Transcription In Eukaryotes

Introduction

  • Transcription occurs in the nucleus, translation in the cytoplasm
  • Eukaryotic genes may have introns
  • Initiation requires more transcription factors
  • Regulation of transcription is complex

Transcription Factors

  • Transcription factors: proteins that bind to DNA near the start of transcription of a gene.
  • Transcription factors either inhibit or assist RNA polymerase in initiation and maintenance of transcription.
  • The basal promoter contains a sequence of 7 bases (TATAAAA) called the TATA box. It is bound by a large complex of some 50 different proteins, including Transcription Factor IID (TFIID)
  • Transcription Factor IID (TFIID): a complex of
    • TATA-binding protein (TBP): which recognizes and binds to the TATA box
    • 14 other protein factors which bind to TBP - and each other - but not to the DNA
  • Transcription Factor IIB (TFIIB): binds both the DNA and pol II.

RNA Processing

  • Bacterial mRNAs can be translated immediately
  • Eukaryotic mRNAs are made in a longer pre-mRNA form that requires processing into mature mRNA
    • Introns: transcribed but not translated
    • Exons: coding sequence found in mature mRNA
  • Splicing: removal of introns
  • Other modifications – addition of tails and caps
  • 5’ Cap: modified guanosine attached to 5’ end
    • Needed for mRNA to exit nucleus and bind ribosome
  • Poly A tail: 100-200 adenine nucleotides added to 3’ end
    • Increases stability and lifespan in cytosol
    • Not encoded in gene sequence