RNA Maturation Flashcards
What are the 4 ways by which pre RNA is modified? Define them
Terminal modifications (addition of a 5’ cap and a poly-A tail), splicing (removal of introns), cutting (removing spacer sequences) , chemical modifications (addition of new chemical groups)
Explain terminal modifications? What structures are added, how, and quantity?
The 5’ cap is added it is made of 7-methyl guanosine residues.
The poly-A tail is added at the internal site of the 3’ end and this is added by a template independent RNA polymerase, around 250 adenosine are added.
Explain the formation of the cap
When RNA has 3 phosphate and a nucleotide this is a very unstable structure, so once the cap is added it is bounded by a 5’-5’ bond. And a capping enzyme is brought onto the RNA molecules by the C terminal domain of the RNA polymerase
Polyadenylation is associated with which step of the mRNA processing ?
The termination
What are 2 traits of the polyadenylation Reaction?
- the RNA polymerase always transcribes a fragment of mRNA that is longer than the mature mRNA, this is because the last portion of mRNA are cut for the addition of polyadenylation.
- polyadenylation is tightly connected to transcription as the C terminal domain of the RNA polymerase II is needed to trigger polyadenylation
Explain the steps in the processing of the 3’ end of mRNA
1- there is DNA, RNA polymerase II and it’s tail (has CPSF, CstF) and the RNA molecule.
2- RNA polymerase reads the cleavage and poly A signals encoded on DNA.
3- the proteins from the tail of RNA polymerase move onto the RNA molecule. There is the Poly A polymerase and RNA is cleaved.
4- then PAP is attached to the end of the 3’ end and there are poly A binding proteins that start to attach onto the 3’ end.
5- then there is poly A length regulation and the removal of PAP.
There are now 200 A present
Define an exon
A segment of eukaryotic DNA that will be present in the mRNA they can the coding or Noncoding
What are the 3 sequences needed for splicing to occur
The donor splice site (5’) and the acceptor splice site (3’) an the branch site in the center of the intron
Explain the splicing reaction
It is done by 2 transesterification reactions, all the ribonucleotides have an OH group of their 2’ position (this reactive group is partly why RNA is unstable) This OH group on the Adenosine of the introns can attack the bond linking the last nucleotide of the exons with the first nucleotide of the intron.
It reforms a bond that joins the 5’ of the nucleotide in the introns with the adenine of the introns.
The OH group that is now part of the exon cause the rest of the intron to be free.
The intron is released as a loop shape
What is a spliceosome
It is a macromolecule made by small nuclear RNA. It needs the assistance of many proteins to function. It does not have a fixed composition, it is formed based on need and availability.
SnRNA joins with proteins to form SnRNP.
Is made of 150 proteins and 5 RNA. To catalyse splicing the spliceosome hydrolysis many ATP molecules.
What are the 2 things that determine whether or not to include an exon in the final transcript or not
1- the intrinsic strength of the splicng sites
2- the combination of positive (enhancers) and negative (silencer) regulatory sequences
What is the exon definition hypothesis
It is that through introns(10^5nt) are very long, exons (200nt) are recognised.
Explain the export of mRNA from the nucleus into the cytosol
After splicing, capping, poly A tail, the 5’ end of the mRNA exits the nucleus first. The ribosomes starts translating the RNA immediately,
What are the 2 ways on which rRNA are modified chemically?
1- by addition of methyl groups onto the 2’ end of the OH of the sugar ribonucleotides.
2- by conversion of uridine into pseudouridine
What are sno-RNA
They’re involved in the chemical modification of methylation sites and pseudouridine sites.they are located in the nucleolus. They can anneals with the modified residue by base pairing with rRNA sequence
