Chapter 17: From Gene to Protein

1. Chapter 17:From Geneto Protein

2. Important Point: If you are having trouble understanding lecture material: Try reading your text before attending lectures. And take the time to read it well!

3. DNA  DNA = Replication • DNA  RNA = Transcription • RNA  Protein = Translation • RNA  DNA = Reverse Transcription • Protein  RNA or DNA: DOES NOT HAPPEN! • Nucleic acid sequence is most readily translated into protein sequence • But protein sequence cannot be translated into nucleic acid sequence • In other words, information flows from Nucleic Acid Sequence to Protein Sequence • Information flows from Genotype to Phenotype • Mutation and Natural Selection supplies the means by which protein sequence can influence nucleic-acid sequence Flow of Information

4. Flow of Information “The DNA inherited by an organism leads to specific traits by dictating the synthesis of certain proteins. Proteins are the links between genotype and phenotype.”

5. Information Flow

6. Information Flow

7. A codon is a sequence of three nucleotides • mRNA is the molecule that presents codons to ribosomes • DNA serves as a codon-storage molecule • DNA serves as a template for RNA synthesis • Ribosomes translate codons, in sequence, into chains of amino-acids (polypeptides) • These amino-acid (and RNA) sequences are precisely controlled • Precision is both costly and requires complex machinery to achieve • Once translated, proteins also are often post-translationally modified • Proteins consist of one or more polypeptide Roles of Central Players

8. Triplet Code

9. 61 sense codons for 20 amino acids Genetic Code Note the degeneracy of the triplet code But also note the lack of ambiguity

10. mRNAs consist of a sequence of nucleotide triplets—codons—that code for amino acids and which together are described as “The Genetic Code” Genetic Code Note 3 stop codons = nonsense codons Note AUG, the start codon, codes for Methionine (Met)

11. Codons don’t overlap, there is no punctuation, each codon codes for at most only one amino acid (lack of ambiguity in the code), many amino acids are coded by more than one codon (= degeneracy in the code) • The cell would need tRNAs with 61 different anticodons to complement the available 61 codons • However, due to the Degeneracy of the genetic code, the third base is less discriminatory for the amino acid than the other two bases • This third position in the codon is referred to as the Wobble Position (and cells get by with ~45 tRNAs) • Us and Cs may be read by a G in the anticodon & As and Gs may be read by a U or y (pseudouridine) • If a tRNA contains an inosine (I) in the anticodon at the wobble position, then this tRNA may read codons having As, Us or Cs in the third position Wobble

12. Wobble – one-codon A.A.s All wobble discussion from http://www.nobel.se/medicine/educational/dna/a/translation/trna_wobble.html

13. Wobble – 2- or 3-codon A.A.s

14. Wobble – 4- or 6-codon A.A.s

15. RNA Diversity • Transcription makes various kinds of RNAs • For example: • Messenger RNA (mRNA) • Ribosomal RNA (rRNA) • Transfer RNA (tRNA) • Other (e.g., snRNA) • RNAs may be matured in various ways (we will concentrate on mRNA maturation) • Translation employs rRNAs and tRNAs to “translate” mRNA nucleotide/codon sequence into amino-acid sequence About 60% of the mass of ribosomes is rRNA

16. Types of RNA

17. RNA Polymerase Note 5’ to 3’ direction

18. Transcription Note that only one strand is serving as template for transcription

19. Initiation of Transcription

20. mRNA Processing Note: Eucaryotes

21. mRNA Splicing Note: Eucaryotes

22. snRNPs & Spliceosomes

23. Exons & Protein Domains

24. Remember that the primary goal of translation is the synthesis of a polypeptide from mRNA-coded information Translation Overview

25. rRNAs have 2° Structure (e.g., 16S)

26. tRNA 2D Structure Note that the anticodon is more or less complementary to the mRNA codon in terms of base-pairing

27. tRNA 3D Structure No, you don’t have to memorize this structure

28. Aminoacyl tRNA Synthetases Aminoacyl-tRNA synthetases are responsible for tRNA’s ability to precisely translate codon-based code into amino-acid sequence

29. Ribosome (in 3D) You don’t have to memorize structural detail (e.g., the various bumps)

30. Ribosome Schematic (“empty”)

31. Ribosome Schematic (functioning)

32. Translation, Initiation

33. Translation, Elongation

34. Translation, Termination

35. Protein Targeting to ER Polypeptides are subjected to a number of post-translational modifications whether or not they end up in the ER

36. Secreting Proteins

37. Polyribosomes

38. Works this way in bacteria Translation-Transcription Coupling

39. Mutation Mutations are alterations in DNA sequence that result either in modified transcription (since smaller target means less likely) or in modified translation (which we shall dwell upon)

40. Some mutations can be beneficial! Ultimate Source of Variation Some mutations are “silent”, not changing a.a. sequence Mutations are typically detrimental, but not always

41. Understand the concept, don’t memorize the sequences!!! Mutation: BP Substitution Point mutations Specifically, a missense mutation

42. Sickle Cell Anemia: Point Mutation

43. Understand the concept, don’t memorize the sequences!!! Insertion / Deletion Reading frames start with AUG, have numerous sense codons, and end with a stop codon

44. What is a Gene?

45. Summary

46. The End