LEC 3: Transcription II

Question 1

Prokaryotic Transcription

Answer 1

mRNA transcript is mature, and used directly for translation without modification
- polycistronic: contain amino acid coding information for more than one gene

Question 2

Eukaryotic Transcription

Answer 2

mRNA transcript is not mature (pre-mRNA) and must be modified
- monocistronic: contain amino acid for just one gene

Question 3

Eukaryotic Transcriptional Modifications

Answer 3

• 5’ capping
• 3’ maturation
• Intron splicing
• Editing
• Stabilization/destabilization of mRNA

Question 4

RNA Capping (5’ Cap)

Answer 4

• Recognition signal for translation machinery
• Protection from exonucleases
• Essential for the ribosome to bind to the 5’ end of the mRNA

Question 5

Pre-mRNA Polyadenylation (polyA Tail)

Answer 5

• Most cytoplasmic mRNAs have a polyA tail (3’ end) of 50-250 Adenylates, added post-transcriptionally by polyA polymerase(s)
• Promotes mRNA stability
• Enhances translation

Question 6

RNA Splicing

Answer 6

RNA splicing is the removal of Intervening Sequences (IVS) that interrupt the coding region of a gene
- Excision of the IVS (introns) is accompanied by the precise ligation of the coding regions (exons)

Question 7

Introns vs. Exons

Answer 7

Intron = non-coding DNA sequences between exons in a gene.
Exon = expressed DNA sequences in a gene, code for amino acids.

Question 8

Steps to Splicing

Answer 8

Introns typically begin with a 5’-GT(U) and end with AG-3’
- The free G joins with an A at a specific branch
point sequence in the middle of the intron, using a 2’ to 5’
phosphodiester bond
- Intron forms a lariat-shaped structure which is excised, and the exons are joined to form a spliced mRNA
- 3’ end Splicing is mediated by spliceosomes

Question 9

Splicesomes

Answer 9

Spliceosomes contain snRNPs (small nuclear ribonuclearproteins)
- A snRNP contains a small, nuclear U-rich RNA

Question 10

Roles of snRNA’s

Answer 10

1. U1 base-pairs with the 5’ splice-site
2. U2 binds/pairs with the branch point; also pairs with U6 in the assembled spliceosome
3. U4 pairs with U6 in SnRNPs, but unpairs during spliceosome assembly
4. U5 interacts with both exons (only 1-2 nt adjacent to intron); helps bring exons together
5. U6 displaces U1 at the 5’ splice-site (pairs with nt in the intron); it also pairs with U2 in the catalytic center of the spliceosome

Question 11

RNA Editing

Answer 11

Any process of addition or deletion of nucleotides from a pre-RNA, or chemically altering of the bases

• mRNA bases do not match the DNA sequence!
• Can result in the substitution, addition, or deletion of amino acids (relative to the DNA template)