eukaryotic mRNA splicing

Question 1

what is the saying for introns?

Answer 1

INTrons go IN the Trash

Question 2

what is the saying for exons?

Answer 2

EXons are EXpressed

Question 3

what are the caharcteristics of group 1 introns?

Answer 3

Found in bacteria and viruses, microeukaryotes and plants
They self-splice

Question 4

what are the characteristics of group 2 introns?

Answer 4

Found in bacteria and in organelle DNA of plants and fungi
Self splice

Question 5

what are the characteristics of group 3 introns?

Answer 5

Found in eukaryotes
Require a protein-RNA complex (the spliceosome) for splicing

Question 6

what is the name of the reaction by which introns are removed?

Answer 6

Transesterification

Question 7

what type of reaction is Transesterification?

Answer 7

SN2 nucleophilic displacement reactions with special properties

Question 8

describe the characteristics of transesterification reactions;

Answer 8

Splicing reactions are conservative
There is no change in the number of phosphodiester bonds
ATP is not required
Easily reversible

Question 9

describe the basic transesterification reaction;

Answer 9

Step 1: the intron is detached from the exon at the 5’ end .
The nucleophile attacks the upstream exon-intron junction
The nucleophile will end up covalently joined to the 5’ end of the intron
The number of phosphodiester bonds has not changed
Step 2: exon 1 is joined with exon 2 thereby releasing the intron
Some internal rearrangement might be needed for this step to occur

Question 10

what could the purple box be?

Answer 10

Free guanine nucleoside or internal adenine nucleoside
Free guanine = group 1 self splicing
Internal adenine = group 2 self-splicing and mRNA splicing

Question 11

describe the process of group 1 intron removal;

Answer 11

1: a free guanosine is the attacking species that detaches the 5’ end of the intron from exon 1
2: the free 3’ end of the exon 1 attacks the intron-exon 2 junction, joining the 2 exons together
Discovered by Tom Cech

Question 12

describe Tom Cech;s discovery of introns;

Answer 12

Set out to study transcription by RNAP1 in Tetrahymena
Tetrahymena has a mononucleosis with many copies if the genes that are to be transcribed
He used Tetrahymena cell extracts to observe transcription of rRNA
He also saw a small RNA whose amount increased with time which he identified as an intron

Question 13

describe the process of group 2 and spliceosomal intron removal;

Answer 13

1: the 2’ OH of a specific internal A within the intron attacks the exon-intron junction
A rearrangement may be needed so that the free end of the exon 1 is re-positioned close to the other intron-exon junction
2: the free end of exon 1 attacks the intron-exon 2 junction, joining the 2 exons together
Spliceosomes manage the movement and positioning of the intron and exon ends. They regulate the timing of snRNP mediate splicing. It is made up of 5 nuclear ribonucleoprotein particles (snRNP)

Question 14

what is RNase T2?

Answer 14

s a ribonuclease with little to no sequence/structural specificity

Question 15

what are the 4 defining motifs for intron splice sites?

Answer 15

The 5’ splice site (GU)
The 3’ splice site (AG)
A branch-point adenosine nucleotide within the intron
A polypyrimidine region upstream of the 3’ splice site
Spliceosomes recognize the splice site sequences in pre-mRNA

Question 16

what is the consensus sequence for spliceosomal introns?

Answer 16

GU — A — Y10 — AG