Primer Extension Assay

[Danja]

Could somebody please explain to me how the primer extension assay detects the 5' end of the transcription start site? I found the following diagram of the assay:

http://www.biochem.arizona.edu/classes/ ... ension.htm

However, that diagram is unlabeled and I'm really confused at to what hybridizes to what, and at what end. Could somebody please explain how the assay detects the transcription start site, or draw a diagram with the ends labeled? Also, why do this assay instead of just sequencing the transcript?

Thanks!

[JackBean]

didn't you ask already? You have specific primer with which you amplify fragment of specific length. Then with simple math you calculate, where does the mRNA start.

[Danja]

I definitely didn't ask already; I just registered on this forum yesterday. I also searched for primer extension and found no results. If you could show me the thread where this question has already been asked, I would appreciate it.