Semi-conservative DNA Replication
Each new DNA molecule keeps one original strand and one newly built one. Helicase, DNA polymerase and the split into leading and lagging strands explain how base pairing gets copied.
What a learner can do afterwards
- States what each new molecule inherits from the parent molecule.
- Names the enzymes that unwind and that build, and says what each one needs.
- Explains why one strand is built continuously and the other in fragments.
1 · Read
DNA replication is semi-conservative. You unzip the parent, keep one old strand in each daughter, and build the partner from free nucleotides. Each new molecule is half history and half new print. The Meselson Stahl experiment proved it: bacteria grown in heavy nitrogen shifted DNA density across generations exactly as half old, half new predicts, ruling out fully kept and fully mixed models.
Meet the enzyme crew and what each one demands. Helicase needs ATP to break the bonds and open the helix. Primase lays the short primer that starts each new stretch, because polymerase cannot start from nothing. DNA polymerase needs that primer plus nucleotides and proofreads as it goes. Ligase later stitches fragments into one unbroken strand.
The two strands at each fork behave differently, and the cause is direction. Polymerase only adds bases one way, so the strand pointing the right way grows smoothly as the leading strand. The other must loop back and build in short Okazaki fragments, the lagging strand, which ligase joins together.
Assign each job and confusion ends: unwinding belongs to helicase, starting to primase, building to polymerase, and joining to ligase. Eukaryotes fire many origins along each long chromosome at once, and telomere tips need special handling. When a question asks what an enzyme needs, answer with its fuel.
Unzip, keep one old strand each, and build the partner with primed polymerase and ligase.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.