L7/8 Post translational control of gene expression

Question 1

When the pre-mRNA has been transcribed, post translational control occurs. What are the 4 post translational modifications?

Answer 1

1. Capping
2. Polyadenylation
3. Splicing
4. editing

Question 2

Describe the 5’ m7G Cap.

Answer 2

Added to 5’ end of mRNA post transcriptionally. It is not encoded in the genome. Capping has two events - adding the GpppN structure and methylation. Methylation of phosphate bond and base changes chemical behaviour. The Cap is added before splicing occurs. It is added to the pre-mRNA.

Functions of Cap -
1. facilities export of mRNA from nucleus to cytoplasm. Cap binds to CBP80/CPB20 complex in nucleus.

2. Protects mRNA from degradation by 5’-3’ nucleases
3. Facilitates splicing
4. Cap is critical for translation. Cap binds to eIF4 in cytoplasm

Question 3

What is similar about the polyA tail and the Cap?

Answer 3

They are both added post transcriptionally therefore are not encoded in the genome

Question 4

Once the cap has been added to the pre-mRNA, what happens?

Answer 4

Splicing and polyA tail is added.

Question 5

how many conserved sequences are in introns and what are they?

Answer 5

3. 5’ spice site on intron
   branch site (contains adenosine residues)
   3’ splice site on intron

Question 6

Describe the two step splicing of introns. What is the name of the type of reaction?

Answer 6

Step 1- 5’ splice site is cleaved and a bond is created between 5’ end of intron and branch site. You are left with exon 1 and intron LARIAT intermediate

Step 2- Cleavage at 3’ splice site- releases lariat. Two exons are ligated together.

Trans-esterification reactions

Question 7

The spliceosome is another way of removing the introns. What is the sliceosome?

Answer 7

an enzymatic complex that catalyses the removal of introns. Catalysed by snRNAs and involves ATP.

The sliceosome contains snRNPs (small nuclear ribonucleoprotein particles), which are RNA-protein complexes in the nucleus.The snRNPs are U1, U2, U4, U5 and U6.

Question 8

What do all snRNPs contain?

Answer 8

An Sm ring which helps to stabilise binding to RNA.

Question 9

U1 and U2 are two snRNPs. What conserved sequence do they both attach to?

Answer 9

U1 attaches to the 5’ splice site, U2 attaches to the branch site.

Question 10

What autoimmune disease is causes by anti-Sm antibodies?

These antibodies would cause destruction to Sm ring in the snRNPs and therefore splicing wouldn’t occur.

Answer 10

Systemic Lupus Erythmatosus (SLE)

Question 11

What does alternative splicing do?

Answer 11

It expands the amount of proteins made from genes - expands the proteosome.

Question 12

How is alternative splicing regulated?

Answer 12

Activators bind to either intronic splicing enhancers (ISE) or exonic splicing enhancers (ESE). Will activate splicing

Repressors bind to intronic splicing silencers (ISS) or exotic splicing silencers (ESS) to repress splicing

Question 13

What are 3 diseases caused by defects in splicing due to mutations?

Answer 13

1. Spinal muscular atrophy
2. Retinitis Pigmentosa
3. Myotonic dystrophy