20: RNA processing Flashcards
What are the 3 processes in RNA processing?
Why are these needed?
- 5’ capping
- 3’ polyadenylation
- splicing
required for cell to keep RNA stable in cytoplasm on in eukaryotes
premRNA -> RNA
What is of 5’ capping? What is the purpose? When does it occur?
adding a “reversed” methylated G nucleotide which can’t be easily cleaved by exonuclease and protects the 5’ end of mRNA from degradation and acts as a signal
this is a 5’-5’ connection with 2-3 phosphates in a row
methyl groups are added to 1st few bases and cap binding protein recognizes and binds to cap to also protect mRNA from degradation
occurs as soon as first few premRNA bases transcribed
What is the poly adenylation regulation?
poly A binding protein(s) can bing to poly A tail, but without protein, the poly A tail shortens over time.
poly A tail extends mRNA time span, but not too long for regulation
How does poly adenylation occur?
- past the stop codon, there is a consensus sequence recognized by poly A polymerase
- poly A polymerase catalyzes 2 reactions: (a) cleaves RNA 30 bases down stream of consensus sequence with exonuclease activity (b) adds 300 A bases for poly A tail
- poly A polymerase interacts with RNA poly 2 causing it to dissociate
Why are the 5’ CAP and 3’ poly(A) tail important?
- mark the 5’ and 3’ ends of the mRNA as being intact, completely transcribed, and ready for export and translation
- protects the mRNA from degradation
Why are a large number of nucleotides wasted on introns?
evolutionary advantage
Why splice?
- increases coding capacity of the genome with differential splicing and make different proteins from 1 transcript
- evolution
What are the 3 reactions occurring in splicing?
- cut 5’ end
- cut 3’ end
- ligation
What does splicing depend on? What organisms is alternative splicing?
cell/tissue type
occurs in all multicellular organisms, but prevalent in vertebrates
What are the important sites for splicing in the genome?
5’ splice site upstream of intron, 3’ splice site down stream of intron (consensus sequence) and branch point (A) closer to 3’ end
What is the splicosome?
RNA + protein complex
RNA carries out the splicing
proteins scaffolding, increasing the likelihood for splicing to occur.
Can splicing occur without splisosome?
yes, energy comes from mRNA being spliced, but splicosome increases likelihood for splicing to occur
What is the process of intron removal?
2 transesterification reactions
- 2’OH group of branch point adenosine residue inside intron attacks phosphate at 5’ end of intron, feeling 3’ end of first exon (SHAPE: LARIAT)
- free 3’ OH of first exon attacks 5’ phosphate of 2nd exon forming PHOSPHODIESTER BOND, uniting two exons (intron diffuses and degraded)
What is B-thalassemia?
a disease characterized by anemia caused by protein that forms the beta subunit of hemoglobin is not properly produced. (excess of the alpha subunit and a lack of beta subunit.
common cause: splicing mutation; splice sites being lost or created in the middle of introns or exons
mRNA export from the nucleus
requires …
proteins that interact with the 5’cap and the polyA tail, and with specific protein carriers (transport receptor on transcript)
