Q.The correct depiction of the centrifugation step of the experiment conducted by Alfred Hershey and Martha Chase on using radioactive labelled phages to prove that DNA is the genetic material is : (A) [Left] No Radioactive (35S) detected in cells + Radioactive (35S) detected in supernatant; [Right] Radioactive (32P) detected in cells + No Radioactivity detected in supernatant (B) [Left] Radioactive (35S) detected in cells + Radioactive (35S) detected in supernatant; [Right] No Radioactive (32P) detected in cells + No Radioactivity detected in supernatant (C) [Left] No Radioactive (35S) detected in cells + No Radioactivity detected in supernatant; [Right] Radioactive (32P) detected in cells + Radioactive (35S) detected in supernatant (D) [Left] Radioactive (35S) detected in cells + No Radioactivity detected in supernatant; [Right] No Radioactive (32P) detected in cells + Radioactive (35S) detected in supernatant

Alfred Hershey and Martha Chase conducted their experiment to definitively prove that DNA, not protein, is the genetic material. They used bacteriophages, viruses that infect bacteria, and labelled their components with different radioactive isotopes.

• They grew some phages in a medium containing radioactive sulfur (35S), which labels proteins because sulfur is present in amino acids but not in DNA.
• They grew other phages in a medium containing radioactive phosphorus (32P), which labels DNA because phosphorus is present in DNA but not in proteins.

These labelled phages were then allowed to infect E. coli bacteria. After infection, the viral coats were agitated off the bacterial cells using a blender. The mixture was then centrifuged.

• In the 35S experiment, the radioactive protein coats remained outside the bacterial cells. After centrifugation, the heavier bacterial cells formed a pellet at the bottom and showed no radioactivity. The lighter viral coats remained in the supernatant and were radioactive. …

Before we delve into the ingenious experiment that settled a fundamental question in biology, let's briefly recall the structure of DNA itself. DNA, the blueprint of life, is a double helix composed of nucleotides. A crucial aspect of its structure, discovered later by Watson and Crick, is complementary base pairing, where adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). This precise pairing is vital for DNA's ability to store and transmit genetic information accurately. However, for many years, scientists were unsure if it was DNA or protein that carried this genetic information.

The scientific community in the mid-20th century was divided. Proteins, with their complex and diverse structures, seemed like strong candidates for carrying genetic information. DNA, on the other hand, was considered too simple, a mere repeating polymer. It took a series of elegant experiments to definitively prove DNA's role. Among the most conclusive was the work of Alfred Hershey and Martha Chase in 1952.

Hershey and Chase designed an experiment using bacteriophages (viruses that infect bacteria) to determine whether DNA or protein was the genetic material. Their approach was brilliant because bacteriophages inject their genetic material into a host bacterium to replicate, leaving most of their structural components outside.

The core of their experiment involved selectively labeling the DNA and protein components of the bacteriophages with different radioactive isotopes:

• Radioactive Phosphorus (32P): DNA contains phosphorus but no sulfur. By growing phages in a medium containing 32P, their DNA became radioactively labeled.
• Radioactive Sulfur (35S): Proteins contain sulfur but no phosphorus. By growing phages in a medium containing 35S, their protein coats became radioactively labeled.

They then allowed these labeled phages to infect E. coli bacteria. After a short period of infection, they subjected the mixture to a crucial step: blending. This blending step was designed to agitate the solution and detach the viral protein coats (capsids) from the surface of the bacterial cells. The idea was to separate the material that entered the cell from the material that remained outside.

The final and most critical step, which directly addresses your question, was centrifugation. Centrifugation is a process that uses centrifugal force to separate components of a mixture based on their density.

• When the blended mixture was centrifuged, the heavier bacterial cells, which had been infected, settled at the bottom of the test tube, forming a pellet.
• The lighter viral particles and detached protein coats remained suspended in the liquid above, forming the supernatant.

Now, let's consider the two experimental setups and their expected outcomes:

• Experiment 1: Phages labeled with 35S (protein label)

  • If protein was the genetic material, it would enter the bacterial cells, and the radioactivity would be found in the pellet (with the cells).
  • If protein was not the genetic material, it would remain outside the cells (or detach during blending) and be found in the supernatant (with the viral coats).
  • Result: Hershey and Chase found that the vast majority of the 35S radioactivity was detected in the supernatant, not in the bacterial cells. This indicated that protein did not enter the cells to direct the synthesis of new viruses.

• Experiment 2: Phages labeled with 32P (DNA label)

  • If DNA was the genetic material, it would enter the bacterial cells, and the radioactivity would be found in the pellet (with the cells).
  • If DNA was not the genetic material, it would remain outside the cells and be found in the supernatant.
  • Result: Hershey and Chase observed that almost all the 32P radioactivity was detected in the bacterial cells (the pellet). Furthermore, these cells, when allowed to grow, produced new phages that also contained 32P, confirming that the DNA had indeed directed the synthesis of new viral particles. …