the replication fork, and the C-terminal end, comprising the ATPase motors, behind it — the opposite of what was long thought.