Question

Assertion (A): DNA cannot pass through cell membrane of a bacterial cell.
Reason (R): DNA is a hydrophilic molecule.

• A. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false.
• D. Assertion (A) is false, but Reason (R) is true.

Hint:

Remember the rule of solubility: "Like dissolves like." Cell membranes have a greasy, hydrophobic interior. Because DNA is covered in negatively charged phosphate groups, it is highly hydrophilic and cannot dissolve into or pass through the greasy membrane core.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question looks at the biochemical property of a DNA molecule that restricts its movement across biological plasma membranes, specifically the bacterial cell membrane. We must determine if both statements are correct and if the chemical property explains the membrane barrier.

Step 2: Key Formula or Approach:
The plasma membrane is a semi-permeable lipid bilayer composed primarily of amphipathic phospholipids. Its core interior is highly hydrophobic (lipophilic). Consequently, only non-polar, hydrophobic molecules can dissolve in and diffuse freely across the lipid core, while polar, charged, or hydrophilic molecules face an energetic barrier.

Step 3: Detailed Explanation:
Evaluating Assertion (A): In recombinant DNA technology and bacterial transformation experiments, exogenous DNA molecules cannot cross the bacterial plasma membrane through simple passive diffusion. To force a bacterial cell to take up an external plasmid, laboratory protocols must make the cells "competent" using chemical treatments (like divalent $Ca^{2+}$ ions) combined with thermal shock. This demonstrates that the intact membrane acts as an impassable barrier to naked DNA. Thus, Assertion (A) is true.
* Evaluating Reason (R): A DNA molecule is a polymer composed of nucleotide units. Its structural backbone consists of alternating sugar and phosphate groups. Each phosphate group carries a full negative charge at physiological pH. Because of these abundant negative charges along its exterior backbone, DNA interacts strongly with water molecules, making it a highly hydrophilic (water-loving) and polar macromolecule. Thus, Reason (R) is true.
Evaluating the Link: Because the interior core of the bacterial cell membrane is made of non-polar fatty acid chains, it acts as an energetic barrier to highly charged, hydrophilic molecules like DNA. The hydrophilic nature of DNA directly explains why it cannot pass through the hydrophobic membrane bilayer without assistance. Therefore, the reason provides the correct chemical explanation for the assertion.

Step 4: Final Answer:
Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A), matching option (A).