RNA Processing During RNA Transcription

Question 1

How are RNA transcripts that are destined to become mRNA molecules processed?
(3 ways)

Answer 1

RNA capping (5’ end)

Polyadenylation (throughout synthesis)

RNA splicing (3’ end)

Question 2

When does RNA processing happen?

Answer 2

During transcription

Question 3

How does RNA capping work?

3

Answer 3

The modification of the 5’ end of the mRNA transcript

RNA is capped by addition of an atypical nucleotide

Occurs after RNA pol has produced 25 nts of RNA (long before it has completed transcribing the whole gene)

Question 4

What is RNA capped with?

Answer 4

A guanine nucleotide with a methyl group attached

Question 5

At what point does RNA capping occur?

Answer 5

At the 25th nucleotide

Question 6

How does polyadenylation work?

4

Answer 6

Newly transcribes mRNA is provided with a specialised structure at the 3’ end (tail)

First the tail is trimmed by an enzyme which cuts mRNA at a particular sequence of nucleotides

A second enzyme then adds a series of repeated adenine nts onto the cut end (Poly-A tail)

Poly A tail is a few 100 nts long

Question 7

What does the first enzyme do during polyadenylation?

Answer 7

It trims the tail (3’ end) of RNA at a particular sequence of nucleotides

Question 8

What does the second enzyme do during polyadenylation?

Answer 8

It adds a series of repeated adenine nucleotides onto the cut end

Question 9

Why is capping and polyadenylation done?

4

Answer 9

It increases the stability of the mRNA molecule

It aids the export of mRNA from the nucleus to cytoplasm

It identifies mRNA molecule as mRNA

It indicates to the protein synthesis machinery that both ends of mRNA are present and the message is complete

Question 10

What percentage of RNA that is transcribed reaches the cytoplasm?

Answer 10

Only 5%

Question 11

Why does only 5% of RNA transcribed reached the cytoplasm?

Answer 11

Eukaryotic genes have their protein coding sequences (exons) interrupted by non-coding sequences (introns)

Question 12

What is splicing?

Answer 12

Removing non protein coding sequences from the transcribed mRNA

Question 13

What happens after splicing?

Answer 13

The exon sequences are ‘stitched’ together

Question 14

List the order of events in RNA processing.

3

Answer 14

Capping occurs on 5’ end first

Then splicing occurs

Then as mRNA is released and the 3’ end is exposed a Poly-A tail is added

Question 15

How does the cell determine which parts of the RNA transcript to remove during splicing?

Answer 15

Each intron contains a few short nucleotide sequences that act as cues for its own removal

Question 16

Where are the nucleotide sequences that act as cues for an intron’s removal found?

Answer 16

They are found at or near each end of the intron

Question 17

Name the two splice sites on an intron.

Answer 17

5’ splice donor site

3’ splice acceptor site

Question 18

What are the three types of short sequences involved in splices?

Answer 18

Splice donor (5’)

Splice acceptor (3’)

Branch site

Question 19

What is a branch site?

Answer 19

The AT rich region with at least one A that’s about 30 nucleotides upstream from the splice acceptor

Question 20

What molecule is mainly in charge of splicing?

Answer 20

Spliceosome

Question 21

What is the spliceosome?

2

Answer 21

Small nuclear RNAs (snRNA)

Combined with proteins (snRNP)

Question 22

How does the spliceosome work?

2

Answer 22

It recognises the beginning and end of introns

It brings the two ends of the intron together so that splicing can take place

Question 23

Explain how RNA splicing works, explaining how the adenine branch site works
(6)

Answer 23

A particular adenine (A) branch site in intron sequence attacks the 5’ splice donor site

The sugar-phosphate backbone of RNA is cut at the 5’ site

The cut 5’ end becomes covalently linked to adenine to form a branched structure

Free 3’-OH end of exon reacts with start of next exon sequence

2 exons join together into a continuous coding sequence

Intron is then released and eventually degrades

Question 24

What first happens in splicing?

3

Answer 24

A particular adenine (A) branch site in intron sequence attacks the 5’ splice donor site

The sugar-phosphate backbone of RNA is cut at the 5’ site

The cut 5’ end becomes covalently linked to adenine to form a branched structure

Question 25

What happens after the 5’ splice donor site is cut off from the sequence during splicing?
(2)

Answer 25

Free 3’-OH end of exon reacts with start of next exon sequence

2 exons join together into a continuous coding sequence

Question 26

What is alternative splicing?

Answer 26

The different ways exons can be reconnected to produce different mRNAs which can be translated into different proteins

Question 27

How does mRNA leave the nucleus?

Answer 27

Leaves through the nuclear pore complex

Question 28

What does mRNA need to be bound to in order to be transported?
(3)

Answer 28

Poly-A-binding protein

Cap-binding complex

Nuclear transport receptor

Question 29

Where does Poly-A-binding protein bind to mRNA?

Answer 29

Binds to the poly-A tail

Question 30

Where does Cap-binding protein bind to on mRNA?

Answer 30

Binds to the 5’ cap

Question 31

How does nuclear transport receptors work?

2

Answer 31

It only associates with mature mRNAs

It actively guides them through nuclear pores

Question 32

What eventually happens to all mRNA molecules even if they are translated many times?

Answer 32

All mRNA degrades into nucleotides by cellular RNases

Question 33

What in part controls the lifetime of mRNA?

2

Answer 33

The nucleotide sequences contained in the mRNA itself

Especially those in the 3’ untranslated region (area between 3’ end of coding sequence and poly-A tail)

Question 34

Where is the 3’ untranslated region found?

Answer 34

Between 3’ end of coding sequence and poly-A tail