RNA Synthesis Flashcards
What are the different types of RNA polymerase enzymes and what genes do they transcribe?
RNA polymerase I > Most ribosomal RNA (rRNA)
RNA POLYMERASE II > PROTEIN-CODING, microRNA (miRNA), non-coding RNA
RNA polymerase III > Transfer RNA (tRNA), 5S rRNA, other small RNAS
What is a gene?
It’s a DNA segment containing instructions for making a particular product. It’s a unit of heredity; it contains instructions for an organism’s phenotypes.
Describe the process of TRANSCRIPTION.
1) TF2D (transcription factor) has a TBP (tata binding protein) which binds to TATA on the DNA strand
2) TF2A and TF2B bind, and the TF2A stabilises the complex (the binding of the TF2D enabled the adjacent binding of the TF2B)
3) Then TF2E and TF2H bind onto the DNA strand, followed by the RNA Polymerase II assembling at the promoter. This creates the Transcription Initiation Complex (TIC)
4) TF2H pulls apart the DNA helix and phosphorylates the RNA Polymerase II
5) The RNA Polymerase II will move along the DNA strand until it recognises a start codon
6) Transcription starts
Describe the process of CAPPING.
(the 5’ end is the one being modifed here)
A guanine nucleotide is inserted and methylated at carbon position 7, forming a triphosphate bridge (5’ to 5’).
This is catalysed by a capping enzyme complex.
This process happens throughout, while the RNA is being transcribed.
Describe the process of POLYADENYLATION.
(the 3’ end is the one being modified here)
A cleavage signal is a specific DNA sequence which is recognised and “cleaved” by specific endonucleases.
This is a signal for another enzyme, poly (A) polymerase, to add a sequence of A nucleotides to the end of the RNA molecule to make a poly A tail.
The longer the tail, the more stable the mRNA molecule, and the longer it is present in the cytoplasm.
Why do we cap and polyadenylate?
- For stability
- For it to be transported to the cytoplasm (won’t happen unless all the steps are done correctly)
- Integrity is needed prior to translation
Describe the process of SPLICING.
Splicing is the removal of introns and the joining of exons.
There is cleavage at the 5’ site by a splicosome.
There is a formation of a lariat-like intermediate (GU joining to A at the branch point, making an AGU)
Then, ligation (joining together (of exons occurs.
What is alternative splicing?
The ligation of exons can differ and so the sequence of the mRNA also changes.
The proteins made may have similar functions as there are common exons, but different function as the sequence of exons differs.
What are splicosomes?
A type of ribonuclear complex (made up of RNA and protein) which removes introns.
Why do we need introns?
We need it so that alternative splicing can take place.
What happens when the mRNA is to be exported form the nucleus?
The relevant proteins bind to the mRNA strand
The recognition of these enzyme binding complexes allows the mRNA strand to leave via the nuclear pore.
As it leaves, the cap on the 5’ end is switched with another protein. This along with the Poly-A binding protein on the poly A tail are initiation factors that are needed to continue on to translation.
Why is this step necessary for export out of the nucleus?
This is a control step and will, therefore, ensure that the mRNA strand is stable enough to travel through the cytoplasm to be translated.
