HRR: RNA processing

Question 1

Name the 3 main steps involved in the processing of pre-mRNA to mRNA.

Answer 1

1.Capping: Addition of 7-methylguanosine to 5 ́-end of pre-mRNA
2. Polyadenylation: Addition of Poly A tail to 3 ́-end of pre-mRNA
3. Splicing: Removal of introns/joining of exons in pre-mRNA

Question 2

describe the capping of mRNA and the function of the cap

Answer 2

• 7-methylgunylate methyltransferase attached the cap to the 5’ end. the cap is then further modified via methylation. the cap helps protect the RNA and helps it be recognized by the ribosomes.

Question 3

Specify the sequence element and the main proteins involved in the cleavage and
polyadenylation of the 3’-end of pre-mRNA

Answer 3

A2UA3; it tells the CPSF complex to cleave off a 3’ end downstream of the site. The polyA polymerase then comes in and adds the polyA tail.

Question 4

Describe the 2 transesterification reactions and their corresponding roles in mRNA splicing.

Answer 4

Step one: branch site A attacks the 5’ end splice donor site (GU), and creates a linkage.

Step two: to address a free 5’ OH from the first step, exon 1 at the 5’ end connects to exon 2 at the 3’ end via phosphodiester bond. This removes the intron and connects the exons.

Question 5

the splice donor site is __

Answer 5

GU

Question 6

Describe the formation of a ribozyme, as well as the function of the subunits (snRNPs)

Answer 6

U1 snRNP: binds to the splice donor site (GU at 5’ end)
U2 snRNP: binds to the A branch site
U1 forms a complex with U5 and U6, kicking out U2 and U4. This then forms a ribozyme.
U2/U6 snRNP catalyzes the first esterification rxn, U5 catalyzes the second.

Question 7

identify the splice-site consensus sequences of an intron.

Answer 7

Always starts with GU at 5’ and always AG at 3’ and an A somewhere variable that is a branch site

Question 8

Describe how cis-acting elements in pre-mRNA regulate splicing.

Answer 8

There are sequences within the mRNA near the GU at the 5’ end that will either attract splicing proteins which will bind and recruit the U1 snRNP, or prevent the binding of snRNPs.

Question 9

Describe how alternative splicing enhances gene expression.

Answer 9

Alternative splicing: generation of different mRNAs from the same pre-mRNA by splicing exons together in various combinations.

Question 10

Describe how a mutation to a splice site can alter expression of the b-hemoglobin gene

Answer 10

A splice that includes an intron can cause a frameshift that results in a nonsense mutation due to a premature stop codon. Because of this, the b-hemoglobin mRNA degrades and the protein is not produced.

Question 11

Describe an example of RNA editing

Answer 11

Deaminase in the small intestine convert CAA to UAA, generating a new stop codon.