RNA Processing

Question 1

When RNA processing occurs

4 changes transcript undergoes

Answer 1

• Occurs before translation
• 4 changes;
  1. capping
  2. addition of polyA tail
  3. Removal of introns
  4. RNA editing

Question 2

Features of mRNA as it leaves nucleus

Answer 2

• only contains exons
• has a cap
• has a polyA tail

Question 3

RNA capping

• What it is
• other functions

Answer 3

• Is a methylated guanine base added in reverse orientation at the 5’ end of primary transcript
  
  • bases methlyated = unusual
• cap’s purpose is to protect mRNA from nucleases w/in cell that may destroy it

Other functions;

• enhances splicing of mRNA
• required for transport of mRNA to cytoplasm
• enhances tranlatability

Question 4

Polyadenylation of mRNA

• What it is
• Functions
• Enzyme responsible

Answer 4

• towards end have consensus sequence (11-30 nucleotides) where cleavage occurs after
• Lots of Adenines added (200-250)
• Enzyme Poly A polymerase adds poly A tail (Doesn’t require a template)

Functions:

• Protection against nucleases
• helps transport
• helps ribosome recognise mRNA as molecule to be translated

Question 5

Removal of Introns

• why
• how specificity in removal achieved
• what carries out splicing

Answer 5

• Introns: intervening sequences that don’t code for proteins
  - are removed before translation
• mRNA only has exons (whereas pre-mRNA contains both)
• No. of introns and exons vary in different genes
• Removal of introns needs to be v. precise - otherwise mutations will be introduced
  
  • ends of nuclear introns defined by GU-AG rule (start with GU and ends with AG)

-Introns removed by spliceosome (made up of snurps = snRNPs)

Question 6

4 types of introns

Answer 6

• Group 1 and group 2 (can splice by themselves - don’t need a protein [aka ribosomes])
• Nuclear pre-mRNA (spliceosome needed)
• tRNA (enzymatic)

Question 7

Self Splicing introns (group 1 & 2) differences

Answer 7

Group 1: requires guanosine or its phosphate [is the larger class]
Group 2: doesn’t need free guanosine or its derivatives (v. similar to nuclear RNA splicing - doesn’t require snRNAs)

Question 8

Why do Eukaryotes have introns? (3)

Answer 8

1. Some of miRNA come from introns
2. Alternative splicing can generate diff types of mRNA and hence polypeptides from same gene (human’s 35000 genes code for >100000 proteins)
3. Role in evolution: often code for a domain in a mulitidomain polypeptide chain - presence of introns make exon shuffling easier

Question 9

RNA Editing

• when
• 2 examples

Answer 9

• Process in which info changes at level of mRNA (NOT happening at DNA level)
• Happens after transcription of pre-mRNA

1. mRNA produced - guide RNA hybridises with pre-edited mRNA
   
   • are some bases in guide that isn’t in mRNA - mRNA adds extra bases
2. Base conversion - may create a base codon (UAA) which results in a shorter protein
   
   • called deanimation
   • seems to be changes to uracil

Question 10

tRNA processing

Answer 10

• Splicing of ends and loop
• bases added to 3’end (CCA = where a.a. attach)
• introns removed
• some bases altered