2. The Central Dogma of Molecular Biology replication transcription translation

4. Why RNA?

5. 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

6. 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

7. Why RNA?

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

9. Gaining Access to DNA

10. 4 Phases of Transcription 1. Initiation 2. Elongation 3. Termination 4. Processing (Eukaryotes Only)

11. 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

12. RNA Polymerase II

14. Enhancers

15. 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)

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

18. Elongation S U P S G PS G PS T PS C PS C PS G PS A PS T PS C P 5´ 3´ Template Strand S C PS G PS A PS T PS C PS G PS G PS A PS C PS C P 3´5´ S A PS G PS C PS C PS U PS G PS G P 3´ Coding Strand

20. More Detail: Elongation

21. 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

22. Lots of copies for lots of ribosomes

23. Electron Micrograph

24. 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

26. 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

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

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

29. Splicing

30. 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

33. Cap and Tail S G PS G PS U PS C PS C PS G PS A PS U PS C P 5´ 3´ S G PS G P S G PS G P S A PS A P S A PS A P S A PS A P S A PS A P cappoly A tail mRNA transcript

34. 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