In nucleic acid hybridization, which Wallace rule expression correctly estimates the melting temperature (Tm) of a short oligonucleotide under standard conditions?

Introduction / Context:
The Wallace rule provides a quick mental estimate of the melting temperature (Tm) for short DNA oligonucleotides in typical salt conditions. It helps select hybridization stringency in PCR, cloning, and probe design without complex calculations.

Given Data / Assumptions:

• Short oligonucleotides (about 14–20 bases).
• Standard monovalent salt conditions.
• We only compare formulae; no sequence is given.

Concept / Approach:
G:C base pairs contribute more to duplex stability than A:T because G:C forms three hydrogen bonds and stronger stacking. The Wallace approximation weights G:C pairs by 4 and A:T pairs by 2 to reflect this increased stability.

Step-by-Step Solution:
1) Identify correct weighting: G:C should contribute more to Tm than A:T.2) Examine options: only one assigns 4 per G:C and 2 per A:T.3) Select that expression as the Wallace rule suitable for quick estimates.

Verification / Alternative check:
For longer oligonucleotides, nearest-neighbor models or empirical equations (e.g., Tm = 69.3 + 0.41 * %GC - 650 / length) provide better accuracy, but the Wallace rule remains appropriate for short primers.

Why Other Options Are Wrong:
Option A swaps weights and underestimates G:C stability.Option B greatly overemphasizes A:T contributions.Option D is a different empirical formula for longer sequences, not the Wallace rule.

Common Pitfalls:
Applying the Wallace rule to very long primers, ignoring salt/mismatch effects, or confusing %GC equations with the short-oligo heuristic.

Final Answer:
Tm = 4 * (number of G:C pairs) + 2 * (number of A:T pairs)