I was wondering whether the presence of the methyl group on thymine vs uracil has any bearing on the relative stability of DNA vs RNA. Any thoughts?

RNA is pretty unstable. Say, for example I'm trying to measure the concentration of DNA in a sample with a spectrophotometer but some RNA's "getting in the way", I just leave my sample sitting at room temperature for a bit (10 mins) and it'll have degraded sufficiently.

 

This makes sense as in the cell, you don't want mRNA hanging around for too long, otherwise that wouldn't allow for very good control of e.g. protein expression levels.

 

While the A-T base pair IS "stronger" than the A-U pair, this doesn't actually contribute much to the stability differences. Instead I would guess that the 2’ OH group on the ribose sugar makes RNA more prone to nucleophilic attack.

I know RNA is considerbaly less stable - I was just wondering why. the OH groups are a source of instability I hadn't considered, thanks!

A major source of RNA degradation is also of biological origin- RNAses are everywhere. Moreover, RNAses are much more stable than DNAses. The mentioned fast degradation of RNA within a few minutes is almost certainly caused by minute RNAse contamination.

 

The use of thymine instead of uracil has less to do with molecule stability (in terms of degradation) but is most likely a mechanism to reduce mutations. Deamination of cytosine form uracil. Its presence in the DNA would therefore complicate the repair of this kind of mutation.

Great, thanks! It's really interesting