DNA replication begins at origins of replication where proteins recognize specific sequences and attach to DNA. The two strands then separate, forming a replication bubble. DNA helicase unwinds the strands to prevent knot formation while topoisomerase prevents rewinding. Leading strand synthesis proceeds continuously from 5' to 3' toward the replication fork, using RNA primers made by primase. Lagging strand synthesis occurs discontinuously away from the fork, producing Okazaki fragments joined by ligase. DNA polymerase and ligase complete the process, forming two new double helix strands.