7.2 gene expression

• transcription factor


  proteins that bind to DNA
• how do transcription factors work?

  • bind to promoter region
  • RNA polymerase binds to DNA-transcription factor complex
  • allows for transcription to occur
• transcription factors can be...

  • activators (switch on gene expression)
  • repressors (switch off gene expression)
• RNA splicing:

  • gene is transcribed
  • introns (non coding regions) removed
  • exons joined together using spliceosomes
• Alternative splicing

  • spliceosome make different forms of mature mRNA
  • translated to make different proteins with a different structure and function.
• how does splicing result in different products from same gene?

  when exons joined back together, they are joined in different ways. resulting in different versions of RNA
• Epigenetics


  changes in gene expression without changing the base sequence
• Heterochromatin


  
  

  • nucleosome tightly packed
  • DNA inaccessible for transcription
  • DNA methylated
  • Histones de-acetylated
• Euchromatin


  • nucleosome loosely packed
  • DNA accessible for transcription
  • DNA de-methylated
  • Histone acetylated
• methods of epigenetic modification?


  • DNA methylation
  • Histone acetylation
• DNA methylation

  • involves nucleotide with C base next to G base
  • methyl group added to C base
  • prevents activation of RNA polymerase
  • stops transcription - chromatin is tightly packed so not accessible
• Histone acetylation

  • acetyl group added to leucine of histones
  • causes chromatin to be loosely packed
  • allows transcription to occur - RNA polymerase can bind to promoter
• non coding RNA can be used to regulate translation.

  2 types:

  • siRNA
  • miRNA
• Micro RNA (miRNA)

  • bind to mRNA in the cytoplasm
  • complete complimentary base pairing = mRNA destruction
  • incomplete binding = ribosome activity inhibited, so no translation
• Silencing RNA (siRNA)

  • protein complex RISC undergoes cleavage
  • separates DNA into single stranded siRNA
  • coordinates binding of siRNA to target mRNA
  • target mRNA degraded