29. Processing and translation

Question 1

What is the purpose of RNA processing in eukaryotes?

Answer 1

To generate mature mRNA from premRNA in the nucleus

Question 2

What are the 3 key processing types that eukaryotes have between transcription and translation?

Answer 2

1. 5’ capping
2. 3’ polyadenylation
3. RNA splicing

Question 3

What does 5’ capping do?

Answer 3

Addition of 7-methyl guanosine cap protects RNA from degradation by nucleases and is recognised by translation machinery

Question 4

What does 3’ polyadenylation do in processing?

Answer 4

A poly A tail of up to 200 is added by poly(A) polymerase which enhances RNA stability and regulates nucleus to cytoplasm transport

Question 5

What does RNA splicing do?

Answer 5

It involves the removal of introns and joining of the exons

Question 6

What is the process of introns to be removed?

Answer 6

1. snRNP (small nuclear RNA and protein) finds and binds to the splice sites
2. snRNP and other proteins associate to dorm the spliceosome
3. The spliceosome cuts at the exon intron boundary and the intron forms a loop
4. The two cut ends are joined and the intron is degraded

Question 7

What is alternative splicing?

Answer 7

Alternative splicing can make different mRNA from the same gene
- Means not always cutting out the same parts

Question 8

What are the four ways alternative splicing can occur?

Answer 8

1. Exon skipping
2. Mutually exclusive exon
3. Alternative 5’ splicing donor sites
4. Alternative 3’ splice donor sites

Question 9

What does exon skipping involve?

Answer 9

Leaving out an exon

Question 10

What does mutually exclusive exon mean?

Answer 10

Multiple slight different exon options

Question 11

What does alternative 5’ donor sites mean?

Answer 11

Multiple options of a splice start

Question 12

What does alternative 3’ acceptor sites mean?

Answer 12

Multiple options of a splice end

Question 13

What does it mean for the genetic code to be degenerate?

Answer 13

There is more than one codon per amino acid

Question 14

What are synonymous codons?

Answer 14

Codons that encode the same amino acids are synonymous codons

Question 15

What does ‘wobble of the code’ describe?

Answer 15

Wobbles of the code describes codons where the last base is irrelevant

Question 16

How many start and stop codons are there?

Answer 16

There is only 1 start codon AUG and 3 stop codons

Question 17

What are the 3 key processes of translation?

Answer 17

1. Initiation - ribosome finds and binds to start codon on mRNA
2. Elongation - Polypeptide chain grows by addition of individual amino acids carried to the ribosome by tRNA
3. Termination - translation finds stop codons and dissociates

Question 18

In prokaryotes how is translation initiated?

Answer 18

1. Recognition of the consensus ribosome binding site - Shine-Dalgarno sequence located in the 5’ UTR
2. Binding of small ribosome unit and initiation factors
3. A special initiator tRNA in prokaryotes: fMet-tRNA binds
4. Initiation factors are released and the large ribosome unit binds forming the initiation complex

Question 19

What are the steps in the translation initiation of eukaryotes?

Answer 19

1. Small ribosome unit binds to initiator tRNA and initiation factors, forming pre-initiation complex.
2. This complex finds and binds to the 5’ cap.
3. The pre-initiation complex moves along mRNA scanning for Kozak consensus sequence, indicating the start site
4. Initiation factors dissociate and the large ribosome unit binds forming initiation complex

Question 20

During translation elongation what are the binding sites for tRNA in the ribosome?

Answer 20

1. A site is for codon recognition between anticodon and codon (mRNA)
2. P site is for peptide bond formation and amino acid translocation
3. E for Exit of uncharged tRNA and translation machinery progresses

Question 21

When does translation terminate?

Answer 21

When a stop codon is encountered.

1. A release factor recognised the stop codon and enters the A site
2. Stop codons do not encode amino acids and the release factors do not carry AA
3. Release factor separates the last tRNA from the last AA