Gene transcription 2

Question 1

What is mRNA processing?

Answer 1

Splicing out introns before it exits the nucleus and enters the cytoplasm

Question 2

What bases do introns begin with and end with and what do exons end with?

Answer 2

Splice donor at 5' and accpetor at 3'
Start: GU
End: AG
GU-------AG
Exons end with: AG

Question 3

what is the splice donor site and splice acceptor site

Answer 3

Splice donor site: junction between exon and intron

Splice acceptor site: junction between intron and exon.

Question 4

What type of RNA do RNA processing use?

Answer 4

snRNPs

small Ribonuclear Proteins

Question 5

Describe how small ribonuclear proteins (snRNPs) bind to the mRNA.

Answer 5

U1 binds to the splice donor site, U5 binds to the splice acceptor site, U2, U4 and U6 bind in the middle of the intron.
Binding of U2,U4,U5 and U6 completes formation of the splicing complex/spliceosome.
This results in a cleavage of the splice donor sequence.

Question 6

Describe how the intron is removed from the mRNA strand.

Answer 6

The RNA cleaves at the splice donor site and the first G loops round and forms a phosphodiester bond with an adenine residue in the middle of the intron (branchpoint). The mRNA then cleaves at the splice acceptor site and the intron is removed as a lariat structure.
Adjacent exons are ligated together by ligase enzyme

Question 7

What two structures are added during post-translational modification and why?

Answer 7

Cap is added to the 5’ end of mRNA.
Cap structure:
-is formed by hydrolysis of terminal triphosphate of mRNA to a diphosphate
-cap is a guanosine molecule that is methylated at the 7’ position by methyltransferase so it is 7-methylguanylate cap.
-protects RNA from degradation and greatly enhances translation

Poly-A tail:
Addition of a POLY A TAIL to mRNA at 3’ end
occurs 11-30 bases downstream of an AAUAAA sequence in the mRNA
– also acts as protection
– added one base at a time

Question 8

Give an example of a disease that affects one of the structures added during post-translational modification.

Answer 8

Polio Myelitis:

• interferes with recognition of cap during translation
• highly infectious disease
• virus can cause total paralysis very quickly
• mainly affects children

Question 9

Give an example of a mutation in a splice site which features in human disease.

Answer 9

Thalassaemia:
-imbalance in the amount of alpha and beta chains in heamoglobin.
-common in SE Asia and the Mediterranean
Features:
-iron overload, causing cardiomyopathy
Several types of Beta- thealassemia feature splice site mutations in B-globin gene.

Duchenne Muscular dystrophy ( DMD):
-Mutation in Dystrophin gene
Most DMD caused by deletions of one or more exons
Causes premature end during translation
TREATMENT: to favour Alternative Splicing