Class Notes (807,479)
Canada (492,657)
Biology (Sci) (2,417)
BIOL 300 (90)
Lecture 13

Lecture 13 October 5.pdf

4 Pages
Unlock Document

McGill University
Biology (Sci)
BIOL 300
Siegfried Hekimi

th BIOL 300 October 5 2012 Lecture 13 Dr. Schӧck The main components of post-transcriptional regulation will all determine how much, and in what levels, RNA will be translated into proteins. RNA Polymerases walk along the DNA in order to transcribe RNA; the main one is Pol II, we won’t consider the other polymerases. This takes place within the nucleus. Very early on, the transcript is capped at the 5’ co- transcriptionally (i.e. while transcription is still going on) • Other processes related to the transcript (splicing and polyadenylation) are also tightly coupled to specific processes in transcription • Alternative splicing results in different functional proteins resulting from one pre-mRNA Next, the mRNA is exported into the cytoplasm from the nucleus; there are quality control mechanisms by certain proteins to make sure only correctly processes mRNAs are exported Once in the cytoplasm, there are other mechanisms, like poly A extension, which can regulate levels of translation • Every mRNA has a certain half-life, determined by the poly A tail which is attacked by an enzyme, which determines how long the mRNA lasts in the cell before it is degraded All of the steps mentioned above can be regulated • Another regulatory mechanism use miRNAs, which actually modify RNA stability • Very often, there is localization of mRNAs before translation in the cytoplasm in order to increase efficiency of the proteins, so that are translated in the area where are needed Review of eukaryotic pre-mRNA processing: • Capping at the 5’ end • Polyadenylation at the 3’ end • Splicing to remove introns This is a diagram of a primary RNA transcript produced by RNA Pol II; the exons denoted in red and the introns denoted in blue. The Poly(A) site is at the 3’ end. • Introns are generally much larger than exons. First, the 5’ cap is added, followed by cleavage of the Poly(a) site, followed by 3’ polyadenylation and then 1 th BIOL 300 October 5 2012 Lecture 13 Dr. Schӧck RNA splicing • It is important to remember that a lot of these steps are coupled and occur more or less at the same time (not sequentially, as shown in the diagram) • 5’ capping usually takes place as soon as transcription takes place, while splicing and polyadenylation happen further on during transcription once the correct sequences are present RNA Pol II is responsible for coupling the three events mentioned before. Remember, RNA Pol II has a globular portion which carries out the main transcription function, and a linear CTD connected to the globular portion by a linker peptide; the CTD is in fact much larger in diameter than the globular portion • In yeast, the CTD is about 65 nm long, in humans a bit longer • The CTD of Pol II extends behind the globular portion, and is able to bind a series of factors which can carry out various processing mechanisms • These factors are present even during transcriptional initiation, they are just waiting for their target sequences to be exposed • This is an efficient processing mechanism because the binding of the factors to the CTD increases the local concentration of these proteins near the target mRNAs, rather than having the factors freely floating in the cytoplasm • These factors are only able to bind the CTD when Pol II I actively transcribing, i.e. when the CTD is phosphorylated on certain residues. If the CTD is not phosphorylated, the factors will not be able to bind. The CTD is a series of heptapeptide repeats (7 amino acids); some amino acids, like serines, are able to be phosphorylated. The main serines in the repeat which are able to be phosphorylated are serines 2 and 5 • Phosphorylation of serine 5 is done following formation of the pre-initiation complex and is mediated by TFIIH which allows for binding of the capping factor • Later on during elongation, serine 5 is de- phosphorylated and serine 2 is phosphorylated which allows factors for spicing to bind to the CTD • This makes sense because capping enzymes are needed before splicing machinery Summary: • THIIH first phosphorylates serine 5, allowing binding of the capping enzymes • THIIH later on phosphorylates serine 3, allowing binding of splicing machinery 2 th BIOL 300 October 5 2012 Lecture 13
More Less

Related notes for BIOL 300

Log In


Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.