Describe how the Hershey-Chase experiment demonstrated that DNA is the genetic material, and not protein as previously thought. Explain how they used radioactive isotopes and bacteriophages in their experiment and summarize their conclusions. Distinguish between inducible operons and repressible operons and explain how they work. Describe the three different types of genetic recombination (conjugation, transformation, and transduction). Explain how recombination might interfere with the metabolic functions of operons, such as the lac operon or trp operon of E. coli. Please include websites so that I can refer back to them. Thanks.
The Correct Answer and Explanation is:
1. Hershey–Chase experiment (1952): DNA is genetic material
Hershey and Chase used T2 bacteriophages (viruses infecting E. coli). They labeled phage DNA with radioactive phosphorus-32 (^32P, specific to DNA) and phage protein with sulfur-35 (^35S, found in proteins) khanacademy.orgen.wikipedia.org+12embryo.asu.edu+12bio.libretexts.org+12. Each labeled batch infected separate E. coli cultures. After infection, they used a kitchen blender to shear off phage “ghosts,” then centrifuged to separate cells from supernatant.
- In the ^32P experiment, the pellet (cells) was radioactive—indicating DNA entered the cells and directed viral replication.
- In the ^35S experiment, radioactivity remained in the supernatant—showing protein stayed outside the cell en.wikipedia.org+1embryo.asu.edu+1.
They concluded: only DNA enters bacteria and carries genetic instructions, proving DNA (not protein) is the hereditary material byjus.com+8embryo.asu.edu+8en.wikipedia.org+8.
2. Inducible vs. repressible operons
- Inducible operons (e.g., lac operon): off by default; turned on by substrate (e.g., lactose binds the repressor, allowing transcription).
- Repressible operons (e.g., trp operon): on by default; turned off by the end-product (tryptophan binds the repressor, activating it to block transcription).
3. Genetic recombination types
- Conjugation: plasmid transfer between bacteria via sex pili.
- Transformation: uptake of naked DNA from the environment (e.g., Griffith, Avery experiments) bio.libretexts.org+10embryo.asu.edu+10en.wikipedia.org+10.
- Transduction: bacteriophage transfers bacterial DNA during infection cycles.
4. Recombination impacting operons
Recombination can disrupt regulatory sequences.
- If donor DNA lacks a functional promoter/operator or contains a mutated version, the operon may be non‑functional.
- For lac operon: insertion of non-inducible operator sequence could prevent induction even in the presence of lactose.
- For trp operon: gaining a defective repressor-binding site might keep the operon on even with high tryptophan, causing wasteful overproduction.
Further reading:
- Hershey–Chase summary (LibreTexts): https://bio.libretexts.org/Bookshelves/…/5.02%3A_The_Hershey_-_Chase_Experiments khanacademy.org+10bio.libretexts.org+10en.wikipedia.org+10
- Wikipedia overview: Hershey–Chase experiment en.wikipedia.org
- Khan Academy: “Classic experiments: DNA as the genetic material” en.wikipedia.org+15khanacademy.org+15youtube.com+15
