Chemical Mutagenesis

To determine the correctness of the Assertion [a] and the Reason [r], let's first understand the concepts involved:

• Assertion [a]: Nitric oxide is involved in transient paracrine and autocrine signaling.
  • Nitric oxide (NO) is indeed a crucial signaling molecule in the body, involved in various physiological processes.
  • It acts as both a paracrine and autocrine signaling molecule. In paracrine signaling, it affects nearby cells, while in autocrine signaling, it acts on the cell that releases it.
• Reason [r]: Nitric oxide is highly reactive, with a lifetime of few seconds, yet can diffuse freely across membranes.
  • Nitric oxide is a small, lipophilic molecule, allowing it to diffuse rapidly across cell membranes.
  • Its reactivity leads to a short biological half-life, typically lasting only a few seconds, which limits its signaling range to nearby cells and contributes to its role in transient signaling pathways.

Now, we evaluate the truth and connection between [a] and [r]:

1. Both the assertion (a) and the reason (r) are correct individually.
2. The reason provided (r) indeed explains why nitric oxide can function effectively in transient paracrine and autocrine signaling. Its ability to diffuse across membranes and its reactivity suits its role in these signaling types, where quick, localized responses are necessary.
3. Therefore, [r] correctly explains [a].

Based on the analysis above, the correct answer is: Both [a] and [r] are true and [r] is the correct reason for [a].