Question

Difficulty: HardProcedural Steps

To isolate plasmid DNA from bacterial cells, researchers use the alkaline lysis method, which relies on a precise sequence of chemical treatments. The process begins by centrifuging the bacterial culture to concentrate the cells into a pellet and discarding the supernatant liquid. Next, the cell pellet is resuspended in a glucose-Tris-EDTA buffer, which destabilizes the cell membrane without lysing the cells. Subsequently, an alkaline lysis solution containing sodium dodecyl sulfate (SDS) and sodium hydroxide is added. The SDS solubilizes the cell membrane lipids, while the sodium hydroxide denatures both the plasmid DNA and the larger chromosomal DNA. Crucially, before the mixture can degrade the DNA entirely, a potassium acetate neutralizing buffer is introduced. This acidic buffer restores neutral pH, causing the small circular plasmid DNA to rapidly renature and remain in solution, while the large chromosomal DNA and proteins precipitate out as a tangled white mass. After centrifuging this mixture to pellet the precipitate, the supernatant containing the dissolved plasmid DNA is transferred to a fresh tube. Finally, isopropanol is added to precipitate the plasmid DNA from the supernatant, which is then collected by a final centrifugation step.

Based on the passage, if a researcher transfers the liquid to a fresh tube prior to centrifuging the neutralized mixture, which of the following is the most likely consequence for the final isolated product?

  1. The final plasmid DNA product will be contaminated with chromosomal DNA and proteins.Answer
  2. B
    The cell membrane lipids will remain bound to the plasmid DNA in the final solution.
  3. C
    The plasmid DNA will fail to renature and will remain permanently denatured.
  4. D
    The sodium hydroxide will degrade the circular plasmid DNA before it can be collected.

Answer

The final plasmid DNA product will be contaminated with chromosomal DNA and proteins.
The correct answer is correct because the neutralizing buffer causes the chromosomal DNA and proteins to precipitate while the plasmid DNA remains in solution. Without centrifuging to pellet this precipitate, the tangled mass of chromosomal DNA and proteins remains suspended in the liquid and is transferred along with the plasmid DNA, resulting in a contaminated final product.

Step-by-Step Solution

1
Analyze what happens immediately after the potassium acetate neutralizing buffer is introduced.
The buffer restores neutral pH, causing the circular plasmid DNA to renature and stay dissolved, while the chromosomal DNA and proteins precipitate as a tangled mass.
This establishes the state of the components in the mixture before centrifugation.
2
Determine the role of the centrifugation step that follows neutralization.
Centrifugation pellets the precipitated chromosomal DNA and proteins, allowing the liquid supernatant containing the dissolved plasmid DNA to be separated.
This identifies the specific purification purpose of this procedural step.
3
Evaluate the consequence of transferring the liquid to a fresh tube before performing this centrifugation.
Because the precipitate has not been pelleted to the bottom of the tube, transferring the liquid will also transfer the suspended chromosomal DNA and protein precipitate.
This links the omitted step directly to the logical outcome for the final collected product.

Key Concept

Determining the logical outcome of omitting a physical separation step in a multi-stage chemical purification process.
Rate this question