5 - RNA coding and noncoding

Question 1

RNA as genetic material experiment

Answer 1

Could mix RNA and protein from different TMV strains and produce viral progeny identical to the strain of the RNA donor, not the protein donor.

Question 2

Differences between RNA and DNA

Answer 2

• Bases differ (U in RNA not T)
• Sugar is ribose not deoxyribose
• RNA is more flexible but less stable

Question 3

Can uracil base pair with other ribonucleotides

Answer 3

Yes. including itself. However 2 hydrogen bonds between U - G for example are weaker than two between U - A (normal)

Question 4

2 classes of RNa

Answer 4

• Coding: Encodes information to makr proteins (mRNA)
• Noncoding: functional RNAs (e.g. RNA itself is the final functional product)

Question 5

What are the three key types of RNA involved in protein synthesis

Answer 5

• mRNAs: Messenger RNAs, code for proteins
• rRNAs: Ribosomal RNA, form the basic structure of the ribosome and catalyse protein synthesis
• tRNA: Transfer RNAs, adaptors between mRNA and amino acids

Question 6

explain process of making mRNA (RNA processing steps)

Answer 6

1. Introns, exons and a long 3’ end are all transcribed into pre-mRNA
2. A 5’ cap is added
3. Cleavage at the 3’end is downstream of the consensus sequence
4. Polyadenylation at the cleavage site produces the poly(A) tail
5. Introns are removed producing mature mRNA

Question 7

3 co transcriptional processing methods

Answer 7

• 5’ RNA capping
• Splicing
• 3’ polyadenylation

Question 8

RNA capping

Answer 8

Addition of 7-methylguanosine to primary transcript via 5’-5’ phosphodiester bond (triphosphate linkage)

Question 9

Function of RNA capping

Answer 9

• Protects against 5’-3’ exonuclease attack
• Facilitates transport from nucleus
• Facilitates RNA splicing

Question 10

Function of poly A tail

Answer 10

Important for nuclear export, translation and stability of mRNA

Question 11

What are the three sequences required for splicing

Answer 11

• Splice donor site (5’ end of intron)
• Splice acceptor site (3’ end of intron)
• Branch point sequence in intron

Question 12

Y nomenclature

Answer 12

C or U

Question 13

R nomenclature

Answer 13

A or G

Question 14

N nomenclature

Answer 14

any base

Question 15

What carries out the splicing reaction

Answer 15

Spliceosome-Ribonucleoprotein complex, forms RNA intermediate (Lariat)

Question 16

Exon skipping

Answer 16

Whole exon can be deleted as a result of mutations in splice signals

Question 17

Abnormal splicing of b-globin RNA in patients with beta-thalassemia

Answer 17

Different single base mutations cause different splicing defects and disease
- Exon skipping
- Cryptic splice site selection
- New splice site created

Question 18

Splicing reaction

Answer 18

• 2’ OH of branch A forms covalent bond with 5’ Phos
  of 5’ intron sequence
• 3’ OH of 5’ exon joins 3’ exon (Phosphodiester
  bond) to complete splicing
• Lariat is degraded in cell