Exam 3: RNA Processing

Question 1

Define RNA splicing

Answer 1

The process of excising introns from RNA and connecting the exons into continuous mRNA

Question 2

Define spliceosome

Answer 2

A complex formed by snRNPs and additional protein factors that is required for RNA splicing

Question 3

True or false: prokaryotes have the highest number of interrupted genes

Answer 3

False: interrupted genes predominate in eukaryotes

Question 4

How much of the human genome consists of coding regions?

Answer 4

1% of the human genome is exons

Question 5

How much of the human genome is introns?

Answer 5

24% of the human genome is introns

Question 6

How much of the human genome is alternatively spliced?

Answer 6

about 60%

Question 7

Define cap

Answer 7

• The structure at the 5’ end of eukaryotic mRNA which is introduced after transcription by linking the terminal phosphate of 5’ GTP to the terminal base of mRNA.
• Capping protects mRNAs at their termini against attack by phosphatases and other nucleases and promotes mRNA function at the level of initiation of translation.

Question 8

What is the function of guanylyl transferase?

Answer 8

generates a 5’ cap by adding a G residue to the terminal base of the transcript via a 5’-5’ linkage

Question 9

List the purposes of the mRNA 5’ cap

Answer 9

1. Protects mRNA from degradation of exonucleases
2. The 5′ cap can enhance splicing of the first intron
3. Important for nuclear export
4. elF4F binds cap to initiate translation

Question 10

True or false: intron-exon boundaries have short consensus sequences in the exon

Answer 10

False: intron-exon boundaries have short consensus sequences in the intron. The 5’ end has the consensus sequence GU and the 3’ end has the consensus sequence AG

Question 11

What is the basic problem with pre-mRNA splicing?

Answer 11

Splicing junctions are recognized only when in correct pairwise combinations. All 5’ splice sites are functionally equivalent and all 3’ splice sites are functionally equivalent.

1. Splice sites are genetic: The do not have specificity for individual RNA precursors and individual precursors do not convey specific information that is needed for splicing.
2. The apparatus for splicing is not tissue-specific: An RNA can usually be properly spliced by any cell, regardless of whether it is synthesized in that cell.

Question 12

Describe the process of splicing

Answer 12

Splicing occurs in two stages:
Stage 1:
1. The 5’ exon is cleaved off by splicing at the 5’ site
2. The 5’ end of the intron forms a lariat when the GU consensus sequence binds to the 2’ A of the branch site.
Stage 2:
3. The 3’ exon is cleaved off by splicing at the 3’ site.
4. 5’ and 3’ exons are ligated together

Question 13

Describe the composition of the spliceosome.

Answer 13

• The spliceosome is a large complex.
• 5 snRNA (small nuclear RNA) account for almost half of the mass.
• snRNPs, splicing factors, and other proteins account for the rest