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Transcription

Transcription. … from DNA to RNA. The Central Dogma of Molecular Biology. DNA. RNA. Protein. transcription. translation. replication. Why RNA?. Why RNA?. Not all genes need to be turned on at once. We can make an RNA transcript of just ONE GENE

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  1. Transcription … from DNA to RNA

  2. The Central Dogma of Molecular Biology DNA RNA Protein transcription translation replication

  3. Why RNA?

  4. Why RNA? • Not all genes need to be turned on at once. • We can make an RNA transcript of just ONE GENE • Now we can make the right protein at the right time in the right location

  5. Why RNA? In EUKARYOTES… • 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

  6. Why RNA?

  7. mRNA • Messenger RNA (mRNA) is a complementary copy of a gene that CAN leave the nucleus

  8. Gaining Access to DNA

  9. 4 Phases of Transcription • Initiation • Elongation • Termination • Processing (Eukaryotes Only)

  10. Initiation • RNA polymerase (RNAP) binds to the double stranded DNA molecule at a promoter sequence (with the help of initiation factors) • It is able to locally unzip DNA with its own built in helicase activity as it constructs an RNA transcript of the DNA

  11. RNA Polymerase II

  12. Enhancers

  13. Promoters • DNA sequence upstream of the gene being transcribed • Determines where RNAP binds and where transcription begins • Usually rich in Thymine and Adenine (“TATA” box)

  14. 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´ Elongation • One strand of the unzipped DNA acts as a template for RNA synthesis Template Strand

  15. T C G G U C T G G A U C C A A G C G C G C T A G C C G 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´ Elongation 3´ 5´ Coding Strand 5´ 3´ Template Strand

  16. More Detail: Elongation

  17. 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

  18. Lots of copies for lots of ribosomes

  19. Electron Micrograph

  20. Termination • A terminator sequence on the coding strand tells RNAP when to stop transcribing the mRNA • RNAP is released and reused and mRNA is released

  21. Processing • In Eukaryotic cells the RNA transcript is called pre-mRNA (or primary RNA) because it must still be modified before it leaves the nucleus • Why processing? • Remove introns • Protects from degradation in the cytoplasm

  22. Introns and Exons Genes contain both coding regions (exons) and non-coding regions (introns)

  23. Introns and Exons • To produce a final mRNA transcript, introns must be removed

  24. Splicing

  25. Splicing • Small nuclear RNA (snRNA) in complex with proteins are called small nuclear ribonucleic particles (snRNPs) • These assemble with other proteins to form the Spliceosome • snRNA binds to specific mRNA sequences at the beginning and end of an intron forming a loop • The loop is removed and the remaining exons are linked

  26. 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 Cap and Tail 5´ 3´ cap mRNA transcript poly A tail

  27. Cap and Tail • To protect RNA from restriction endonucleases in the cytosol, a poly-A tail is added to the 3‘ end of the pre-mRNA • As an attachment site for the ribosome, a 5‘ cap of modified Gs is added

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