B2.072 Prework 1 Basic RNA Processing Mechanisms

Question 1

three types of mRNA processing

Answer 1

1. capping (5’ end)
2. cleavage and polyadenylation (3’ end)
3. splicing (occurs co-transcriptionally)

Question 2

what is splicing

Answer 2

removing introns and joining exons

Question 3

when does mRNA capping occur

Answer 3

shortly after transcriptional initation

Question 4

describe the process of mRNA capping

Answer 4

guanylyltransferase adds G to 5' end
cap has 5' to 5' triphosphate bond
methyltransferase adds methyl groups
-5' terminal G
-ribose 2' OH on next 2 nucleotides

Question 5

what are some function of the mRNA cap

Answer 5

increased stability
export to cytoplasm
required for translation
identifies mRNA as cellular (vs. viral)

Question 6

describe the 3’ cleavage of RNA

Answer 6

complex recognized polyA signal sequence and cleaves 25 nts 3’ of the signal

Question 7

describe the polyadenylation process

Answer 7

catalyzed by polyA polymerase complex
adds 150-200 A residues
no DNA template

Question 8

function of polyA tail

Answer 8

translational efficiency
export to cytoplasm
stabilization of mRNA

Question 9

what does it mean that processing is co-transcriptional

Answer 9

chromatin structure and modifications contribute to efficiency and timing of RNA processing
splicing, gapping, and polyA machinery “ride along” with RNA pol II

Question 10

what are the 2 steps of splicing

Answer 10

1. 2’-OH of A at “branch point” attacks 5’-G of intron

2. OH-G at 3’ end (donor) of exon 1 attacks 5’-G of exon 2 (acceptor)

Question 11

evidence for important of consensus sequence

Answer 11

mutation of splice signal = loss of splicing

splice signals often mutated in genes inactivated in disease

Question 12

what are snRNPs

Answer 12

small nuclear RNAs (snRNAs) associated with proteins = small nuclear ribonucleo-proteins (snRNPs)

Question 13

what are the snRNPs

Answer 13

U1, U2, U4, U5, U6

Question 14

what is the function of snRNPs

Answer 14

force pre-mRNA into arrangement that enables activation of an RNA catalytic center > intron removal

Question 15

function of U1 and U2

Answer 15

U1: donor exon/intron boundary
U2: adenosine at the “branch point”

Question 16

spliceosome

Answer 16

U1, U2, U4, U5, U6 all together

Question 17

function of spliceosome

Answer 17

1. set up proper geometry from splicing
2. hold together exons prior to joining
3. catalyze cleavage and formation of bonds

Question 18

core elements (RNA)

Answer 18

demarcate where splicing occurs

bound by snRNPs and other general factors to influence splice site utilization

Question 19

auziliary splicing regulatory elements

Answer 19

splicing enhancer and silencers
within introns and/or exons
bind proteins that promote or repress splice site use