semiconservative replication
When DNA is copied, you might guess the cell builds two brand-new helices and keeps the old one untouched. It does not. Instead, each new double helix is a hybrid: one strand is the old, original parental strand, and one strand is freshly made. This is what “semiconservative” means — half of every copy is conserved from the parent.
Because the two strands of the double helix are complementary, each separated strand carries all the information needed to rebuild its missing partner. Keeping one original strand as a template is what makes replication so reliable: the new strand is dictated, base by base, by the old one.
This was not obvious in advance — competing ideas imagined the original helix staying fully intact (conservative) or being chopped and mixed (dispersive). A classic experiment by Meselson and Stahl in 1958 used DNA of different densities to show clearly that replication is semiconservative.
If you label all of a cell's DNA with a heavy isotope and then let it replicate once in normal medium, every resulting helix has one heavy (old) strand and one light (new) strand — the hallmark of semiconservative copying.
The Meselson–Stahl result: after one round, all DNA is of intermediate density.