Holliday junction

[Marta]

Hello, I'm new here and I hope you can help me.
I've had a look into a couple of books in Polish but they didn't give me any tips to understand the topic.
First of all, I didn't get clear information when and where that process occures. I imagine that it's between chromatides in homologous chromosomes during meiosis, when the chromosomes create chiasmas. Is that process also called crossing-over? Or is crossing-over only a type of Holliday junction?
How is that possible, that two not identical strands from two different molecules join together? I think that there may be some correcting process, especially, that I found that:
http://www.youtube.com/watch?v=1JynBEX_kg8&feature=fvw
but then- How is it decided which strand is the correct one? Or are the mismatching pairs repaired randomly?
Then there is also a problem of DSBR, SDSA and the dofference between them as well as NCO and CO. I have a lot of crossing lines and arrows here in the book and can't really make much sense of it. If it's not possible to give me siple instructions I would be grateful for some good source of information (link or something).
I've already finished high school and haven't started university yet, so while reading books on genetics I sometimes come across such problems. I can't solve them by myself. Please, help!

[Marta]

I'm sorry, I didn't mean that link, I don't know how it happened ;p
There was to be a short animation showing that after H-J some nucleotydes are replaced with some other to create correct pairs A-T, G-C.
Still, as far as I can remember such processes are quite ATP-consuming, so why does the cell allow them to happen? If it's connected with crossing over- then recombination will be "payment" for the ATP-lost? Then, doen't the newly created DNA strand loose it's function with exchanged nucleotydes? At least- some genes? Is it possible that the H-J couses recombination inside one gene, or affects oly some small part of it?

i'm sorry if I'm not clear enough. English is not my native language... at all ;p

[Marta]

Is it really impossible to give me an answer?

[MrMistery]

Marta,

A Holliday junction is like the molecular mechanism of crossing-over if you will. While crossing over is something described mostly by cytogenetisists, Holliday junctions are the topic of molecular biologists.

Now, what you are missing is that there is a very elaborate enzyme machinery working in this process. The three most important enzymes that function in homologous recombination are RecA ( http://en.wikipedia.org/wiki/RecA ), RecBCD ( http://en.wikipedia.org/wiki/RecBCD ) and RuvABC ( http://en.wikipedia.org/wiki/RuvABC ). Let me know if there is anything you don't quite understand from those. Also, just a heads up: you will find a lot about double strand break repair when reading on those enzymes. Double strand break repair is mechanistically the same thing as homologous recombination.