RNA Splicing

Question 1

similarity of prokaryotic DNA to its protein

Answer 1

direct copy
no post-translational modification in prokaryotes

Question 2

similarity of eukaryotic DNA to its protein

Answer 2

not the same
eukaryotic genes are split by introns

Question 3

why must introns be removed

Answer 3

introns often contain STOP codons that would terminate translational
even if no STOP codons are present the intron sequence may shift the translational reading frame

Question 4

what is the spliceosome

Answer 4

RNA-protein complex that catalyses splicing of introns from pre-mRNAs

Question 5

what is the composition of the spliceosome

Answer 5

snRNPs (small nuclear ribonuclear proteins)
~ 200 polypeptides

Question 6

how are introns recognized

Answer 6

via splice site consensus sequences conserved across all introns
RNA-RNA base pairing

Question 7

U1 and U2 snRNA pairing sites

Answer 7

U1: base pairs across the 5’ exon-intron junction
U2: base pairs across the intron branch point

Question 8

how are separate ends of the intron brought together

Answer 8

snRNPs indicate to the spliceosome where each end of the intron is and form a higher order structure

Question 9

spliceosome cycle overview

Answer 9

dynamic and stepwise
occurs of nascent pre-mRNA
separate spliceosome assembles independently on each intron
does not need to happen in a 5’ to 3’ order

Question 10

step 1 of pre-mRNA splicing

Answer 10

trans-esterification where the 2’ hydroxyl group of the branch site A residue attacks the phosphodiester bond between exon 1 and the 5’ end of the intron

Question 11

step 2 of pre-mRNA splicing

Answer 11

another trans-esterification where the 3’ hydroxyl group of the terminal ribose of the free 5’ exon attacks the phosphodiester bond between the 3’ end of the intron and exon 2

Question 12

products of pre-mRNA splicing

Answer 12

spliced mRNA and an excised intron lariat

Question 13

why is pre-mRNA splicing isoenergetic

Answer 13

the two trans-esterification reactions swap phosphodiester bonds with no overall change in the number of bonds formed/broken

Question 14

despite being isoenergetic why does splicing still require energy

Answer 14

required to drive multiple assembly steps of the spliceosome

Question 15

how do multiple protein isoforms come from a single gene

Answer 15

alternative splicing

Question 16

what can insertion/loss of protein sequences affect

Answer 16

insertion/loss of protein sequences that affect:
- nucleic acid binding/recognition
- protein targeting/stability
- protein-protein interactions
- post-translational modifications and associated activities