Transcription

• Initiation


  Transcription starts when RNA poly molecules attach to a promoter (upstream of the desired gene to transcribe). RNA poly doesn't need a primer to attach

  The promoter is the starting point

  Transcription unit: stretch of DNA upstream that is transcribed into RNA molecule
• Initiation in Eukaryotes vs Prokaryotes


  E= promoter region is called TATA box. Transcription factors help RNA poly to bind

  P= RNA poly can bind directly to promoter
• Elongation


  RNA poly opens the DNA and reads triplet code of template strand.

  Moves in the 3-5 direction, but the mRNA transcript elongates 5-3.

  RNA poly moves downstream and only opens small sections of DNA at a time. Pairs complementary RNA nucleotides, with the growing mRNA strand peeling away from DNA template.

  A single gene can be transcribed simultaneously by many RNA poly. Helping to increase the amount of mRNA synthesized and increasing protein production
• Termination: Prokaryotes


  Proceeds through termination sequence: a termination signal, RNA poly detaches, mRNA transcript gets released and proceeds to translation. mRNA do NOT need modification
• Termination: Eukaryotes


  RNA poly transcribes a sequence of DNA called the polyadenylation signal sequence. Coding for a polyadenylation signal (AAUAAA). pre-RNA gets released from DNA and must undergo modifications before translation
• Pre-mRNA modifications


  3 types of mods:

  5' cap: the 5' end of pre-mRNA receives a modified guanine nucleotide cap

  Poly-A tail: the 3' end receives 50-250 adenine nucleotides

  Both of these help mature mRNA leave the nucleus, protects mRNA from degradation, and helps ribosomes attah to 5' end of mRNA in cytoplasm

  RNA splicing: sections of pre-mRNA called introns are removed and exons joined together
• RNA Splicing


  Introns: intervening sequence, do not code for AA

  Exons: expressed sections, code for AA

  Spliceosomes: Complex that cuts the introns out

  A single gene can code for more than one kind of polypeptide. Exons from same gene can lead to different mRNA transcripts depending on their combination (alternative splicing)