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Transcription: From DNA to RNA

Transcription: From DNA to RNA. Chapter 8.2. Why RNA?. DNA cannot leave the nucleus BUT proteins are built by the ribosomes in the cytosol! We need a messenger to transfer the genetic code to the ribosomes mRNA is a complementary copy of a gene that CAN leave the nucleus. Messenger RNA.

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Transcription: From DNA to RNA

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  1. Transcription:From DNA to RNA Chapter 8.2

  2. Why RNA? • DNA cannot leave the nucleus • BUT proteins are built by the ribosomes in the cytosol! • We need a messenger to transfer the genetic code to the ribosomes • mRNA is a complementary copy of a gene that CAN leave the nucleus Messenger RNA

  3. Transcription:Making mRNA from DNA Four phases • Initiation • Elongation • Termination • Processing

  4. RNAP 1) Initiation • RNA polymerase (RNAP) binds to the double stranded DNA molecule at a promoter sequence • Promoters • always upstream of the gene being transcribed • Determines where RNAP binds and where transcription begins • Usually rich in Thymine and Adenine (“TATA” box) • RNAP has its own built-in helicase activity TATATT

  5. 3´ 5´ A T G T G A C T A C G G G C C C C G P P P P P P P P P P P P P P P P P P S S S S S S S S S S S S S S S S S S 3´ 5´ 2) Elongation • One strand of the unzipped DNA acts as a template for RNA synthesis Template Strand

  6. T C G T G G G A C U G G C G C A C C A C G C U A C G T P P P P P P P P P P P P P P P P P P P P P P P P P P P S S S S S S S S S S S S S S S S S S S S S S S S S S S 3´ 5´ 2) Elongation 3´ 5´ 5´ 3´ Template Strand

  7. 2) Elongation • mRNA is transcribed in the 5’ to 3’ direction • DNA unwinds only in the region of transcription • After transcription DNA recoils • Several RNAPs can work on a single gene at once

  8. 3) Termination • A terminator sequence on the non-transcribed DNA strand tells RNAP when to stop transcribing the mRNA • RNAP is released and reused • mRNA released

  9. 3´ 5´ G G T G C U G G C C A U C C G A C C G A C T A C G G T P P P P P P P P P P P P P P P P P P P P P P P P P P P S S S S S S S S S S S S S S S S S S S S S S S S S S S 3´ 5´ 3) Termination mRNA Template Strand

  10. 4) Processing • Occurs only in eukaryotic cells • Why processing? • Remove introns • Protects from degradation in the cytoplasm

  11. 4) Processing • Splicing • mRNA still has bothintrons and exons • We need to removeintrons • Spliceosome - a complex of multiple proteins and snRNA (small nuclear) • snRNA binds a specific mRNA sequences at the beginning and end of an intron forming a loop • Loop is removed by spliceosome

  12. restriction enzymes in cytosol 4) Processing • Protecting the mRNA • From what? • 5’ cap • Modified G added to 5’ endof mRNA • Poly-A tail • Many A added to 3’ end of mRNA

  13. G G G C U C G A U C G A A A A A A A G G A P P P P P P P P P P P P P P P P P P P P P S S S S S S S S S S S S S S S S S S S S S Final mRNA Molecule 5´ 3´ cap mRNA transcript poly A tail

  14. video http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter15/animations.html

  15. Better Video

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