Molec Bio CH. 15

At what end of the mRNA does capping occur?

5' end

What structure is added to the 3' end of eukaryotic mRNAs?

Poly-A tail

Where is the majority of mRNA methylation clustered?

At the 5'-end

Which specific modified guanine makes up the cap structure?

7-methylguanosine (m7G)

What type of chemical linkage connects the cap to the mRNA?

Triphosphate linkage

In the cap structure, which component contributes a positive charge?

m7G

How many negative charges does the triphosphate linkage contribute to the cap structure?

3

Which enzyme removes the terminal phosphate from pre-mRNA during the first step of cap synthesis?

RNA triphosphatase

Which enzyme adds the capping GMP from GTP?

Guanylyltransferase

Which enzyme methylates the N7 of the capping guanosine?

Methyltransferase

How early in transcription does cap synthesis occur?

Before the chain is 30 nucleotides long

Name two of the four primary functions of the mRNA cap.

• Protecting mRNA from degradation

• enhancing translatability

What is the term for a long chain of AMP residues at the 3' end of mRNA?

Poly (A) tail

What is the precursor to mRNA called?

Heterogeneous nuclear mRNA

Approximately how many nucleotides long is a standard poly(A) tail?

250 nucleotides

Which enzyme adds the poly(A) tail post-transcriptionally?

Poly(A) polymerase

Why is poly(A) not considered a product of transcription?

It is not encoded in the DNA

What effect does the poly(A) tail have on the lifetime of mRNA?

It enhances the lifetime

Does the poly(A) tail assist in the transport of mRNA out of the nucleus?

Yes, it is required for efficient transport

What happens to eukaryotic transcripts before they are polyadenylated at the 3' end?

They are cleaved (cut)

What is the specific hexamer motif located about 20 nucleotides upstream of the polyadenylation site?

AAUAAA

Which motif follows the AAUAAA sequence by 23 or 24 base pairs in a mammalian polyadenylation signal?

GU-rich motif

What are the two main events involved in polyadenylation?

Pre-mRNA cleavage and polyadenylation at the cleavage site

What does the acronym CPSF stand for?

Cleavage and Polyadenylation Specificity Factor

Which protein binds directly to the AAUAAA motif to initiate the process?

CPSF

After the poly(A) tail reaches 10 nucleotides in length, what does further polyadenylation depend on?

The poly(A) itself

Which specificity factor is required for the elongation of the poly(A) tail to 250 nucleotides?

Poly(A)-Binding protein II (PAB II)

Which specific domain of RNA polymerase II is involved in all three types of RNA processing?

The carboxy-terminal domain (CTD)

What is the first step of transcription termination by RNA polymerase II?

Co-transcriptional cleavage (COTC)

What serves as a platform for capping, polyadenylating, and splicing enzymes to bind directly to?

The CTD of the Rpb1 subunit of RNA polymerase II