Joel Huberman and Arthur Riggs used pulse–chase labeling to examine the replication of DNA in mammalian cells. Briefly describe the Huberman–Riggs experiment, and identify how the results exclude a unidirectional model of DNA replication.

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1. The Huberman–Riggs experiment used a technique called pulse–chase labeling to study DNA replication. In this method, cells were first exposed to a short 'pulse' of radioactive thymidine, which gets incorporated into newly synthesized DNA strands during replication.
2. After the pulse, the radioactive thymidine was replaced with non-radioactive thymidine for a 'chase' period. This allowed the researchers to track the movement of the labeled DNA over time.
3. The DNA was then extracted and analyzed using autoradiography, a technique that visualizes the location of the radioactive label on the DNA. This helped determine the pattern of replication along the DNA molecule.
4. The results showed that the radioactive label appeared at multiple points along the DNA molecule and spread bidirectionally from these points. This indicated that replication starts at specific origins and proceeds in both directions, forming replication forks.
5. These findings excluded a unidirectional model of DNA replication because, in a unidirectional model, the radioactive label would have moved in only one direction from the origin of replication. The observed bidirectional movement of the label provided strong evidence for the bidirectional replication model.

Key Concepts

Pulse-Chase Labeling

Pulse-chase labeling is a technique used to study the dynamics of biological molecules, particularly DNA and proteins. In this method, cells are exposed to a labeled compound (the 'pulse') for a short time, allowing the incorporation of the label into newly synthesized molecules. After this period, the labeled compound is replaced with an unlabeled one (the 'chase'), enabling researchers to track the fate of the labeled molecules over time.

DNA Replication Models

DNA replication can occur through different models, including unidirectional and bidirectional replication. The unidirectional model suggests that DNA strands are replicated in one direction from a single origin, while the bidirectional model proposes that replication occurs simultaneously in both directions from multiple origins. Understanding these models is crucial for interpreting experimental results related to DNA synthesis.

Huberman-Riggs Experiment

The Huberman-Riggs experiment involved using pulse-chase labeling to track the incorporation of labeled nucleotides into DNA during replication in mammalian cells. Their findings showed that labeled DNA was present in both leading and lagging strands, indicating that replication occurs bidirectionally. This evidence contradicted the unidirectional model, demonstrating that DNA replication involves simultaneous synthesis from multiple origins.