NAGE 4

Question 1

What is pre-mRNA aka?

Answer 1

Primary transcript or heterogenous nuclear RNA (hn RNA)

Question 2

Where does RNA processing occur?

Answer 2

Nucleus

Question 3

Where does the gene promotor lie in the gene?

Answer 3

Towards the 5’ end

Question 4

What is the splice donor site?

Answer 4

Junction between exon and intron.

AGGU which ends the exon and starts the intron is the splice donor sequence.

Question 5

What is the splice acceptor site?

Answer 5

Junction between intron and exon

Pyr15NCAG which ends intron is the splice acceptor sequence.

Question 6

What do introns start and end with?

Answer 6

Start with GU

End with AG

Question 7

What do exons usually end in?

Answer 7

AG

Question 8

What general sequence do introns have?

Answer 8

GU———Pyr15NCAG

Question 9

What does RNA processing use?

Answer 9

small nuclear Ribonucleoproteins (snRNPs)

Question 10

Explain how splicing works.

Answer 10

1. U1 (snRNP) binds to splice donor sequence (AG/GU).
2. U2, U4, U5 and U6 bind to pre-mRNA strand. U5 binds to splice acceptor sequence (Pyr15NCAG). This forms the “spliceosome” (splicing complex).
3. Spliceosome formation results in cleavage of the splice donor sequence. AG separates from GU (phosphodiester bond hydrolysed).
4. End of G intron bends around. “A” residue acts as a branchpoint (intermediate step in splicing).
5. Phosphodiester bond forms between 5’ phosphate on “G” and 2’ OH of “A” residue. UG-A.
6. Causes splice acceptor site to be cleaved. Phosphodiester bond between last G of intron and first base of exon is cleaved.
7. Intron removed as LARIAT structure.
8. Adjacent exon are ligated together.

Question 11

Describe what happens in post-transcriptional modification of mRNA.

Answer 11

1. “Cap” structure added to 5’ end of mRNA.

2. Poly-a tail added

Question 12

Describe the addition of the “CAP” to mRNA.

Answer 12

1. Cap formed by hydrolysis of terminal triphosphate of mRNA to diphosphate.
2. This diphosphate then reacts with the a-phosphate of GTP to form a 5’-5’ phosphate linkage. (done by guanyl transferase
3. Further modification as the N7 position in the purine ring is methylated (methyl transferase). Forms a 7-methylguanylate cap.
4. Cap protects mRNA at 5’ end and greatly enhances mRNA translation
5. Viruses (e.g. polio interfere with cap recognition during translation)

Question 13

Explain the addition of the poly-a tail.

Answer 13

1. Poly-a tail added to pre-mRNA.
2. Poly-a tail added one base at a time, (roughly 200 “A” bases at the 3’ end)
3. Poly-a tail found 11-30 bases downstream from AAUAAA, found in all mRNAs.