RNA Processing

Question 1

What are the 3 steps in mRNA processing?

Answer 1

1. Capping of 5’end of pre-mRNA
2. Polyadenylation of the 3’ end of pre-mRNA
3. Splicing of Pre-mRNA

Question 2

What is the name of the enzyme that adds GTP to the 1st nucleotide of pre-mRNA? And what kind of unique bond does it form?

Answer 2

1) Guanylyltransferase (capping enzyme)

2) 5’-5’linkage

Question 3

When does the capping rxn occur?

Answer 3

Immediately after the 1st phosphodiester bond is formed

Question 4

What is the role of methyltransferaces in the capping of the 5’end of pre-mRNA?

Answer 4

Methylation of N-7 on the terminal guanine and the 2-OH on riboses of the next 2 nucleotides.

Question 5

What is the function of 7-methylguanosine cap?

Answer 5

1) Required for initiation step of translation

2) Enhances stability

Question 6

What is the polyadenylation sequence in the 3’end of pre-mRNA?

Answer 6

AAUAAA in the 3’-UTR last exon

Question 7

What is the name of the enzyme that recognizes and cleaves the 3’end for polyadenylation?

Answer 7

Cleavage and Polyadenylation Specificity Factor

Question 8

What enzyme add the poly A tail to the 3’end?

Answer 8

PolyAPolymerase

Question 9

What are the functions of the polyA tail?

Answer 9

1) Binding sit for PolyA Binding Protein
2) mRNA stability
3) Regulation of initiation step of translation

Question 10

What are the components of a splicesome?

Answer 10

Pre-mRNA (hnRNA) and snRNPS (small nuclear ribonucleoprotein particles)

Question 11

What consensus sequence is at the donor 5’ splice site?

Answer 11

GU

Question 12

What is the consensus sequence at the acceptor 3’ splice site?

Answer 12

AG

Question 13

What is the mechanism for the splicing reaction?

Answer 13

1) First cleavage-ligation reaction (transesterification): 2’OH-A from branch site cleaves the GU on the 5’donor site
2) Second C-L rxn (transesterfication): -OH from 5’ attacks A-G on the 3’ end

Question 14

What is a product of splicing?

Answer 14

Lariat

Question 15

What is the unique bond that forms btw branch site A in the intro and the 5-end donor site of the intron?

Answer 15

2’-5’ phosphodiester bond

Question 16

What is the function of U1 snRNP?

Answer 16

Pairs with donor 5’ splice site of pre-mRNA

Question 17

What is the function of U2 snRNP?

Answer 17

Pairs with A-branch site; one of the components of the ribozyme

Question 18

What is the function of U4?

Answer 18

Interacts with U6 snRNP

Question 19

What is the function of U5?

Answer 19

Pairs with 3’ acceptor site; aligns splice sites for catalysis by ribozyme.

Question 20

What is the function of U6?

Answer 20

Displaces U1 at 5’ splice site

Forms with U2 the ribozyme

Question 21

What are the two snRNPs that form the ribozyme to catalyze splicing reactions?

Answer 21

U2 and U6

Question 22

What are the 2 main roles of enhancers and silencers?

Answer 22

1) Regulate splicing at specific exon-intron boundaries

2) Enable correct splice sites in pre-mRNA to be distinguished from cryptic splice sites

Question 23

What is the function of Exon Splicing Enhancers (ESE) and Intron Splicing Enhancers (ISE)?

Answer 23

Cis-acting elements that facilitate recognition of an exon by the splicesome

Question 24

What is the function of Exon Splicing Silencers (ESS) and Intron Splicing Silencers (ISS)?

Answer 24

Cis-acting elements that DO NOT facilitate recognition of an exon by the splicesome.

Question 25

Do they type, #, and position of cis-acting elements (e.g. silencers and enhancers) vary at each exon-intron junction within each pre-mRNA?

Answer 25

YES

Question 26

What is the generation of differenct mRNAs from the same pre-mRNA by splicing exons together in different combos?

Answer 26

Alternative splicing

Question 27

What is B+-thalassemia?

Answer 27

Partial reduction in synthesis of B-hemoglobin chains; a hereditary anemia that results from a splicing defect

Question 28

What is B0-thalassemia?

Answer 28

No synthesis of B-hemoglobin chains; a hereditary anemia that results from a splicing defect

Question 29

What is the effect of a splicing error?

Answer 29

A frameshift results in a non-sense mutation bc the reading frame contains a stop-codon so the mRNA of B-hemoglobin degrades and the protein isn't produced.

Question 30

What is RNA editing?

Answer 30

Change in the nucleotide sequence of RNA by a process other than splicing

Question 31

Give an example of RNA editing?

Answer 31

Deamination results in a different base. E.g. small intestine C to U shorted form of Apo-B-100 protein