Lecture 14 - Post transcriptional modifiations

Question 1

List the reasons why MRNA Capping is important

Answer 1

• Important for translation initiation

- Increases the stability of messenger RNA

Question 2

What is the 5’ Cap

Answer 2

It is a specially altered nucleotide found on the end of a messenger RNA transcript

Question 3

List the 5 main functions of the 5’ Cap

Answer 3

• Increases the stability of messenger RNA
• Required for efficient splicing
• Prevents MRNA degradation by exonucleases (enzymes)
• Promotes initiation of translation into proteins.

Question 4

Which parts of the messenger RNA are edited

Answer 4

The ends

Question 5

State two main types of post transcriptional modifications made to the messenger RNA

Answer 5

The polyadenly tail and the 5 prime cap

Question 6

Which direction does the RNA polymerase act

Answer 6

RNA polymerase moves in the 3’ to 5’ direction.

Question 7

Which end of the messenger RNA does the capping enzyme act

Answer 7

The 5’ end

Question 8

State two functions of the M7G cap

Answer 8

• M7g cap protects against degradation of the Mrna from exonuclease attack.
- M7g cap also interacts with eukaryotic translation initiation factors, helps to hold MRNA in a circular form to allow several Ribosomes to bind the molecule simultaneously and translate the code into proteins, thus makes the process of translation a lot more efficient!

Question 9

What does the polyadenylation process do

Answer 9

It is the addition of a poly A tail to the RNA in eukaryotes.

Question 10

What is a poly A tail

Answer 10

It is an AAUAAAA nucleotide sequence.

Question 11

What does the Poly A tail do to eukaryotic messenger RNA

Answer 11

• Increased stability of messenger RNA
• increases the amount of time that the ribosome is engaged with messenger RNA, therefore increases the number of times that the MRNA can be translated, thus increasing translation efficiency
• helps in nuclear export of messenger RNA from the nucleus to the rough endoplasmic reticulum

Question 12

Explain how addition of the Poly A tail to eukaryotic messenger RNA increases efficiency of translation

Answer 12

Addition of the Poly A tail increases the amount of time the messenger RNA interacts with the ribosome subunit thereby increasing the number of times the mRNA can be translated, thereby increasing the number of protein products

Question 13

Which structure is responsible for carrying out splicing of messenger RNA

Answer 13

The spliceosome

Question 14

Which molecules are involved in messenger RNA splicing

Answer 14

The spliceosome
Small nuclear snRNPs
protein splicing factors

Question 15

What is an snRNP

Answer 15

small nuclear Ribonucleic protein

Question 16

What is the function of an snRNP

Answer 16

Involved in splicing of pre MRNA, to remove introns.

Question 17

What is the purpose of MRNA Splicing

Answer 17

• Removal of introns (non coding regions)
• exon ligation
• carried out by the spliceosome in a series of enzyme controlled reactions

Question 18

Describe the structure of the spliceosome

Answer 18

• it is an RNA- Protein complex

- Formed by small nuclear RNPs.

Question 19

Describe the stages in the 2 step mechanism of splicing

Answer 19

Step 1) Cleavage at the 5’ splice site and lariat formation at the branchpoint sequence.
Step 2) Cleavage at the 3’ splice site and removal of intron region, exon ligation follows/

Question 20

List the types of small nuclear RNPs involved in splicing

Answer 20

U1, U2,U4,U5,U6.

However, only three of these snRNPs are used to form the spliceosome active site.

Question 21

List the three small nuclear RNPs that form the active site of the spliceosome

Answer 21

• U2
• U5
• U6

Question 22

Which features on the intron are required for splicing to occur

Answer 22

• The 5’ donor site within the intron
• The 3’ acceptor site within the intron
• The branch sequence

Question 23

The 5’ donor site, 3’ acceptor site and branch sequence are all examples of conserved sequence. What is meant by this

Answer 23

Means these are the same for every individual in every person?

Question 24

What are snRNPs made of

Answer 24

an RNA skeleton with a lot of proteins attatched to it.

Question 25

What is the significance of alternative splicing

Answer 25

• Alternative splicing is the recombination of different exons
• Also known as differential RNA splicing
• This allows a single gene to encode several different protein products

Question 26

What does alternative splicing lead to

Answer 26

it means that a single gene can lead to production of multiple different gene products.