Eukaryotic Transcription Part 2

Question 1

What is mRNA capping

Answer 1

only added to RNA polymerase II (mRNA) transcripts

5’ cap participates in binding of mRNA to ribosome to
initiate protein synthesis

-may also stabilize mRNA by protecting it from nucleases

Question 2

what is poly A tailing

Answer 2

An enzyme complex (including endonuclease, polyadenylate polymerase, and other proteins) binds to the cleavage signal sequence (AAUAAA) in the mRNA and the CTD of RNAP II.

The endonuclease cleaves the transcript 10–30 bases downstream of AAUAAA, releasing it from RNAP II.

Polyadenylate polymerase adds 80-250 A residues (polyA tail) to the 3’ end of the mRNA.

Importance of the 3’ polyA tail:
Facilitates mRNA export from the nucleus
Enhances protein translation (associates with ribosomes)
Increases stability (protects mRNA from nucleases)

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Question 3

How does splicing occur in group 1 introns

Answer 3

Initiation:
Splicing begins with the binding of an essential guanosine cofactor.
This cofactor initiates a nucleophilic attack on the phosphodiester bond at the 5′ splice site.

First Transesterification Reaction:
The attack by the guanosine cofactor starts the first transesterification reaction, cleaving the 5′ splice site.

Exon Joining:
The intron is removed, and the two exons are joined.
The new 3′-OH nucleophile of the exon attacks the 3′ splice site to complete the splicing.

Question 4

How does splicing occur in group 2 introns

Answer 4

For group I introns, the excised intron is linear, whereas group II introns have a covalently joined lariat structure after the excision reaction.