Summary
Each new DNA molecule contains one original strand and one newly built strand, shown by Meselson and Stahl.
In depth
Meselson and Stahl (1958) grew E. coli in heavy nitrogen (15N) then moved them to 14N. After one generation, DNA had intermediate density; after two, half intermediate and half light. This ruled out conservative and dispersive models.
Each daughter helix retains one old strand, which ensures accurate copying.
Alternatives that Meselson and Stahl ruled out were conservative replication (one old and one new double helix) and dispersive replication (mixed fragments in each strand). After one generation a single band at intermediate density disproved the conservative model, and after two generations two bands (intermediate and light) disproved the dispersive model.
Centrifugation in a caesium chloride density gradient separated the DNA by weight. The experiment is often called the most beautiful in biology. Each daughter helix keeps one parental strand, which helps preserve information and also lets epigenetic marks on the old strand guide their copying.
Remember: semi-conservative describes the outcome, not the mechanism; in each new helix one strand is old and one new.
Frequently asked questions
What is Semi-Conservative Replication?
Each new DNA molecule contains one original strand and one newly built strand, shown by Meselson and Stahl.
Which area of biology covers Semi-Conservative Replication, and how advanced is it?
Semi-Conservative Replication is one of the 52 topics in Molecular Biology. It is rated Intermediate, so it suits learners who already know the core vocabulary. Related topics in this area include DNA Replication, Homologous Recombination, Helicase and Topoisomerase.
Can you give an example of Semi-Conservative Replication?
Centrifugation in caesium chloride separated the DNA bands by density.
What should I learn before Semi-Conservative Replication?
Work through the Beginner topics in Molecular Biology first, such as Central Dogma, DNA Replication, Base Pairing, then return to Semi-Conservative Replication. Intermediate topics assume the Beginner vocabulary.
What topics are related to Semi-Conservative Replication?
The closest topics to Semi-Conservative Replication are DNA Replication, Homologous Recombination, Helicase and Topoisomerase, CRISPR-Cas9 Mechanism. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Semi-Conservative Replication?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Semi-Conservative Replication. To practise the whole of Molecular Biology, use the Molecular Biology quiz.
How is Semi-Conservative Replication different from DNA Replication?
Semi-Conservative Replication: Each new DNA molecule contains one original strand and one newly built strand, shown by Meselson and Stahl. DNA Replication: The copying of DNA before cell division so each daughter cell receives a full set of genes. Read both side by side and note what each one explains.
How should I study Semi-Conservative Replication step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with DNA Replication and explain the difference aloud. Finally, take the Molecular Biology quiz and come back to this page for any question you miss.