OECG

TRANSCRIPTION IN EUKARYOTES General process: Initiation, elongation and termination Initiation Enzyme involved: RNA polymerase II proteins involved: General Transcription factors ● TBP ( Tata binding protein) ● Other general transcription factors So what does TBP do? ● TBP binds to promoter at the TATA box ● Recruits other GTF which helps with binding of RNA polymerase to bind to the promoter ● Forming preinitiation complex What does RNA polymerase do? ● Unwinds and unzips the DNA strands Why does RNA polymerase need to do that? ● To expose the nitrogenous base so that it can act as a template for transcription ● For complementary base pairing for free nucleotides Elongation Enzyme involved: rna polymerase What does rna polymerase do? ● Reads the DNA template from 3’ to 5’ direction ○ The rna synthesised from 5’ to 3’ ○ Free nucleotides are paired with their complementary bases via hydrogen bonds ● Catalyses formation of phosphodiester bonds between a 3’ hydroxyl end to incoming 5’ phosphate group Termination Enzyme involved: RNA polymerase Products formed: Pre-mRna

Whats the terminator sequence called: polyadenylation sequence signal What does RNA polymerase do? ● transcribes a polyadenylation code AAUAAA past its signal sequence ● the growing chain is then Cleaved downstream 10-30 nts, RNA dissociates from DNA What happens after transcription is done? ● RNA transcript is released ● RNA polymerase dissociates from DNA Pre-mRNA modification (only in eukaryotes) Why is it important? ● to produce mature mRna ● helps with translation Note: can occur simultaneously with transcription Addition of 5’ caps (ALWAYS THE FIRST) when does it occur? ● during synthesis of pre-mRNA Enzymes involved: capping enzymes What does it do? ● Capping enzymes catalyse the covalent attachment of 7-methylguanosine to the 5’ end of the mRNA transcript, forming a 5’-5’ triphosphate linkage. Why does it do it? ● protects mRNA from degradation by 5’ exonucleases, conferring stability to the capped mRNA ● serves as a binding site for translation initiation factors and ribosome to initiate translation. ● facilitates the export of mature mRNA from the nucleus to the cytoplasm.

addition of 3’ poly(A) tails process: Polyadenylation Enzyme involved: poly-(A)-polymerase What does the enzyme do? ● The enzyme catalyses the addition of 50-240 adenine nuecleotides to the 3’ end Why does the enzyme do that? ● to facilitate the transport of mRNA from nucleus to cytoplasm ● prevent degradation by the 3’ exonucleases ● facilitate bin