1. 3.5 (SL)/ 7.3 (HL): Transcription

7. Compare the structure of DNA and RNA

17. Compare the processes of transcription and translation

20. DNA transcription (higher level)

21. Let’s begin with some animations… 1.from the Wellcome trustfrom the Wellcome trust 2.From PBSFrom PBS

22. Exceptions to the central dogma of genetics 1: retroviruses (thanks Max)

23. Prion protein replication Prions are proteins that propagate themselves by making conformational changes in other molecules of the same type of protein

24. And finally….thanks, Ilona! exceptions

26. 7.3.1: State that transcription is carried out in a 5’ – 3’ direction

27. Remember that BOTH transcription AND translation are carried out in a 5’ to 3’ direction

28. 7.3.2: Distinguish between the sense and antisense strands of DNA Sense strand is the genetic code Sense strand is not copied Antisense strand is the template strand Antisense strand is used for transcription Antisense strand is complementary to sense strand

29. Sense and antisense DNA SENSE strand has same base sequence as transcribed mRNA, except that T is replaced by U RNA polymerase enzyme adds RNA nucleotides to sugar-phosphate background in 5’ – 3’ direction

30. Where does transcription begin? Promotor region: contains ‘promotor’ sequence: specific DNA recognised by activator proteins wh

31. Promotor Region Upstream of transcription site Contains specific DNA (TATAAA boxes) recognised by activation factors which recruit RNA polymerase 100 – 100 bases long Very diverse in eukaryotic DNA

32. Coding Region Provides specific triplet code for specific protein Coding region produces mRNA which will be modified after transcription mRNA produced by RNA polymerase

33. Terminator Region Site signals the end of DNA transcription Triplet code makes both RNA polymerase AND mRNA strand fall off the template Fold up mRNA strand to unlock both mRNA and polymerase

34. Process of transcription Animation of transcription more complete animation of transcription

35. Post-translational modification Eukaryotes only mRNA is processed in the nucleus before exportation to the cytoplasm Introns are removed (spliced) Mature mRNA is exported to the cytoplasm EXONS are joined together

36. Post-translational modification - splicing Animation on RNA splicing

37. Translation! Let’s start with some animations… From John Kyrk From Wiley From Harvard

38. Welcome to the world of RNA!

41. Ribosomal RNA

42. Ribosomal structure form the PDB

43. Transfer (tRNA) ‘Adaptor molecules’: one end can read the triplet code on mRNA (anticodon), one end(3’) attaches to a specific amino acid (Crick, 1958) Each tRNA has a specific shape, determined by looping of helical sections Each tRNA specific shape fits one of 20 aa-tRNA synthase enzymes

44. Structure of tRNA Amino acid attachment site (3’) ( acceptor loop/stem ) :always CCA Complementary base pairing (H-bonds) Non-base pairing loops (7 or 8 bases) Anticodon (3 bases): atttaches to mRNA codon Mammals have 150 tRNA molecules

45. Activation of tRNA

46. How does the correct amino acid link to tRNA? 1.The shape of each tRNA is different, defined by folding of the loop and helical structures RNA folding 1.The shape of the tRA determines which of 20 specific amino-acyl tRNA synthase enzymes it attracts 2.ATP is needed to attach the amino acid to the 3’ CCA end of the tRNA 3.Each amino acid has >1 tRNA molecule (degenerate code)

47. Structure of a ribosome (ribozymes) Ribosomes are actually ribozymes They catalyse translation of mRNA into a polypeptide The substrate is mRNA Each ribosome is multifunctional – it is not used up, and catalyse translation of many different mRNA codes

48. Structure of a ribosome

49. Prokaryotes and eukaryote ribosomes differ

50. Polysomes

51. Cluster of ribosomes bound to mRNa strand Read mRNa simultaneously to increase efficiency of protein synthesis Direction of translation is 5’ to 3’

52. Free and fixed ribosomes Free ribosomes make proteins for ‘internal use’ RER-bound ribosomes synthesise proteins which will be placed invesicles and exported from the cell

53. Translation has 4 stages (leader sequence) 1.Initiation 2.Elongation 3.Translocation 4.Termination

54. (Leader sequence) Leader sequence contains a ribosome binding site In bacteria, called the Shine-Delgado box (AGGAGG) In vertebrates, the KOZAK box (170 bases) Leader sequence contains many regulatory sequences, including protein binding sites, that can affect stability of the mRNA or the efficiency of translation

55. Initiation mRNA has the start codon (AUG) Only one start codon tRNA has anticodonUAC Small ribosomal subunit associates with the tRNA Large subunit moves over mRNA

56. Initiation (2) ONLY AUG can occupy the P site ‘de novo’ Next codon on A site if free to attract appropriate tRNA

57. Elongation Bond between tRNA and methionine is broken Energy released is used to form peptide bond GTP is also used as an energy source Large ribosomal subunit moves 3 base pairs (one codon) towards the 3’ end

58. Elongation (2) Both subunits are now over the P and A sites tRNA for methionine is on E site and is released for re-charging tRNA for proline is in P site A site is free and attracts tRNA for alanine

59. Elongation (3) Bond between tRNA and proline is broken GTP is provided as a source of energy Energy released forms peptide bond Ribosome moves along the mRNA towards 3’

60. Termination Ribosome encounters s STOP codon: UGA, UAA, UAG Not recognised by tRNA Instead, release factors (proteins) bind and facilitate release of the polypeptide from the translation process Two subunits move apart and separate Protein moves to ER and Golgi for processing