DNA Structure and Replication

ID #1782

As mentioned in our notes there is a OPO3- group on the 5'end of the DNA strand, and an OH group on the 3'end. BUt I also know that there are phosphodiester linkages to the incoming dNTP to the 3'end. So with that being said, is that why the 5'cap can only bond with the 5'end and not the 3'end because of the OPO3-? Also, what is holding the 5'cap to the DNA strand, I mean which type of bond it is?

First, please be very careful with your terminology. 5' caps are structures found on eukaryotic mRNAs, nothing else. DNA molecules don't have 5' caps, so they in turn have absolutely nothing to do with DNA replication. There is a figure in Lecture 12 that shows part of a DNA double helix, and there is a phosphate group at the 5' ends, but this isn't the same thing as a 5'-methylguanosine cap structure. If you're asking why cap structures aren't added to the 3' ends of mRNAs instead of the 5' ends, take a careful look at the figure from Lecture 12 that shows the double helix. There is no available phosphate group at the 3' end, and a free phosphate group (actually a triphosphate group) is required to form that unusual phosphate-phosphate bridge. So it's physically impossible to add the cap to the 3' end.

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