DLA post-transcription

Question 1

What are the 3 important modification of mRNA?

Answer 1

1. Addition of 7- methyl-guanosine cap of the 5’ end of mRNA
2. Addition of poly-A tail at 3’end of RNA
3. Removal of introns and splicing together of exons

Question 2

Where does 7-methyl-guanosine capping occur? and by what protein? and when?

Answer 2

in nucleus by a group of proteins that are initially bound to the RNA polymerase II enzyme, prior to completing transcription.

Question 3

What is capping important?

Answer 3

It is required for the efficient binding of the ribosome to the mRNA and translation

Question 4

What kind of enzymes are involved in capping? (4)

Answer 4

1. Phosphatase
2. Guanylyl transferase
3. Guanine-7-methyl transferase
4. 2’O-methyl transferase

Question 5

Where and when does polyadenylation occur?

Answer 5

It occurs in nucleus following transcription.

Question 6

What are the three functions of poly A tail?

Answer 6

1. Aiding mRNA transport out of the nucleus to cytoplasm
2. Stabilize mRNA in the cytoplasm so that they can serve as messages for translation for a longer time
3. increase efficiency of the initial steps of translation

Question 7

What kind of enzymes are involved in polyadenylation? (4)

Answer 7

1. Cleavage and polyadenylation specificity factor (CPSF)
2. Cleavage stimulating factor F (CstF)
3. Cleavage factor
4. Poly-A-polymerase (PAP)

Question 8

Cleavage and polyadenylation specificity factor (CPSF)

Answer 8

Binds to the polyadenylation

signal (AAUAAA) in the 3’ UTR of the mRNA

Question 9

Cleavage stimulating factor F (CstF)

Answer 9

binds to a GU-rich region located in the 3’ UTR

past the point where the 3’UTR is cleaved

Question 10

Cleavage factors

Answer 10

bind to a CA sequence located after the polyadenylation signal and
cleaves the mRNA at this location. The small cleaved fragment is degraded and the
remaining mRNA is polyadenylated.

Question 11

Poly-A-polymerase (PAP)

Answer 11

adds approximately 200 A nucleotides to the new 3’ end of the
mRNA produced by the cleavage of the 3’ UTR.

Question 12

Poly-A binding protein (PABP)

Answer 12

binds to the poly-A tail and assists in directing

translation by the ribosome.

Question 13

Where does splicing occur?

Answer 13

at splice donor and splice acceptor sites located on the 5’ and 3’ ends of introns.

Question 14

What sequence does 5’junction site possess?

Answer 14

GU sequence

Question 15

What sequence does 3’ junction site possess?

Answer 15

AG sequence

Question 16

What is the name of conserved sequence located 18-38 nucleotides upstream of 3’ junction site, where it always possess an A nucleotides?

Answer 16

The branch point sequence

Question 17

What are name of proteins that are involved in splicing?

Answer 17

small nuclear ribonucleoproteins (snRNP). They form spliceosome.

Question 18

Steps of splicing (4)

Answer 18

1) Cleavage of the pre-mRNA occurs at the 5’ splice junction site.
2) The G nucleotide located at the 5’ of the intron folds back and forms a bond with the A
nucleotide of the branch point sequence. This produces a lariat structure.
3) Cleavage of the pre-mRNA occurs at the 3’ splice junction site thus removing the intron.
4) The two exons are ligated together.

Question 19

What is the purpose of alternate splicing?

Answer 19

Alternative splicing ultimately increases the level of molecular diversity of both mRNA and proteins
in the cell.

Question 20

How is alternate splicing regulated?

Answer 20

by binding of activator or repressor proteins at the splice junction site.

Question 21

What is RNA editing?

Answer 21

change of genetic information at the level of mRNA

Question 22

2 examples of RNA editing in mammal

Answer 22

single base editing of apolipoprotein-B mRNA and the glutamate receptor mRNA

Question 23

Which enzyme carries out RNA editing of apolipoprotein-B mRNA?

Answer 23

Cytidine deaminase

Question 24

Which enzyme carries out RNA editing of glutamate receptor mRNA?

Answer 24

Adenosine deaminase

Question 25

For parasites, what kind of enzymes are involved in RNA editing?

Answer 25

• Guide RNA binding
• enzyme complex that has endonuclease activity,
• a terminal uridyltransferase (inserts uracil bases)
• RNA ligase

Question 26

What is the clinical relevance of abnormal splicing of mRNA to lupus erythematosus?

Answer 26

Lupus patients
possess auto-antibodies that cross react with the U1 RNA component of the spliceosome. This may
prevent the normal splicing of the mRNA of some genes.

Question 27

How is beta thalassemia and limb girdle muscular dystrophy related to abnormal splicing?

Answer 27

mutations in genes destroy splice donor/acceptor

sites or generate new splice sites.