Gene organisation and transcription II

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?

Answer 2

GU —-AG

Question 3

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

Answer 3

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

Question 4

Describe how the intron is removed from the mRNA strand.

Answer 4

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.

Question 5

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

Answer 5

7-methylguanylate cap – protects the RNA from degradation – enhances translation
Poly-A-tail – occurs 11-30 bases downstream of an AAUAAA sequence in the mRNA – also acts as protection – added one base at a time

Question 6

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

Answer 6

Poliomyelitis affects 7-methylguanylate cap recognition during translation

Question 7

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

Answer 7

Beta Thalassemia – splice site mutation in the beta globin gene – caused by imbalances in the relative amounts of alpha and beta chains