7. Transcription, mRNA processing (excluding splicing) and translation Flashcards
What happens in transcription?
RNA synthesised in 5’ to 3’ direction as a single strand, complimentary to the template (antisense) strand
Sam base sequence as sense strand, but U instead of T
How is transcription inititated?
Transcription factors bind to promoter and position RNA polymerase
Name 3 promoter elements
TATA box - 25-35bp upstream of TSS, defines direction of transcription and indicates strand to be read
GC box - in housekeeping genes, functions in either orientation
CAAT box - strongest determination of promoter efficiency
How do enhancer elements work?
Cis-acting, variable distance from TSS
Bind regulatory proteins - DNA between promoter and enhancer loops out, proteins bound to enhancer interact with promoter-bound TFs or RNA polymerase
Control specificity of gene expression
Give two ways that mRNA is processed
- 5-capping - after initiation of transcription, methylated nucleoside linked to 5’ of mRNA
- protects it from exonuclease activity and facilitates transport from nucleus to cytoplasm - 3’ poly adenylation - cleavage of pre-mRNA 15-30 bp downstream of AAUAAA signal - polyA tail added
- facilitates transport to cytoplasm, stablises mRNA, facilitates translation
What happens in translation?
mRNA engages with ribosome in the cytoplasm
tRNAs mediate translation of mRNA to protein
Anticodon loop recognises complementary mRNA - corresponding amino acid lnked to 3’ OH
What are the 3 stages of translation?
- Initiation
- small ribosomal subunit binds to mRNA, scans along 5’ UTR to find start codon
- tRNAMet pairs with start codon (AUG) and binds to pepidyl site of ribosome - Elongation
- ribosome shifts in 3’ mRNA direction to next codon
- translocation of tRNAs and mRNA facilitated by binding of GTPase - Termination
- Elongation ends at stop codon - no complementary tRNA
- polypeptide and tRNA releases, ribosomal subunits and template disassociate
