Sequence the steps of DNA replication and explain why the process is described as semiconservative.
Every time a cell divides, its entire DNA sequence must be copied precisely during S phase. DNA replication is described as semiconservative: each resulting double helix contains one original (parental) strand and one newly built strand, rather than one fully old helix and one fully new helix. This was confirmed directly by the Meselson-Stahl experiment in 1958, which grew bacteria first in a heavy nitrogen isotope, then transferred them to a normal, lighter nitrogen isotope and tracked the density of their DNA across generations of replication -- the results matched the semiconservative prediction precisely, ruling out competing models.
Mechanically, the enzyme helicase unwinds and unzips the double helix at an origin of replication, breaking the hydrogen bonds between paired bases; single-strand binding proteins hold the separated strands apart. DNA polymerase then reads each exposed template strand and adds complementary free nucleotides to build a new strand, working continuously on one strand (the leading strand) but only in short, repeated segments called Okazaki fragments on the other (the lagging strand), since it can only build in one consistent direction along the antiparallel template; DNA ligase later joins those fragments together. DNA polymerase also proofreads its own work as it goes, catching and fixing the great majority of mismatched bases, which is why replication is remarkably, though not perfectly, accurate.
Mastery looks like: They can sequence the major steps of DNA replication, explain what "semiconservative" means, and describe how the Meselson-Stahl experiment supported it.
Common stumbles: Thinking replication produces one entirely new and one entirely untouched double helix rather than a mix of old and new in each; forgetting DNA polymerase's proofreading role.