biochem DNA things

Replication

DNAa binds origin of replication, twists DNA, separates adjacent region. Helicase continues separation. Topoisomerase relieves supercoils. SSBs prevent strands rejoining. Primase synthesises RNA primer, reverse complement of 3’ end of DNA. DNA pol III builds on primer.

Gene starts at…

The promoter: not just start of mRNA transcript on gene (i.e. +1 region). Prokaryotes have consensus sequence at -10 and -35. Eukaryotes have TATATTT box at -30.

Initiation of transcription (eukaryotes) (general TFs)

TBP binds to TATA box. TFIIB binds TBP and both strands of DNA, and RNA pol II - assembles preinitiation complex. TFIIH unwinds DNA, extending replication fork, and phosphorylates CTD to activate RNA pol II.

CTD activities

Adds 5’ methyl cap: the 7-methyl guanine, with its cap-synthesising complex. Adds poly A tail, as an endonuclease associated with CTD cleaves at AAUAAA. Spliceosomes also assemble at CTD as mRNA intron emerges from RNA pol II.

Spliceosome structure

snRNAs: bind to specific mRNA sequences in introns: GU at 3’ end of intron, AG at 5’ end. Associate with proteins to form snRNPs: subunits of the spliceosome. U1 binds GU, U2 binds AG. These two come together, forming lariat loop as GU binds A near 3’ end.

Translation initiation

aminoacyl-tRNA synthetases charge methionine tRNA. methionine tRNA binds mRNA and small ribosomal subunit. Large ribosomal subunit joins - met tRNA in A site. Ribosome moves. rRNA catalyses bond breaking between tRNA and met in P site, as amine in A site attacks carboxyl in P site. Ribosome moves so now tRNA in P site holds polypeptide chain.