RNA Processing

Question 1

What are the 4 main types of RNA processing?

Answer 1

1. Transcription and capping at the 5’ end.
2. Cleavage
3. Polyadenylation
4. Splicing

Question 2

How does RNA capping occur?

Answer 2

1. When the C-terminal domain of RNAP is phosphorylated, it recruits the capping enzyme complex.
2. Guanylyl-transferase removes the γ-phosphate of the 5’ nucleotide and the β and γ phosphates of GTP- adds a guanosine to the 5’ end of mRNA via a 5’ to 5’ triphosphate linkage.
3. Guanine methyltransferase converts the added guanosine to 7-methylguanosine.

Question 3

How does polyadenylation occur?

Answer 3

1. Poly(A) hexanucleotide signal binds to the Cleavage and Polyadenylation Specifity factor and the GU-rich downstream element binds to the Cleavage Stimulation factor, both on the mRNA.
2. This positions the mRNA 3’end for cleavage of the GU-rich region at the 3’ end of pre-mRNA, by an endonuclease.
3. Poly(A) polymerase produces the poly(A) tail, using ATP.
4. Poly(A) binding protein binds.

Question 4

What is the purpose of polyadenylation and the poly(A) binding protein?

Answer 4

To stabilise and prevent degradation of the mRNA.

Question 5

What is the main type of intron found in eukaryotes?

Answer 5

GU-AG intron.

Question 6

How many classes of introns are there in eukaryotes?

Answer 6

7.

Question 7

What type of reactions are involved in the splicing mechanism?

Answer 7

Transesterification reactions.

Question 8

What is the spliceosome mechanism?

Answer 8

1. U1 binds 5’ splice site and U2 binds to the A branch site.
2. U4, U5, U6 bind to the intron, completing spliceosome assembly.
3. 5’ splice site is cut, 5’ end of intron is connected to the branch site- forming a lariat structure. U1 and U4 are released, U5 and U6 change positions.
4. 3’ splice site is cut and exons are ligated together.
5. Lariat intron is released with the remaining parts of the spliceosome.

Question 9

What is the spliceosome made of?

Answer 9

Small ribonucleoproteins (snRNPs), e.g. U1 snRNP.

Question 10

What are snRNPs?

Answer 10

snRNAs + proteins + mRNA.

Question 11

Give an example of how mutations in the splice site can cause disease.

Answer 11

Incorrect splicing of the β globin gene can cause thalessemia.

Question 12

Give an example of a gene that undergoes alternative splicing.

Answer 12

α-tropomyosin- can have at least 9 different mRNA transcripts, different tropomyosins are found in different cell types.

Question 13

What is alternative splicing?

Answer 13

Different combinations of exons producing different mRNA transcripts. This alters the structure and function of the proteins produced.

Question 14

Where are group I introns found?

Answer 14

In pre-mRNA.

Question 15

Where are group II introns found?

Answer 15

In organelle genomes.

Question 16

What feature of both group I and group II introns supports the RNA world theory?

Answer 16

They are autocatalytic so can catalyse their own removal from RNA.

Question 17

Does Poly(A) polymerase require a template to synthesise the poly(A) tail?

Answer 17

No.

Question 18

How are mRNAs degraded after translation?

Answer 18

Degradation of eukaryotic mRNAs usually starts with shortening of the poly(A) tail then decapping of the 5’ end and then degrading the RNA in the 5’ to 3’ direction.