transcription factors

• controlling transcription of target genes:

  • in eukaryotes, transcription factors move from cytoplasm to nucleus
  • bind to specific DNA sites near start of their target genes
  • control rate of transcription by either
  • activators help RNA polymerase bind to activate transcription
  • repressors prevent RNA polymerase binding, stopping transcription
• oestrogen as a transcription factor:

  • binds to oestrogen receptor (transcription factor)
  • forms oestrogen-oestrogen receptor complex
  • moves from cytoplasm to nucleus where it binds to specific DNA sites at start of target gene
  • acts as activator
• RNA interference (RNAi):

  • small double stranded RNA molecules that stop target genes being transcribed (in eukaryotes)
  • do not code for proteins
  • two types: small interfering RNA (siRNA) or micro RNA (miRNA)
• siRNA in mammals and miRNA in plants:

  • after mRNA is transcribed, it leaves nucleus to cytoplasm
  • in cytoplasm, siRNA unwinds and associates with several proteins
  • a single strand binds to target mRNA (complementary base sequence)
  • the associated proteins cut mRNA into fragments
  • fragments enter processing body and are degraded
  • so mRNA section is not translated
• miRNA in mammals:

  • miRNA not fully complementary to target mRNA
  • so can target more than one mRNA molecule
  • associated proteins physically block translation of target mRNA
  • mRNA is moved into processing body, where it can be degraded or stored
  • if stored, can be returned and translated at another time