BN - Splicing

Question 1

1.

How is the spliceosome loaded during transcription?

Answer 1

Co-transcriptional loading, where RNAP II carries proteins involved in splicing, and the spliceosome assembles as the gene is transcribed

Question 2

What are some features of the spliceosome? (5)

Answer 2

1) Comprised of both RNA (5) and protein (150)

2) RNA component: small nuclear RNAs (snRNAs)

• U1 U2 U4 U5 U6

3) Each RNA is complexed with 6-10 proteins to form small nuclear ribonuclear proteins (snRNPs)

4) Different complexes come in at different stages

5) Uses ATP

Question 3

What occurs during spliceosomal splicing? (4)

Answer 3

1. First U1 snRNP binds to 5’ splice site

• U1 snRNP consists of U1 snRNA + proteins

2. BBP binds at branch point

U2AF65/35 binds at 3’ splice site

3. U2 snRNP binds to branch site

• Aided by U2AF
• Displaces BBP

Branch point A residue is excluded creating a single nucleotide bulge

4. U4,5 and 6 snRNPs now bind

• The U2AF proteins are displaced
• This brings the 5’splice site close to the branch point and 3’splice site

Question 4

What are features related with self-splicing? (6)

Answer 4

• Can occur in rare introns in which the intron codes for a ribozyme which splices itself out of the RNA
• The ribozyme structure mimics the functions of the spliceosome and shares similarity in reaction mechanisms
• Hairpin and hammerhead structures
• Could be considered the first parasitic elements
• Can re-organise elements of genes to produce more than one product from a limited RNA genome
• Exon-exon shuffling can have an evolutionary effect

Question 5

What is Alternative splicing and how does it occur?

Answer 5

Alternative splicing can result in multiple products arising from the same gene

Alternative splicing

• A process by which the RNA can be spliced in more than one way
• This can result in the formation of 2 or more proteins from the same gene

The 1st gene known to be alternatively spliced

• Calcitonin/Calcitonin Gene Related Peptide (CGRP)

Question 6

What is RNA editing and what is an example?

Answer 6

A process in which information changes at the level of mRNA

Codon changed at the RNA level

E.g CAA → UAA

• UAA is a stop codon and so alters structure

Not a mistake (specific controlled reaction)

Question 7

What are examples of Substitution editing?

Answer 7

• Cytidine deaminases convert a C in the RNA to uracil (U)
• Adenosine deaminases convert an A to inosin (I) which the ribosome translates as a G

Both enzyme classes recognise target nucleotide sequence

Question 8

What are 3 features of Insertion/deletion editing?

Answer 8

• Mediated by guide RNA molecules
• These base-pair with RNA
• Serve as template for addition/removal of nucleotides

Question 9

How is Cytosine converted to Uracil? And where may this occur?

Answer 9

Cytidine deaminase action

• Amino group on nucleotide ring is removed and converted to carbonyl

Also occurs in glutamate receptors

• transmitter gated ion channel (in nervous system)
• here there is an A to I (inosine) change
  Catalysed by ADAR

→ Alters the Ca2+ permeability of the channel