lecture 4: gene organisation and transcription ii - mRNA processing

Question 1

what is the primary transcript/ pre mRNA?

Answer 1

this is the initial RNA produced from a gene

Question 2

what happens to the pre mRNA? (RNA processing)

Answer 2

• the pre mRNA needs to be processed before it can be used as mRNA in translation - RNA processing occurs in the nucleus before it can be used at mRNA in translation - the fully processed mRNA is moved to the cytoplasm to be translated - RNA processing is basically removing the introns and leaving the exons

Question 3

* what is the anatomy of the gene?

Answer 3

insert a pic - the gene promotor lies at the 5’ end of the gene - the segments of the DNA which contains sequences that form part of the final mRNA are called exons - introns are transcribed but they are edited out of the final mRNA

Question 4

what is the splice donor site?

Answer 4

this is the junction between the exon and the intron

Question 5

what is the splice acceptor site?

Answer 5

they is the junction between the intron and the exon

Question 6

what do exons normally end with?

Answer 6

they end with AG

Question 7

what do introns start and end with?

Answer 7

they start with GU and they end with AG

Question 8

what does RNA processing use?

Answer 8

uses small ribonucleic proteins (snRNPs)

Question 9

* what is the first stage of RNA processing?

Answer 9

1. the snRNP U1 binds to the splice donor sequence insert the pic

Question 10

* what is there second stage of RNA processing?

Answer 10

after U1, SnRNP U2, U4, U5, U6 also bind to the splice donor sequence - this completes the formation of the spliceosome (because all of the SnRNPs make the spliceosome) - and it results in the cleavage of the splice donor site insert the pic

Question 11

* What is the third stage of RNA processing?

Answer 11

the end G of the intron bends round, and a A residue in the intron acts as a branch point in the intermediate step in splicing. insert the pic

Question 12

* what is the fourth stage in the RNA processing?

Answer 12

• phosphodiester bonds form between the G and the A residues - the bond is between the 5’ phosphate on the G and the 2’OH of the branch point of the A insert the pic

Question 13

* what is the fifth stage of RNA processing?

Answer 13

• the phosphodiester bond between the end G of the intron and the exon breaks so the intron is removed as a lariat structure.

Question 14

what is the sixth stage of RNA processing?

Answer 14

• the adjacent axons are ligated together

Question 15

what is the post transcriptional modification?

Answer 15

• a cap is added to the 5’ end of the mRNA

Question 16

what is the structure of the cap?

Answer 16

• a cap is formed by hydrolysis of the terminal triphosphate of mRNA to a diphosphate - this then reacts with the alpha phosphate of GTP to form 5’ - 5’ phosphate linkage - the cap is then further modified by methylation at the N7 position in the purine ring to form 7- methylguanylate cap

Question 17

what is the function of the cap?

Answer 17

• the cap acts to protect mRNA at the 5’ end - it greatly enhances the translation of mRNA

Question 18

what does polio do to the cap?

Answer 18

polio can interfere with the recognition of the cap during translation

Question 19

what are the symptoms of polio myelitis? who does it affect ? how does the disease enter?

Answer 19

• initial symptoms are fever, fatigue, headache, vomiting, stiff neck - highly infectious disease cause complete paralysis - this mainly affects children under 3 - the virus enters the body through the mouth and multiples in the intestine

Question 20

what is polyadenylation?

Answer 20

• this is the addition of the poly A tail to the pre mRNA - the poly A tail is added one base at a time - poly A tail is added 11-30 bases downstream of the sequence AAuAAA - AAuAAA is found in all mRNAs

Question 21

whats an example of a mutation in the splice site featuring in human disease?

Answer 21

• for example thalassemia

Question 22

what is thalassemia?

Answer 22

• this is an inherited disorder in which there is an imbalance in the relative amounts of alpha and beta chains making haemoglobin

Question 23

what are the features of thalassemia?

Answer 23

• theres an iron overload due to the chronic anaemia - the iron overload results in hepatic fibrosis and cirrhosis, the darkening of the skin and cardiomyophaty

Question 24

what mutation happens in beta thalassemia?

Answer 24

• there is a splice site mutation in the beta global gene