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• #11537

Hello Biologists.

I have a questions that has stumped me and I am hoping someone can offer some support.

Q: DNA is isolated from two species. Both DNA samples are found to be of the same size (ie. equal quantities). The samples have G+C/A+T rations of 2.0 and 2.5

a. Which DNA samples has a higher G+C content? Explain.

WHAT?!?!?! I totally don’t get it, can anyone explain this to me?

b. Which sample contains the smaller number of H bonds between strands.
I would guess that both have the same number of H bonds between strands, because both have the same size and equal quantities. Furthermore DNA always has H bonds between bases, therefore it would be equal.

Can anyone clear this question mess up for me, I would be so grateful!!

New to biology

• #91783

So you have Species A and Species B. They both have the same amount of DNA, but have different ratios of the nucleotide bases. Do remember that Guanine is base paired with Cytosine (G+C), and Adenine is base paired with Thymidine (A+T).
Species A has a G+C/A+T ratio of 2.0 (for every 2 G+C there is 1 A+T)
Species B has a G+C/A+T ratio of 2.5 (for every 2.5 G+C there is 1 A+T, or better yet for every 5 G+C there is 2 A+T))

Thus Species B has a higher ratio of G+C (2.5 of G+C as oppose to 2.0 G+C for Species A) based on the fact that they both have the same amount of DNA (otherwise you would have to do calculation of the ratio to the amount of DNA present in the samples).

You have to remember the number of H bonds between the bases. There is 3 H bonds between G+C, and only 2 between A+T. In this particular question then, the smaller amount of H bonds would be which species has the less G+C and more A+T base pairs. Species B has the most G+C, while Species A has the least G+C, and thus more A+T pairs. Species A then has the smaller amount of H bonds in the DNA sample.

• #91854

Perfect post by kolean, in terms of hydrogen bonding adenine and thymine are linked by 2 hydrogen bonds while guanine and cytosine are linked with 3 hydrogen bonds, thus the DNA strand with the greater ratio of CG will thus have more hydrogen bonds.

• #91866

actually the greater stability of GC base pairs is due mainly not to the fact that they have more hydrogen bonds, but due to more base stalking in the case of gc base pairs

• #114459

Thank you!!! confusing question and clear answer/explanation!!!