Gene Expression

Key points of 3 ways in which gene expression is controlled

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Transcription Factors

  • Switch genes on/off
  • Interact with promoter sequence of DNA- either initiate or inhibit transcription
  • They move from the cytoplasm to the nucleus
  • Bind to specific promoter DNA sites (near start of target genes)
  • Control expression by controlling rate of transcription
  • Activators: increase rate of transcription, help RNA polymerase bind to promoter sequence and activate transcription
  • Repressors: decrease rate of transcription, bind to start of target gene, prevents RNA polymerase from binding, stopping transcription
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  • Mechanism: gene repressed by inhibitor (method for transcription factors)
  • Lipid soluble hormone has small hydrophobic molecules
  • Diffuses through plasma membrane of cells
  • In cytoplasm- diffuses to nuclueus
  • Binds to hormone receptor molecule on transcriptional factor
  • Releases inhibitor molecule
  • Switches on gene for transcription
  • Occurs with plant hormones (auxins, gibberellin) in same way
  • Oestrogen- lipid soluble mammalian hormone
  • Small hydrophobic molecules- diffuse through plasma membranes of cells
  • Once in cytoplasm, moves to nucleus
  • Binds to type of oestrogen receptor molecule on the transcriptional factor
  • Releases inhibitor molecule
  • Switches on gene for transcription
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  • Molecules are short, double stranded RNA molecules- interfere with the expression of specific genes
  • Bases complementary to specific sections of target gene (and mRNA formed from it)
  • Affects translation through RNA interference:
  • mRNA molecules degraded by enzymes after translation
  • Small sections of double stranded siRNA formed
  • One strand combines with enzyme/ protein (RISC-RNA induced silencing complex)
  • siRNA guides protein to mRNA molecule- pairs up bases with complementary ones on sections of mRNA molecule
  • Enzyme now cuts mRNA into smaller sections
  • mRNA can no longer be translated into a polypeptide
  • The gene is blocked so is no longer expressed
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