RNA processing

Question 1

Compare DNA + RNA helices

Answer 1

-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

Question 2

Where are eukaryotic mRNAs processed and what are the main steps involved?

Answer 2

-in nucleus
-capping 5’ end
-removal of introns (pre-mRNA splicing)
-3’ end processing (cleavage + polyadenylation)

Question 3

What do the caps and poly(A) tail do?

Answer 3

-protect mRNA against degradation
-promotes translation

Question 4

Do eukaryotic mRNAs encode a single or multiple polypeptides/

Answer 4

A single polypeptide - monocistronic mRNAs

Question 5

Do prokaryotic mRNAs encode a single or multiple proteins?

Answer 5

Polycistronic mRNA - translates into multiple, functionally related proteins

Question 6

In prokaryotic genes, what is DNA transcribed into and what is mRNA translated to?

Answer 6

-DNA transcribed to form mRNA
-mRNA translated to form proteins

Question 7

In eukaryotic genes, what is DNA transcribed into and what is mRNA translated to form?

Answer 7

-DNA transcribed to form pre-mRNA
-pre-mRNA spliced to remove introns, forming mRNA
-mRNA translated to form proteins

Question 8

How are intronic and exonic sequences distinguished?

Answer 8

Through the recognition of splice site sequences

Question 9

What is a consensus sequence and what are they?

Answer 9

-sequence that isn’t present at all times but is the most common, prevalent sequence
-GU at 5’ end and AG at 3’ end

Question 10

What else do introns contain?

Answer 10

“branchpoint” A (adenosine)

Question 11

What is pre-mRNA splicing carried out by?

Answer 11

A large ribonucleoprotein (RNP) called the spliceosome

Question 12

What are spliceosomes made out of and are they always assembled?

Answer 12

-assembled, then splicing occurs, then disassembled
-made up of smaller RNA/protein complexes called “snurps” - small nuclear RNPs

Question 13

How many snurp complexes come together to form a spliceosome?

Answer 13

5

Question 14

What is an intron lariat?

Answer 14

When an intron is released from splicing, it has a lariat (lasso) structure and adenosine is on the branchpoint

Question 15

What are the 2 steps involved in pre-mRNA splicing?

Answer 15

-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

Question 16

In vitro, what do some RNAs undergo?

Answer 16

Self-splicing

Question 17

What type of structures do self-splicing RNAs have?

Answer 17

Restricted structures that align the exons together

Question 18

What are ribozymes?

Answer 18

Enzymes with an RNA catalytic subunit

Question 19

All mRNA made by which RNA polymerase has the cap structure?

Answer 19

RNA polymerase II

Question 20

What is the m7G cap structure?

Answer 20

-addition of a G nucleotide to the 5’ end of mRNA
-cap nucleotide is methylated

Question 21

How is the m7G cap linked to mRNA?

Answer 21

By a 5’-5’ triphosphate linkage

Question 22

How does the m7G cap structure protect the mRNA?

Answer 22

-not a phosphodiester linkage
-can’t be hydrolysed by enzymes that hydrolyse the rest of the molecule