NATIONAL SCIENCE FOUNDATION
U.S. DEPARTMENT OF EDUCATION
The second major step in processing
pre-mRNA is the removal of a portion of its 3' end. Clicking
on each of the thumbnail images will bring up a larger, labeled
version of the described scene.
see the Flash movie for the following sequence of images,
Certain signal sites control
where the proteins required for cleavage and polyadenylation
will attach. The upstream poly(A) signal is a specific sequence
The downstream poly(A) signal is not a
specific sequence, rather it is a GU-rich region on the pre-mRNA.
After transcription has been completed,
the first two cleavage factors attach to the up and downstream
poly(A) signal sites.
These cleavage factors interact with each
other to bring the signal sites into proximity with each other,
forming a loop in the pre-mRNA.
Two additional cleavage factors
then attach and help to stabilize the complex.
Poly(A) polymerase (PAP) then interacts
with the complex and cleaves the strand of pre-mRNA at the
Once cleavage has occured, most of the
cleavage factors are released.
--> 3.) POLYADENYLATION