Question
The melting temperature (Tm) is defined as the temperature at which 50% of double-stranded DNA is separated into single-stranded DNA. The greater the guanine-cytosine (poly-GC) of the DNA, the higher the Tm compared to DNA with more adenine-thymine (poly-AT) content.
The following graph shows the Tm of a poly-AT DNA strand, a poly-GC DNA strand, and DNA from two different organisms (A and B).
(a) Describe how the Tm of DNA from organism A compares to the Tm of DNA from organism B.
(b) Describe the differences between the Tm of poly-AT and the Tm of poly-GC DNA.
(c) DNA sequencing finds that DNA from organism A has a GC content of 39% and that DNA from organism B has a GC content of 48%. A student claims that DNA from organism C (with a GC content of 55%) would have a Tm greater than 85° Celsius. Using the data from the figure, evaluate the student’s claim.
(d) Explain how these experimental results relate to the structure of DNA.
▶️Answer/Explanation
Ans:
(a) The Tm of DNA from organism A is lower than the Tm of DNA from organism B.
(b) The Tm of poly-GC DNA is much higher than the Tm of poly-AT DNA.
(c) DNA from organism B has a GC content of 48% and a Tm of approximately 85° Celsius. Since increased GC content leads to a higher Tm and the DNA from organism C has a GC content of 55%, it would follow the pattern of the data that organism C’s Tm would be higher than 85° Celsius.
(d) In DNA, GC pairs form three hydrogen bonds while AT pairs form only two hydrogen bonds. More hydrogen bonds would require more energy to separate, so it makes sense that DNA with a higher GC content would have a higher Tm.