Courses/Bioc 406
RNA processing
5' end of the nascent mRNA is Capped by 7-methylgaunosine Capping enzyme 1. hydrolysis removes 5' y-pi from pre-mRNA 2. Gaunylation of pre-mRNA through 5'->5' triphosphate bridge (PPi released) 3. Methylation of the guanosine base at position N7. Methyl donor is SAM (S-adenosylmethionine) Capping occurs during elongation. Capping enzyme binds to the RNAP II phosphorylated CTD Eukaryotes ONLY! Base one after the cap may be methylated if Adenosine The 3' end of the nascent mRNA is polyadenylated Cleavage by specific endonuclease Addition of tail by poly(A) polymerase, adds a bunch of AAAA in a row Most eukaryotic mRNA transcripts are spliced introns removed, exons remain tissue specific splicing: diff combinations of exon skipping makes different stuff
Two nt's at the beginning and end of n intron are Invariant 5' splice site (GU). 3' splice site (AG)
The lariat mechanism Hydroxyl at an adenine attack cuts splice site at 5', binds to middle of intron, creating a lariat form, then excised exons are spliced, hydroxyl on exon 1 goes to exon 2
mRNA splicing is carried out by Spliceosome, composed of snRNPs (small nuclear ribonucleoproteins) ![[Pasted image 20260303101413.png]]
Group 1 introns are autocatalytic: they splice themselves! 3' hydroxyll on guanosine attacks left exon, connects right exon Intron circles ![[Pasted image 20260303101653.png]]
![[Pasted image 20260303101724.png]]
Stretch of DNA has 6 possible reading frames Inserting or deleting 1 or 2 nucleotides destroys reading frame. Inserting or deleting 3, fucks one amino acid, but keeps the rest
Deciphering the Code: Scientists figured out which codons matched which amino acids by creating synthetic RNAs with known ratios of nucleotides (e.g., 5 parts A to 1 part C) and calculating the statistical probability of certain triplets forming to see which amino acids were produced