Q.58 A double stranded DNA molecule of total 5000 base pairs long, has a melting
temperature of 85 °C. What will be the % AT base pairs in this sample? (up
to one decimal place).

The %AT base pairs in the 5000 bp DNA sample with Tm of 85°C is 50.7%. This uses the standard empirical formula for long DNA duplexes under typical conditions. Detailed calculation follows using verified methods from molecular biology references.

Calculation Method

The melting temperature (Tm) formula for double-stranded DNA longer than 200 bp is Tm = 64.9 + 0.41 × (%GC) – (500 / L), where L is length in base pairs. For L = 5000 bp, the correction term is 500/5000 = 0.1°C. Rearranging gives %GC = [(Tm – 64.9 + 0.1) / 0.41] × 100 = (20.2 / 0.41) × 100 ≈ 49.3%. Thus, %AT = 100 – 49.3 = 50.7%.

Why This Formula?

GC pairs form 3 hydrogen bonds versus 2 for AT pairs, raising Tm with higher %GC. Length correction accounts for end effects in duplex stability; longer DNA (like 5000 bp) has higher Tm due to more bonds. Standard conditions (e.g., ~0.15-1 M Na+) are assumed, as unspecified.

Wallace Rule Context

Wallace rule (Tm ≈ 2×(A+T) + 4×(G+C)) applies only to short oligos (14-20 nt), not 5000 bp genomic-like DNA. It ignores length and salt effects, yielding inaccurate results here (e.g., unrealistically high base counts). Use the empirical %GC-length formula for CSIR NET-level long DNA problems.

DNA Melting Temperature 5000 Base Pairs 85°C: Calculate %AT Base Pairs

Discover how to calculate %AT base pairs in a double stranded DNA molecule of total 5000 base pairs with melting temperature of 85°C. This CSIR NET Life Sciences question tests DNA melting temperature (Tm), %GC content, and base composition. Get the exact answer: 50.7% AT with step-by-step derivation.

DNA Melting Temperature Formula Explained

Melting temperature (Tm) is when 50% of double stranded DNA denatures into single strands. For long DNA (>200 bp), use:
Tm = 64.9 + 0.41(%GC) – 500/L.

• 64.9°C: Base Tm for average composition.

• 0.41(%GC): GC pairs (3 H-bonds) raise Tm vs AT (2 H-bonds).

• 500/L: Length correction; negligible for 5000 bp (0.1°C).

Step-by-Step %AT Calculation for 5000 bp DNA 85°C

1. L = 5000 bp, Tm = 85°C.

2. Correction = 500/5000 = 0.1.

3. 85 = 64.9 + 0.41(%GC) – 0.1 → 20.2 = 0.41(%GC).

4. %GC = 20.2 / 0.41 ≈ 49.3%.

5. %AT base pairs = 100 – 49.3 = 50.7% (one decimal).

Common Mistakes: Wallace Rule vs Long DNA Formula

• Wallace rule (Tm = 2(AT) + 4(GC)): For short primers only (14-70 nt). Wrong for 5000 base pairs long DNA.

• Ignores length/salt; gives absurd results here.

• CSIR NET tip: Match formula to length.

CSIR NET Exam Tips for DNA Tm Questions

• Memorize Tm = 64.9 + 0.41(%GC) – 500/L for genomic DNA.

• Higher %GC → higher Tm (3 vs 2 H-bonds).

• Practice: 85°C, 5000 bp → 50.7% AT.

Keywords: DNA melting temperature, 5000 base pairs, 85°C Tm, %AT base pairs, %GC content, CSIR NET Life Sciences, double stranded DNA formula.