Rna Processing Flashcards
ribosomal rna
- rRNA
- bacterial and eukaryotic
- cytoplasm
structural and functional components of the ribosome
messenger rna
- mRNA
- bacterial and eukaryotic
- nucleus and cytoplasm
- carries genetic code for proteins
transfer rna
- tRNA
- bacterial and eukaryotic
- cytoplasm
- helps incorporate amino acids into polypeptide chain
small nuclear RNA
- snRNA
- eukaryotic
- nucleus
- processing of pre-mRNA
small nucleolar RNA
-snoRNA
-eukaryotic
- nucleus
- processing and assembly of rRNA
small cytoplasmic RNA
- scRNA
- eukaryotic
- cytoplasm
- variable
MicroRNA
- miRNA
- eukaryotic
- cytoplasm
- inhibits translation of mRNA
small interfering RNA
- siRNA
- eukaryotic
- cytoplasm
- triggers degradation of other RNA molecules
RNA processing why
- after transcription, RNA has to be modified before it function
- it goes from being a primary transcript to mature RNA
RNA processing: tRNA
- trimming at 5’ and 3’ ends, addition of nucleotides at 3’ end
- splicing of introns if present, by protein not snRNA spliceosomes)
- base modifications by specialized tRNA-modifying enzymes
RNA processing: rRNA in prokaryotes
- extensive processing
- methylation of bases and 2’-OH or ribose
- cut and trimmed into smaller pieces (16S, 23S, 5s)
- often also contains sequences for tRNAs
RNA processing: rRNA in eukaryotes
- primary 45S rRna transcript is exensively processed
-extensive methylation of bases and 2’OH of ribose - cut and trimmed into smaller pieces (28s, 18s, 5.8s)
snoRNAs
- small nucleolar RNAs
- help cleave and modify eukaryotic rRNA and assemble them into mature ribosomes
- base pairs with the region where the modification will take place
mRNA processing: eukaryotes
pre-mRNAs are modified in three ways
1. 5’ cap
2. splicing
3. cleavage at 3’ and adding polyA tail
- occurs in the nucleus
- mRNA is then exported out of the nucleus and translated by the ribosomes in the cytoplasm
mRNAs can be further modified
- additional methylation of bases
- RNA editing (rare)
5’ cap
- 7-methyl Guanosine attached to the 5’most nucleotide
- 2’OH of nucleotides 1 and 2 can be methylated but it’s not necessary
Addition of 5’cap
- occurs soon after transcription starts
- phosphorylated CTD (of RNA polI) recruits capping enzymes
- above-mentioned recognizes the growing 5’ end
- gamma phosphate of residue 1 removed
- GTP is added in a 5’-5’ bond while releasing PPi
- guanine base of cap is methylated on N7
- possible further modifications
3’ cleavage
- requires protein complex containing CPSF, CstF, cleavage factors (CFI, CFII), PolyA polymerase (PAP), polypeptides
- CPS binds to AAUAAA sequence, CstF binds to U-rich sequence -> loop formation
- mRNA I cleaved between the two sequences -> unknown mechanism
- 3’ sequences still attached to pol II and binding factors are released
- 3’ end is degraded and rna pol II gets signal to stop transcribing
polyadenyltaion
- adding of polyA tail
- PolyA polymerase adds 10 As to the new 3’ end after 3’ cleavage
- PABII binds to polyA tail
- PABII stimutlstes PAP to add even more A
- whole polyA is Ultimatlely coveverd by PABII
processing or mRNA without polA tail
- lack polyA tail but still get processed on the 3’ end
- have a hair pin before cleavage site and a consensus sequence downstream
- protein binds to hairpin and recruits U7 snRNP that interacts with consensus sequence and cleaves
exons
parts of the genes that are transcribe and found in mature RNA
can be protein-coding or non-coding
introns
sequences between exons that are transcribed but not found in mature RNA
mostly found in eukaryotic genes coding for mRNAs
are removed by splicing
group I intron
- rRNA regions
- self-splicing
group II intron
- protein-encoding genes in mito. and chloro
- self-splicing
nuclear pre-mRNA
- protein-encoding genes in nucleus
- spliceosomal
tRNA
-tRNA genes
- enzymatic
what is needed for splicing (cis)
- 5’ splice site: starts with GU
-3’ splice site: ends with AG - branch point A
what is needed for splicing (trans)
- 5’ splice site: starts with GU
-3’ splice site: ends with AG - branch point A
plus
- snRNPs
- additional prtoens
snrnps
splicing principle
- pre mRNA splicing is catalysed by spliceosome
- carried out in two transesterification reactions
- results in fusion of two exons and release of intron
