Transcription 2 - RNA Processing

Question 1

What are the three ways that RNA is processed?

Answer 1

1. 5’ capping
2. 3’ poly-A tails
3. mRna editing (alternative splicing)

Question 2

Role of 5’ cap in RNA processing

Answer 2

1. Protects mRNA from exonuclease activity as it leaves the nucleus
2. Binds to specific cap-binding protein
3. Particpates in binding to the ribosome to initiate translation

Question 3

Describe the structure of the 5’ cap

Answer 3

7-methyguanosine attached to 5’ nucleotide via a triphophosphate linkage

2’-hydroxyls of first and second nucleotides often methylated.

Question 4

What is the role of poly-A tail?

Answer 4

• Protects the RNA from degradation once it leaves the nucleus
• involed in RNA transport

Question 5

T/F: Poly A tail is encoded in the genome

Answer 5

False. Product of enzyme activity triggered by the polyadenylation signal (AAUAAA) which is encoded

Question 6

Describe the four (4) classes of introns

Answer 6

Group 1 introns
Group 2 introns
Spliceosomal introns
tRNA introns

Question 7

T/F: Group 1 and 2 introns require ATP

Answer 7

False. They are autocatalytic.

Question 8

What are Isoforms?

Answer 8

Two or more functionally similar proteins that have a similar but not identical amino acid sequence and are either encoded by different genes or by RNA transcripts from the same gene which have had different exons removed.

Question 9

What to reactions do ribozymes catalyze?

Answer 9

Transesterification and hydrolysis of phosphodiester bonds

Question 10

Define and differentiate between the terms primary transcript, hnRNA, pre-mRNA and mRNA.

Answer 10

primary transcript - immediate product of transcription containing introns and exons and 5’cap
hnRNA - heterogeneous nuclear rna is a collective term for the unprocessed mRNA (pre-mRNA) molecules in the nucleus.
pre-mRNA - unprocessed mRNA
mRNA - mature mRNA after all 3 steps of processing

Question 11

List the steps involved in the maturation of RNA in a eukaryotic cell.

Answer 11

1. 5’ capping
2. 3’ tailing
3. Cleavage
4. Polyadenylation
5. Splicing

Question 12

Define the terms intron and exon.

Answer 12

Intron - coding region of gene
Exon - noncoding region of gene

Question 13

Define the terms “spliceosomal introns” and “spliceosomes.”

Answer 13

Spliceosomal intron - non-coding sequences excised from pre-mRNAs by a special complex called spliceosome during mRNA splicing.

Spliceosome - large RNA-protein complex that catalyses the removal of spliceosomal intron from pre-mRNA

Question 14

Outline the splicing mechanism in group I introns.

Question 15

Outline the splicing mechanism in group II introns.

Question 16

Outline the splicing mechanism in spliceosomes.

Question 17

Compare and contrast the splicing mechanisms in group I, II and spliceosomal introns.

Question 18

Describe the distinctive structure of the 3’ end of a eukaryotic mRNA and explain how it is added.

Answer 18

1. Recognition - of the polyadenylation sequence by PAP
2. Cleavage - of the transcript by PAP endonuclease activity
3. Addition of A residues to free 3’ OH by PAP

Question 19

Name the enzymes involved in “tailing.”

Answer 19

RNAP II
Polyadenylate polymerase

Question 20

List important steps in mRNA maturation and when they occur.

Answer 20

1. 5’ capping after first 20-30 ntds
2. 3’ poly-A tails after AAUAAA sequence
3. mRna editing (alternative splicing)

Question 21

Describe how differential RNA processing generates multiple products from a single gene.

Question 22

Discuss the concept of RNA stability

Question 23

Describe the 3 steps of the 5’ capping process

Answer 23

Hydrolysis of the 5’ phosphate
Addition of GTP
Guanosine methylation