Strength of Bonds between Nitrogenous Bases

[Guest ku ku ku]

I'm not sure if this should go here. Either way...

 

Are the three hydrogen bonds between guanine and cytosine stronger than the two hydrogen bonds between adenine and cytosine?

Page 764 of Peter Russell's iGenetics textbook says, "...GC base pairs are stronger than A-T base pairs."

 

However, on the same page, he also says the following: "The bonds between adenine and thymine are double bond and between guanine and cytosine are triple bonds. The double bonds would be the most difficult to break. The triple bonds would be the easiest because the electron cloud is perpendicular, so it can be attacked from more than one plane."

 

Is this a contradiction, or is there an explanation for it?

[Guest SoHo]

Hey

 

I don't know what textbook you're referring to, but yeah, that does sound contradictory. I know that GC bonds are harder to break because of the fact that they bond via 3 hydrogen bonds as opposed to 2. I think this is generally accepted by most people. It's also evidenced by the fact that DNA with more GC pairs requires a higher temperature to denature. Hope this helps.

[Guest Talon01]

I think it's something like 1 bond in a triple bond is easier to break than 1 bond in a double bond. But when you sum up the bond energies of both, the G-C bonds, overall, have larger bond energies.

 

Man, some of you people go into a lot of detail in your MCAT studies eh. Almost makes me feel underprepared 8o

[Guest therealcrackers]

The most important factor in all of this is the impact it has on the melting temperature of the DNA. The higher the GC content, the higher the temperature required to completely separate the two strands. An organism like Thermtoga maritima, with an ambient temperature of 83oC, has a much higher GC content than E. coli or a psychrophilic organism (cold-loving). Plus, breaking a single hydrogen bond has no significant impact on the structure of a DNA molecule longer than a few nucleotides, so overall bond energy is more important in the three para-linear H-bonds in a GC pair. And this is greater than the overall bond energy in the two linear H-bonds in an AT pair.