Bacteria and other microbes can be used to \"clean up\" an oil spill by breaking

Question

Bacteria and other microbes can be used to "clean up" an oil spill by breaking down oil into carbon dioxide and water. Two samples isolated from the Deepwater Horizon leak in the Gulf of Mexico were labeled A and B. The DNA of each was isolated and the percent thymine measured in each sample. Assume the organisms contain normal double-stranded DNA and predict the composition of the other bases. Both samples are then denatured to remove the secondary structure. Which will have the higher temperature to denature?

Explanation / Answer

Ans. Part A: Using Chargaff’s rule for dsDNA-

            No. of A nucleotides = No. of T nucleotides, -----equation 1

            No. of G nucleotides = No. of C nucleotides, ----- equation 2

For sample A:

Given, A = 19.3 = T                    [Using equation 1]

G + C = 2 G       [G = C, equation 2]

Or, 2C = 100 – (A+ T) = 100 – 38.6 = 61.4

Or, C = (61.4) / 2 = 30.7

Hence, C = 30.7

Hence, A=T= 19.3%     , G=C= 30.7%

Note: All values are assumed to be in % without the symbol, too.

For sample B:

Given, A = 30.9 = T                    [Using equation 1]

G + C = 2 G       [G = C, equation 2]

Or, 2C = 100 – (A+ T) = 100 – 61.8 = 38.2

Or, C = (38.2) / 2 = 19.1

Hence, C = 19.1

Hence, A=T= 30.9%     , G=C= 19.1%

Part B: Sample A has greater GC content (2 x 30.7%) than that of sample B (2 x 19.1%), therefore A will require higher temperature for denaturation. A DNA sample with greater GC content has higher melting point because GC forms 3 H-bonds compared to 2 H-bonds by AT pair. So, greater GC content means relatively laager number of H-bonds in DNA molecule, that requires higher temperature for its denaturation.

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