RNA processing Flashcards
Semester 1 year 1
Compare DNA + RNA helices
-DNA double helix has a broader ‘major’ groove + narrower ‘minor’ groove
-major groove of RNA is deeper than that of DNA
-geometries of DNA + RNA helices are similar
Where are eukaryotic mRNAs processed and what are the main steps involved?
-in nucleus
-capping 5’ end
-removal of introns (pre-mRNA splicing)
-3’ end processing (cleavage + polyadenylation)
What do the caps and poly(A) tail do?
-protect mRNA against degradation
-promotes translation
Do eukaryotic mRNAs encode a single or multiple polypeptides?
A single polypeptide - monocistronic mRNAs
Do prokaryotic mRNAs encode a single or multiple proteins?
Polycistronic mRNA - translates into multiple, functionally related proteins
In prokaryotic genes, what is DNA transcribed into and what is mRNA translated to?
-DNA transcribed to form mRNA
-mRNA translated to form proteins
In eukaryotic genes, what is DNA transcribed into and what is mRNA translated to form?
-DNA transcribed to form pre-mRNA
-pre-mRNA spliced to remove introns, forming mRNA
-mRNA translated to form proteins
How are intronic and exonic sequences distinguished?
Through the recognition of splice site sequences
What is a consensus sequence and what are they?
-sequence that isn’t present at all times but is the most common, prevalent sequence
-GU at 5’ end and AG at 3’ end
What else do introns contain?
“branchpoint” A (adenine)
What is pre-mRNA splicing carried out by?
A large ribonucleoprotein (RNP) called the spliceosome
What are spliceosomes made out of and are they always assembled?
-assembled, then splicing occurs, then disassembled
-made up of smaller RNA/protein complexes called “snurps” - small nuclear RNPs
How many snurp complexes come together to form a spliceosome?
5
What is an intron lariat?
When an intron is released from splicing, it has a lariat (lasso) structure and adenine is on the branchpoint
What are the 2 steps involved in pre-mRNA splicing?
-transesterification steps
-1 ester linkage is made + another is broken
1st step - 5’ exon is released and the intron (still attached to 3’ exon) forms a lariat structure involving branchpoint A
2nd step - exons joined by ester linkage + an ester linkage breaks so the intron lariat is released
In vitro, what do some RNAs undergo?
Self-splicing
What type of structures do self-splicing RNAs have?
Restricted structures that align the exons together
What are ribozymes?
Enzymes with an RNA catalytic subunit
All mRNA made by which RNA polymerase has the cap structure?
RNA polymerase II
What is the m7G cap structure?
-addition of a G nucleotide to the 5’ end of mRNA
-cap nucleotide is methylated
How is the m7G cap linked to mRNA?
By a 5’-5’ triphosphate linkage
How does the m7G cap structure protect the mRNA?
-not a phosphodiester linkage
-can’t be hydrolysed by enzymes that hydrolyse the rest of the molecule
