Ch. 16 RNA Processing Flashcards
Is there a lot of RNA processing in prokaryotes?
No. Most RNA produced during transcription is NOT modified before being used.
Why and how does RNA degradation occur in prokaryotes?
RNA degradation is a normal cellular process because cells don’t want to over-produce proteins. RNases do this.
What is a feature of prokaryotic transcription and translation?
Prokaryotic transcription and translation are coupled because both processes occur simultaneously in the cytoplasm.
How can researchers take advantage of prokaryotic coupled expression?
“We can turn bacteria into protein factories.”
Is there a lot of RNA processing in eukaryotes?
Yes. RNA in eukaryotes is USUALLY modified before use.
What aer the five most common types of eukaryotic RNA processing?
- 5’ capping
- 3’ poly-adenylation
- intron/exon splicing
- RNA editing
- degradation
What is 5’ capping catalyzed by? Describe it.
Guanylyl Transferase: an enzyme that associates ONLY with RNA pol II and caps each mRNA
What does guanylyl transferase add to mRNA and how is it linked?
Adds a 7-methyl-guanosine (7-meG) to mRNA via a 5ʹ,5ʹ triphosphate linkage.
What does 5’ capping do for the mRNA? (3)
It protects mRNA from nucleases, signals for export out of the nucleus, and it signals for ribosome binding to the mRNA.
When will be 5’ cap be placed on mRNA?
It will occur by the time the first 20﹣30 bases are added.
What catalyzes the addition of a 3’ poly(A) tail?
Polyadenylate polymerase (PAP) adds a 3’ poly(A) tail to mRNA.
How does 3’ poly-adenylation occur and why? (3)
- an RNA pol domain cleaves transcripts after the polyadenylation sequence (AAUAAA)
- PAP adds 80﹣250 A residues, number varies by species, independent of a template
- poly(A) tail attracts polyadenylate binding proteins (PABP) that protect against RNA degradation by nucleases
Why might an organism want a longer poly(A) tail?
longer tail=more stable
What other eukaryotic RNA processing event occurs during transcription? (3 in total)
Splicing also occurs during transcription, along with 5’ capping and 3’ poly(A) tail addition.
What is pre-mRNA?
The mRNA sequence prior to splicing. Splicing produces a mature RNA ready for translation (mRNA).
What do all splicing mechanism consist of? (2)
All splicing mechanisms consist of breaking and joining specific phosphodiester bonds.
What does splicing mediate?
Mediates intron removal, while exons remain.
What does splicing rely on?
Relies on base pairing between pre-mRNA and splicing machinery to break the correct bonds.
What are the two main ways/mechanisms splicing can occur by?
Some splicing is mediated by a spliceosome which is a complex of RNA and proteins. Some transcripts are self-splicing.
Where are introns mainly found?
Introns are found in most vertebrate genes, but some eukaryotes lack introns (e.g. some yeasts). They are rarely found in prokaryotes.
What is the evolutionary significance of introns? Theories (2)?
Evolutionary significance is unknown. Some play a role in regulating the amounts of mRNA. Some have a role in mature RNA export from the nucleus.
What is the general rule when it comes to introns and species complexity?
More advanced species have more introns.
What do exons make up?
Exons make up mature RNA.
What are alternative exons and where are they found?
Some genes include alternative exons, which are exons that are sometimes excluded from the mature RNA.
