RNA processing Flashcards
what is RNA processing
-the collection of events which occur on RNA polymerase which modifies their structure
what’s The RNA profile of a Yeast Cell
-Large RNAs (rRNA) can be resolved in agarose gels. Smaller RNAs are resolved in acrylamide gels.
-The major cellular RNAs are:
=messenger RNA (mRNA) -5%
=ribosomal RNA (rRNA)-75%
=transfer RNA (tRNA)-10%
and some small stable RNAs.
-mRNA is not clearly visible on the gel: the 1000s of protein-coding mRNAs vary in size and are generally unstable (can be degraded after being transcribed)
-Almost all cellular RNAs are processed from larger precursor transcripts.
-tRNA is easy to spot as there is more tRNA than rRNA, however tRNA are much smaller
-tRNA and rRNA are much more stable than mRNA
what are the 3 main steps of Eukaryotic mRNA processing
- capping of the 5’ end
-removal of introns (pre-mRNA splicing)
-3’ end processing (cleavage and polyadenylation)- formation
-all poly II receives cap- added to RNA as its being made by RNA polymerase
where does Eukaryotic mRNA processing occur
-Processing occurs in the nucleus. Functional mRNAs are exported to the cytoplasm for translation.
-The cap and poly(A) tail protects the mRNA against degradation and promotes translation-poly A synthesises this reaction
what’s The “m7G cap” structure of eukaryotic mRNA
-A guanosine nucleotide is added to the 5’ end of RNA pol II transcripts.
-This “cap” is linked to the transcript by a 5’-5’ triphosphate linkage- cap structure is the other way around
-The cap nucleotide is methylated (“m7G cap”)- always methylated at 7’
- The first transcribed nucleotide is typically modified in higher eukaryotes.
-first nucleotide incorporated into the RNA tends to be A residue
what are Prokaryotic mRNAs typically
-polycistronic
-in eukaryotes: monocistronic mRNA- transcript can only code for one protein
-in prokaryotes: polycistronic mRNA- transcript can be translated into more than one protein
-Eukaryotic mRNAs encode a single polypeptide. The expression of functionally related genes is coordinately regulated
-A single polycistronic mRNA transcript is translated into multiple, functionally related proteins in bacterial cells
what are “Split Genes” in Eukaryotic Cells
-Protein-coding sequences within the genes of eukaryotic cells are typically discontinuous within the chromosomal DNA.
how do intron-exon boundary sequences help pre-mRNA splicing to occur accurately
-Intronic and exonic sequences are distinguished through the recognition of splice site sequences.
-The 5’ splice site sequence GU and the 3’ splice site sequence AG are highly conserved. Introns also contain a “branchpoint” A.
what is splicing mediated by and how does it work
-spliceosome complexes
-introns are removed by splicosome-> this causes the removal of intronic sequence
-Pre-mRNA splicing is carried out by a large ribonucleoprotein (RNP)(contain RNA and protein) called the spliceosome (composed of different complexes- bigger than ribosomes but not formed within the cell)
-Introns are removed; exons are kept.
-Active splicesomes are assembled and disassembled from smaller RNA/protein complexes called “snurps” (small nuclear RNPs (snRNPs)).
what are the 2 transesterification steps of pre-mRNA splicing
-pre-mRNA splicing involves two sequential reactions.
-Both steps involve transesterification reactions, where one ester linkage is made and another is broken.
-R-OH + R’-O-R’’ -> R-O-R’’ + R’-OH
-In the 1st step, the 5’ exon is released and the intron (still attached to the 3’ exon) forms a lariat structure involving the branchpoint adenosine.
-In the 2nd step, the exons are joined and the intron lariat is released.
how has pre-mRNA spicing evolved from self-splicing introns
-Some RNAs undergo self-splicing in vitro- joining 2 exons together
-Self-splicing RNAs have restricted structures that align the exons together.
-Nuclear pre-mRNA splicing is thought to have evolved from more structurally restricted, self-splicing introns.
-Enzymes with an RNA catalytic subunit are known as ribozymes (catalytic component)
-restricted 3D structure so exons can align specifically so splicing can occur
