Question

Plasmid DNA isolation methods are primarily based on the

• A. Location
• B. Sequence
• C. Size and conformation
• D. Restriction sites

Hint:

Alkaline lysis separates plasmid and genomic DNA based on:
1. Size (small vs. large).
2. Conformation (supercoiled circular vs. sheared linear).
During neutralization, only supercoiled plasmids remain soluble.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Plasmid DNA isolation requires separating small, circular plasmid molecules from the host cell's genomic DNA and proteins.
Most isolation protocols exploit physical and chemical differences between plasmid DNA and chromosomal DNA.

Step 2: Detailed Explanation:
The standard method for isolating plasmid DNA is the alkaline lysis method, which relies on differences in size and conformation:
1. Size: Plasmids are relatively small circular molecules (typically $2\text{ to }20\text{ kb}$), whereas bacterial genomic DNA is much larger (several million base pairs).
2. Conformation: Plasmids exist as tightly supercoiled, covalently closed circular (ccc) double-stranded DNA molecules.
Bacterial genomic DNA is sheared into large, linear fragments during cell lysis.
During alkaline lysis, the addition of sodium dodecyl sulfate (SDS) and sodium hydroxide ($\text{NaOH}$) lyses the cells and denatures both plasmid and chromosomal DNA by disrupting hydrogen bonds (at pH $12.0 - 12.5$).
When a neutralizing potassium acetate buffer (pH $4.8 - 5.2$) is added, the pH drops rapidly, allowing the DNA strands to re-anneal.
The small, supercoiled plasmid DNA molecules re-anneal quickly and remain in solution.
The larger, sheared chromosomal DNA fragments cannot re-anneal properly and aggregate with denatured proteins and potassium-dodecyl-sulfate complexes, forming a precipitate.
Centrifugation removes this precipitate, leaving the purified plasmid DNA in the supernatant.
Thus, the separation is based on the size and conformation of the DNA molecules.

Step 3: Final Answer:
Plasmid DNA isolation methods are based on size and conformation, which corresponds to Option (C).