L7 RNA Processing

Question 1

What are the major types of cellular RNA?

Answer 1

The major cellular RNAs are:
* messenger RNA (mRNA) 5%
* ribosomal RNA (rRNA) 75%
* transfer RNA (tRNA) 10%
* some small stable RNAs

mRNA is generally unstable and not clearly visible on gels.

Question 2

What are the three main steps in eukaryotic mRNA processing?

Answer 2

Eukaryotic mRNA processing consists of three main steps:
* capping of the 5’ end
* removal of introns (pre-mRNA splicing)
* 3’ end processing (cleavage and polyadenylation)

Processing occurs in the nucleus, and functional mRNAs are exported to the cytoplasm for translation.

Question 3

Where does mRNA occur?

Answer 3

In the nucleus.

Question 4

What is the purpose of the cap and poly(A) tail in mRNA?

Answer 4

The cap and poly(A) tail protect the mRNA against degradation and promote translation.

Question 5

What is the structure of the ‘m7G cap’ in eukaryotic mRNA?

Answer 5

The ‘m7G cap’ structure consists of a guanosine nucleotide (where a methyl group is attached to the 7-nitrogen position) added to the 5’ end of RNA pol II transcripts linked by a 5’-5’ triphosphate linkage and methylated.

Question 6

True or False: Prokaryotic mRNAs are typically monocistronic. Give explanation.

Answer 6

False

Prokaryotic mRNAs are typically polycistronic, meaning a single mRNA can translate multiple proteins.

Question 7

What are ‘split genes’ in eukaryotic cells?

Answer 7

‘Split genes’ refer to protein-coding sequences within the genes of eukaryotic cells that are typically discontinuous within the chromosomal DNA.

Question 8

During pre-mRNA splicing, the spliceosome recognises specific sequences also known as splice sites. Name the key splice sites

Answer 8

The splice site sequences recognized are
* 5’ splice site sequence GU
* 3’ splice site sequence AG
* branchpoint A

These sequences are highly conserved.

Question 9

What is the role of the spliceosome in pre-mRNA splicing?

Answer 9

The spliceosome is a large ribonucleoprotein (RNP) that mediates pre-mRNA splicing by removing introns and keeping exons.

Question 10

Fill in the blank: Pre-mRNA splicing involves two _______ steps.

Answer 10

transesterification

Question 11

What happens during the first step of pre-mRNA splicing?

Answer 11

In the first step, the 5’ exon is released, and the intron forms a lariat structure involving the branchpoint adenosine.

Question 12

What is a ribozyme?

Answer 12

Enzymes with an RNA catalytic subunit.

Question 13

What is the significance of self-splicing introns?

Answer 13

Self-splicing introns have restricted structures that align the exons together and are thought to have evolved into nuclear pre-mRNA splicing.

Question 14

What is the function of snurps (snRNPs) in the spliceosome?

Answer 14

snRNPs, or small nuclear RNPs, are the smaller RNA/protein complexes that assemble and disassemble to form active spliceosomes.

Question 15

What is the typical outcome of eukaryotic mRNA processing?

Answer 15

Eukaryotic mRNAs encode a single polypeptide, and the expression of functionally related genes is coordinately regulated.

Question 16

What does the term ‘monocistronic’ refer to?

Answer 16

Monocistronic refers to eukaryotic mRNAs that encode a single polypeptide.

Question 17

What gels are used to see large and small RNAs?

Answer 17

Large RNAs - agarose gels
Small RNAs - acrylamide gels

Question 18

What are splice sites?

Answer 18

Splice sites are the key recognition signals that guide spliceosomes to remove the introns

Question 19

How does pre - mRNA splicing occur accurately ?

Answer 19

Intronic and exonic sequences are distinguished through the recognition of splice site sequences

Question 20

What is the 2nd step of transesterification?

Answer 20

In the 2nd step, the exons are joined and the intron lariat is released

Question 21

What are the major types of cellular RNA and what is their relative abundance?

Answer 21

1. Ribosomal RNA (rRNA) - most abundant 75%
2. Transfer RNA (tRNA) - 10%
3. Messages RNA (mRNA) - 5%
4. Small nuclear RNA (snRNA)
5. MicroRNA (miRNA)

Question 22

What RNA processing events occur to eukaryotic protein coding transcripts? What effects do these have?

Answer 22

1. 5’ capping:
   - Process: A 7-methylguanosine (m7G) cap is added to the 5’ end of the pre-mRNA
2. 3’ Polyadenylation:
   - Process: A poly - A tail (a long string of adenine nucleotides) is added to the 3’ of the pre- mRNA)
3. RNA splicing:
   - Process: Non- coding regions of the pre-mRNA called introns are removed, and the coding regions (exons) are joined together

Question 23

What is pre-mRNA splicing and how does it occur?

Answer 23

1) Intron recognition: The spliceosome recognises specific sequences at the end of the trains called splice sites (sites typically start with GU and end with AG)

2) Lariat formation: Introns is excised in a two step process. First, the 2’-hydroxyl group of an adenosine nucleotide within the introns (branch point) attacks the phosphate at the 5’ splice site. This cleaves the intron from the upstream exon and forms a lariat, a lasso - shaped structure

3) Exon ligation: The 3’ hydroxyl group of the upstream exon then attacks the phosphate at the 3’ splice site, joining the two exons together

4) Lariat release: Lariat is released from the spliceosome