A DNA polymerase can extend a strand only in the 5' to 3' direction. To which position on the growing strand does the enzyme add the next nucleotide?

Core Concept

Each nucleotide carries a 5' phosphate and a 3' hydroxyl on its deoxyribose sugar. DNA polymerase catalyzes attack of the chain's exposed 3' hydroxyl on an incoming deoxynucleoside triphosphate, forming a phosphodiester bond and releasing pyrophosphate. Because growth always presents a fresh 3' hydroxyl at the terminus, elongation is chemically possible only from 5' toward 3'. The opposite end terminates in a phosphate and lacks the reactive hydroxyl. Replication machinery handles this constraint by running one new strand continuously and the other in short fragments.