mRNA processing

Question 1

Types of mRNA processing

Answer 1

1. 5’ cap
2. Splicing
3. Poly A tail

Question 2

The 5’ cap is a modified

Answer 2

7 methyl guanosine with an unusual 5’-5’ bond

Question 3

Functions of the 5’ cap (4)

Answer 3

• regulation of nuclear export
• prevention of degradation
• promotion of translation
• promotion of intron excision

Question 4

Splicing occurs at

Answer 4

consensus sequences

Question 5

The 3’ splice site begins with

Answer 5

AU

Question 6

The 5’ splice site begins with

Answer 6

GU

Question 7

Steps of pre-mRNA splicing (4)

Answer 7

• ADENINE within the intron attacks the 5’ splice site
• cut 5’ end of intron covalently links to the adenine, forming a loop
• 3’ OH exon end attacks the 5’ end of next exon sequence
• intron is released as a lariat

Transesterification

Question 8

Intron removal reactions are called

Answer 8

transesterifications

Question 9

The spliceosome contains

Answer 9

snRPS
- contain snRNAs and 7 proteins

small nuclear RNAs (snRNAs): U1, U2, U4, U5 and U6

Question 10

Steps of snRPS binding in splicing (6)

Answer 10

• U1 binds to 5’ splice site on exon 1
• U2AF and BBP bind to branch point
• U2 kicks off U2AF and BBP
• U4/U6/U5 triple snrp create active site and loop pre-mRNA
• U6 kicks off U1 at 5’ splice site
• 3’ OH of exon 1 attacks 5’ site of exon 2

Question 11

Splicing occurs in the

Answer 11

Nuclear speckles

interchromatin granule clusters + spliceosomal machinery and RNA

Question 12

Alternative splicing is regulated by

Answer 12

SR proteins (enhancers)

hnRNPs (silencers)

Question 13

Serine-arginine rich proteins (SR)

Answer 13

Enhance splicing

in introns and exons by binding to enhancer sequences (ESEs)

Question 14

Heterogenous ribonucleoprotein particles (hnRNPs)

Answer 14

Inhibit splicing

bind to intron and exon silencer sequences (ESS)

Question 15

Splicing enhancer and silencer sequences can be

Answer 15

located far from splice sites but are brought into proximity by looped RNA structure

Question 16

Functions of the polyA tail

Answer 16

• regulation of nuclear export
• promotion of translation
• regulation of degradation

(the tail is shortened over time by ARE’s that determine rate of decay)

Question 17

Polyadenylation begins as

Steps (3)

Answer 17

transcription terminates

The 3’ end of the pre-MRNA is cleaved by CPSF, CtsF and CFI and II proteins
PAP (polyadenylate polymerase) adds the poly As
PABPs circularise mRNA and promote translation

Question 18

CtsF and CPSF bind to

Answer 18

the 3’ end as it emerges

Question 19

Polyadenylate polymerase

Answer 19

adds the poly As

Question 20

Poly A binding proteins (PABPs)

Answer 20

circularise mRNA and promote translation

Question 21

Alternative polyadenylation

Answer 21

adds more variation to mRNA transcripts

Question 22

RNA is actively exported into the

Answer 22

cytoplasm through nuclear pore complexes (NPCs) by hnRNPs

Question 23

hnRNPs unwind the hairpin helices of RNA

Answer 23

so that RNA can be read more easily

Question 24

RNAs are directed to specific sites in the cell by

Answer 24

the 3’ UTR

by directed transport on the cytoskeleton or random diffusion and trapping by docking proteins

Question 25

The 3’ UTR contains

Answer 25

binding sites for regulatory proteins
miRNAs
silencer regions (bind repressor proteins to inhibit the expression of the mRNA)

Question 26

RNA processing is

Answer 26

co transcriptional

transcription, splicing and polyadenylation occur at the same time