RNA processing

Question 1

In eukaryotes, where do transcription and translation occur? What does this allow for?

Answer 1

Transcription occurs in the nucleus
Translation in the cytoplasm
= allows regulation at diff steps in the gene expression pathway

Question 2

In E.coli, transcription is coupled to translation. How does this result in rapid gene expression?

Answer 2

The mRNA is translated into protein whilst it is being made by the RNAP = rapid gene expression e.g antibiotic resistance

Question 3

Describe eukaryotic mRNA processing.

Answer 3

• Occurs in the nucleus
  1. Capping of the 5’ end
  2. Removal of introns (pre-mRNA splicing)
  3. 3’ end processing (cleavage & polyadenylation)

Question 4

Describe the structure of the M7G cap

Answer 4

• On eukaryotic mRNA
• Guanosine nucleotide is added to the 5’end of the RNAPII transcripts
• Cap is linked to the transcript by a 5’-5’ triphosphate linkage
• The cap nucleotide is methylated
• The first transcribed nucleotide is often modified

Question 5

What is the role of the m7G cap?

Answer 5

Cap blocks the ability of nucleases to degrade the 5’ end of mRNA = stabilises and stimulates translation of the mRNA

Question 6

Prokaryote mRNA is polycistronic, what does this mean?

Answer 6

translates into multiple functionally related proteins

Question 7

Eukaryote mRNA is monocistronic, what does this mean?

Answer 7

encodes 1 polypeptide
The expression of functionally related genes is coordinately regulated

Question 8

Describe ‘split genes’ in eukaryotes

Answer 8

sequences of protein-coding transcripts in eukaryotic cells are typically discontinuous within the chromosomal DNA

Question 9

Describe intron-exon boundary sequences

Answer 9

→ sequences are found through the recognition of splice site sequences
5’ Splice site sequence GU and the 3’ splice site sequence AG are highly conserved

Question 10

Describe the splicesome

Answer 10

→ large complex containing RNA and protein that mediates splicing
- Introns are removed, exons are kept
- Active spliceosomes are assembled and disassembled from smaller RNA/protein complexes called ‘snurps’ (small nuclear RNPs)

Question 11

Describe the 2 transesterification reactions in splicing

Answer 11

Lone pair of electrons from -OH group breaks phospohodiester bond
2 exons joined
= intron released and degraded

Question 12

What are Riboxymes?

Answer 12

Enzymes with an RNA catalytic subunit

Question 13

Describe the RNA profile of a eukaryotic cell

Answer 13

Messenger RNA (mRNA) ~5%
Transfer RNA (tRNA) ~75%
Ribosomal RNA (rRNA) ~10%
Some small stable RNAs

Question 14

What type of reaction is needed to release the introns in splicing?

Answer 14

two trans-esterification reactions