Lecture 4- Gene organisation and transcription II (mRNA Processing)

Question 1

What happens during RNA processing?

Answer 1

pre-mRNA (original RNA transcribed from a gene) –> heterogenous nuclear RNA (hnRNA)

Question 2

Where does RNA processing take place?

Answer 2

In the nucleus

Question 3

Where is the gene promoter found?

Answer 3

The Promoter lies at the 5’ end of the gene

Question 4

What is an exon?

Answer 4

Segments of gene which contain sequences that form part of the final RNA = exons

Question 5

What is an intron?

Answer 5

Segments of gene which are transcribed but edited out of final mRNA= Introns

Question 6

What is the splice donor site?

Answer 6

The exon | intron junction. The base sequence is usually AG|GU.

Question 7

What is the splice acceptor site?

Answer 7

The intron | exon junction.

Question 8

What sequence do introns usually end with?

Answer 8

Pyr15NCAG
Pyr= pyrimidine (C or U)
N= any base

Question 9

Which molecule does RNA processing use?

Answer 9

small Ribonuclear Proteins (snRNPs)

Question 10

With the aid of diagrams, explain the sequence in which the snRNPs bind

Answer 10

1) U1 binds to the splice donor sequence
2) U2, U4 and U6 binds to the intron
3) U5 binds to the splice acceptor site thus completing the formation of the splicing complex.

Question 11

What is another name for the splicing complex?

Answer 11

Spliceosome

Question 12

What happens once the spliceosome forms?

Answer 12

1) The splice donor sequence cleaves
2) The end G of the intron curves around to an A residue in the intron which acts as a branch-point
3) A phosphodiester bond forms between the 5’ phosphate on the G and the 2’ OH on the A
4) The phosphodiester bond between the end G of the intron and the exon breaks and the intron breaks off
5) The exons are ligated together

Question 13

What is the name of the shape of the excised intron?

Answer 13

Lariat

Question 14

What structure is added to the 5’ end of pre-mRNA and how?

Answer 14

CAP

1) The terminal triphosphate is hydrolysed to a diphosphate.
2) The diphosphate reacts with the a-phosphate of GTP to form a 5’-5’ phosphate linkage
3) The N7 position of the guanine ring is methylated to form a 7-methylguanylate cap.

Question 15

What is the function of the CAP structure?

Answer 15

• protects mRNA at the 5’ end

- enhances the translation of mRNA

Question 16

Which disease is associated with this structure and how?

Answer 16

Polio- it interferes with the recognition of the cap during translation.

Question 17

Describe some of the features of Poliomyelitis

Answer 17

• highly infectious
• Virus invades the nervous system and can cause paralysis within hours
• mainly affect < 3yrs
• enters through mouth and proliferates in intestines
• symptoms: fever, fatigue, headache, vomiting, stiffness
• Motality: 5-10% when muscles for breathing paralysed.

Question 18

What is added to the 3’ end of pre- mRNA and how?

Answer 18

A poly A tail

• process called polyadenylation
• A’s added one base at a time
• Poly A tail added 11-30 bases downstream of AAUAAA sequence found in all mRNAs

Question 19

What is Thalassemia?

Answer 19

A genetic disorder in which there is an imbalance in the relative amounts of a-chains and B-chains making up haemoglobin.

Question 20

Where is thalassemia common? and why?

Answer 20

Common across Mediterranean and S.E Asia because of its resistance to malaria

Question 21

What could cause B-Thalassemia?

Answer 21

Several types of B-thalassemia feature splice site mutations in B-globin gene

Question 22

What are some of the features of B-Thalassemia?

Answer 22

1) Iron overload (Hemosiderosis)- elevate Fe absorption due to chronic anaemia.
2) –> Hepatic fibrosis, cirrhosis, darkening of skin, cardiomyopathy
3) Hepatosplenomegaly