Genetics: triplex question

[joonkimdds]

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Hi, I have a question about triplex.
It says there can be pyrimidine, purine, pyrimidine triplex.
why can't there be pu-py-pu instead of py-pu-py?

and also, it said there can be py-pu-pu triplex.
how can purine bond with purine?
and if pu-pu bond is possible, why can't there be py-py bond?

 

[doc3232]

Which book did you find this in?
Never seen this info so I am interested in some answers.

 

[joonkimdds]

it's not a question from any of the study guide.
it's just the fact about the triplex and I just cannot understand it.
Since DAT covers DNA, I think it's possible to have this on DAT.

 

[utdent20]

yeah is this question relevant to teh DAT.. because i am lost...

 

[keliao]

lol.....yal lost me.!....can you explain more about the triplex...give an example or somhth ...have no idea what you are talkign bout.

 

[FuzzyChicken]

First off Im not sure what the exact question is, but here is my take on it.

If you have 3 "pieces" consisting of py, pur, py. Then the only combination you can correctly form is py-pur-py. And if you switched the end py's, it would be symmetricaly to the original structure.

Also, how can you have pur-py-pur if the only "pieces" you have to work with are py, pur, py (you are not given 2 pur "pieces").

 

[joonkimdds]

Ok, let me first explain what the triplex is.
Watson and Cranky(?) showed B-DNA with double helix, right?
but not all DNA are double helix. if one of the strand bends and surrounds the other strand, that becomes triple strand or triplex.
Here is a picture.

as u can see, C and T strand (pyrimidine) is outside, and then u see G and A(purine) 2nd, and then u see C and T again third. So this is called Pyr-Pu-Pyr triplex.

my question is why it can't be pu-pyr-pu triplex.

 

[IDK]

Did some little research (mainly because I had no clue what a triplex was), but anyways, the reason pyr-pu-pyr is preferred is due to the hoogsteen base pairing which (I think) is only formed with purines.

http://www.nd.edu/%7Easeriann/hoogsteen.gif

Since pyrimidines do not have a hoogsteen side, they can't be in the center of the triplex as they are incapable of forming the required hydrogen bonds with the other two strands. Someone correct me if I'm wrong 😛

 

[utdent20]

I can't exactly remember which question it was in the destroyer but one of em reflects the fact that Cytosine and Guanine weigh more or carry more % of the strand and that is why they are on the outside.. becuase they are more stable.. Idk i am just guessing this..