Question

Cell Biology Tutorial: RNAs & Post-Transcriptional Gene Control

Answer 1

\({\bf{RNA~subtypes:}}\)
-pre-mRNA: mRNA precursor with introns and uncleaved poly-A site
-hnRNA: heterogeneous nuclear RNAs, contain introns ex. pre-mRNAs
- snRNA: remove introns from pre-mRNA
- pre-tRNA: contains additional bases at the 5' and 3' end and may contain intron in the anti codon loop
- pre-rRNA: precusor to 18S, 5.8S, 28S RNA
- snoRNA: small nucleolar RNAs: base pair with pre-mRNAs, direct RNA cleavage and base modification
- siRNA: short interfering RNAs, perfectly complementary to target RNA and cause cleavage
- miRNA: micro RNAs, base pair extensively but not perfectly with 5' end of miRNA, inhibiting translation

Answer 2

\({\bf{RNA~Processing:}}\)
- 5' capping
- poly-A tail addition
- intron removal/splicing
- addition of hnRNPs (heterogeneous ribonuclear particles) to aid in processing/protection

5' cap: 7 methylguanosine cap containing 5-5' phosphodiester linkage, protects RNA from exonuclease activity

Cleavage/Polyadenylation:
- AAUAAA signal
- requires GU-rich region
- PAP (poly-A polymerase) adds 50-250 A residues before cleavage to promote polyadenylation

Splicing:
GU AG rule: GU at 5' end of intron, AG at 3' end
SR proteins: promote binding of U1, U2, U2AF (srnas)
ESEs: exonic splicing enhancers
- spliceosome: large complex of snRNAs and SR proteins, removes the introns from primary transcript
> U1, U2, U4, U5, U6
> U1 and U2 bind transcript first, activates spliceosome
> two trans-esterification events, forming 2-5 phosphodiester bond and 3-5' bond

Answer 3

\({\bf{Ferrinin~and~Transferrin:}}\)

Ferrinin binds/stores iron, mRNA binding
- when iron low --> IRE-BP activates (RNA binding), inhibits ferritin translation so that ferritin doesn't store iton as well
Transferrin transports iron
- when iron low --> TfR translated, increasing iron transport
- when iron high --> TfR inhibited/degraded, decreasing iron transport

microRNA: represses translation by binding to 3' UTR
RNA inteference: RNA processing enzymes cleave dsRNA into small segments, base pair with mRNA and blocks expression, then the pairs are cleaved by exonuclease

Answer 4

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is Chapter 11 of Molecular Cell Biology, Eighth Edition, Lodish, et. al.