Because hydrogen bonds hold the two strands of a DNA molecul

Because hydrogen bonds hold the two strands of a DNA molecule together, the strands can be separated without breaking any covalent bonds. Every unique DNA molecule \"melts\" at a different temperature. In this context, Tm (melting temperature) is the point at which two strands separate, or become denatured. Order the DNA sequences listed below according to relative melting temperatures (from lowest Tm to highest Tm). Assume that they all begin as stable double-stranded DNA molecules. Explain your answer. GACTCCTG GGCGCACC CTAACT GG TATTGTCT

Solution

The nature of bonds between the nucleotides decides the melting point of the DNA strands. The higher is the number of GC bonds, higher would be the melting temperature of the DNA since there will be more triple hydrogen bonds in that strands.

As it can be seen from the data, there are 5, 7, 4 and 2 GC bonds in 1st, 2nd, 3rd and 4th strand. Thus, the order of Tm from lowest to highest would be:

Strand 4 (lowest), Strand 3, strand 1 and strand 7 (highest)