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.
More to read
14.3 Basics of DNA Replication
DNA replication is semi-conservative: each daughter molecule inherits one original strand and one fresh one. The Meselson Stahl experiment proved it by growing bacteria in heavy nitrogen, then watching the DNA bands shift density across generations exactly as the half old, half new model predicts. Conservative and dispersive models both failed that test.
Hold the picture firmly: unzip the parent, keep one old strand in each daughter, and build the partner from free nucleotides. That single image answers every what is inherited question. Nothing is created from scratch and nothing is fully preserved, each new molecule is half history and half new print.
14.5 DNA Replication in Eukaryotes
Eukaryotes copy the same molecule but on a grander scale, with many origins firing at once along each long chromosome. Helicase unwinds each bubble, primase lays primers, and DNA polymerase extends them using the exposed bases as a template, always reading one way. Telomeres at the chromosome tips need special handling so the ends do not shrink away.
The two strands at each fork behave differently, and the cause is direction. Polymerase can only add bases in one direction, so the strand pointing the right way grows smoothly as the leading strand. The other strand must be built backwards in short Okazaki fragments, the lagging strand, which ligase later stitches together.
11.2 DNA Replication
Meet the enzyme crew and what each one demands. Helicase needs ATP to break the hydrogen bonds and open the helix. Primase needs RNA nucleotides to lay the primer that starts each new stretch. DNA polymerase needs that primer plus a supply of DNA nucleotides, and it proofreads as it goes, cutting its error rate to about one in a billion.
Assign each job and confusion ends. Unwinding belongs to helicase, building belongs to polymerase, starting belongs to primase, and joining belongs to ligase. If a question asks what an enzyme needs, answer with its fuel: ATP for the unwinder, a template plus nucleotides for the builder, and a primer to begin.
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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.