DNA replication
Before division, helicase separates template strands, primase supplies RNA primers, DNA polymerase extends DNA, and ligase joins remaining backbone breaks. Because synthesis has one direction, the leading strand grows continuously toward a replication fork while lagging-strand fragments grow away from it. Each completed duplex contains one parental and one newly synthesized strand. Proofreading and repair lower error rates without eliminating mutation.
MAKE IT CONCRETE
If a template runs 3′-TACG-5′, the paired new DNA runs 5′-ATGC-3′. Follow the labeled ends before adding bases; guessing is not a replication enzyme.
DON’T FALL FOR THIS
Do not say the lagging strand is synthesized 3′→5′. Its fragments are made 5′→3′, just like every other new DNA strand.