Question

Which DNA molecule among the following will melt at lowest temperature?

• A. 5'-A-A-T-G-C-T-G-C-3'
  3'-T-T-A-C-G-A-C-G-5'
• B. 5'-A-A-T-A-A-A-G-C-T-3'
  3'-T-T-A-T-T-T-C-G-A-5'
• C. 5'-G-C-A-T-A-G-C-T-3'
  3'-C-G-T-A-T-C-G-A-5'
• D. 5'-A-T-G-C-T-G-A-T-3'
  3'-T-A-C-G-A-C-T-A-5'

Answer 1

The correct option is B 5'-A-A-T-A-A-A-G-C-T-3'

3'-T-T-A-T-T-T-C-G-A-5'

The nitrogenous bases are purines and pyrimidines. The purines are adenine and guanine. The pyrimidines are thymine and cytosine. Purines pair with the pyrimidines by hydrogen bonds. Adenine pairs with thymine by two hydrogen bond while the cytosine pairs with the guanosine by three hydrogen bonds.

Therefore, the strand having least number of C and G will melt at the lowest temperature. In options (a), (c) and (d) given strands have 4, 4 and 3 GC pair while option (b) strand has two GC pair. Therefore, it will melt at the lowest temperature.

So, the correct answer is option B.