RNAs that become part of a protein structure
Ribonucleic Proteins (RNPs)
rRNA genes are found in regions with many copies called…?
Tandem Repeats
rRNA Processing (6 steps)
- rRNA gene is transcribed (except for 5S rRNA)
- Transcript is methylated to identify splice points
- Methylation signals factors (including snoRNPs) for splicing
- rRNA is spliced into 28S, 18S, and 5.8S rRNAs, which form 3D structures
- Several nucleotides are modified by snoRNPs
- rRNAs are combined with ribosomal proteins, helped by more snoRNPs
tRNA Processing (7 steps)
- Large precursor is transcribed with several tRNA genes
- tRNAs form secondary structure
- tRNA is spliced and separated (rough cut)
- 5’ and 3’ ends are trimmed (fine cut)
- (Eukaryotes) RNA is spliced out from near the anti-codon
- 3’ end is modified to accept amino acids (CCA part of prokaryote transcript, added to eukaryotes)
- Several bases are modified to form mature tRNA
Enzyme that helps transport tRNA out of the nucleus
Exportin
Charging of tRNAs
Amino acids attached by aminoacyl-tRNA synthetase, used, and then re-charged
Synonym for pre-mRNA
heterogeneous nuclear RNA (hnRNA)
Pre-mRNA Processing
- GTP is attached backwards as soon as 5’ end is free, and then methylated to form Guanine Cap
- End of transcript is recognized and cut, adding a poly-A tail using poly-A polymerase and adding Poly-A binding proteins (PABPs) every 10-20 bases in MOST transcripts
- Introns are excised out, either spontaneously or using spliceosomes
Functions of G-Cap
- Blocks RNase activity at 5’ end
- Cap binding proteins (CBPs) help with initiation of translation
- Signals translation location
- Improves splicing efficiency
Functions of Poly-A Tail
- Protects mRNA, as RNases shorten tail
- PABPs help initiate translation
Intron Types
Spontaneous - Need no introns
Automatic - Spliced by general spliceosomes
Gene-specific - Spiced only in the presence of specific splicosomes
Signals for Spliceosome Activity
- 5’ splice signal (part of previous exon and the intron) of 9 nucleotides
- Branch point - YNYYRAY
- 3’ splice signal (3 nucleotides of intron and 1 of next exon)
Spliceosome Mechanism
Composed of proteins and snRNPs; breaks 5’ end, creates lariat at branch point, and then swaps out the 3’ intron with 3’ exon
RNA Editing Types
- C to U editing (apolipoprotein B gene, CAA to UAA)
- A to Inosine editing (I pairs like G)
- Addition of U
siRNA and miRNA Function
Start as dsRNA, dicer enzyme splices, bind to RNA-induced silencing complex (RISC), which binds to mRNA to stimulate mRNA cleavage or ribosomal blockage
