From DNA to Protein

1. From DNA to Protein Chapter 13

2. Proteins • All proteins consist of polypeptide chains • A linear sequence of amino acids • Each chain corresponds to the nucleotide base sequence of a gene

3. The Path From Genes to Proteins 1. Transcription • Enzymes use the base sequence of a gene as a template to make a strand of RNA 2. Translation • Information in the RNA strand is decoded (translated) into a sequence of amino acids

4. Prokaryotes and Eukaryotes • In prokaryotic cells (no nucleus) • Transcription and translation occur in cytoplasm • In eukaryotic cells • Genes are transcribed in the nucleus • Resulting mRNA is translated in the cytoplasm

5. Key Concepts:INTRODUCTION • Life depends on enzymes and other proteins • All proteins consist of polypeptide chains • Chains are sequences of amino acids that correspond to sequences of nucleotide bases in DNA called genes • The path leading from genes to proteins has two steps: transcription and translation

6. 13.1 Transcription: DNA to RNA • Two DNA strands unwind in a specific region • RNA polymerase assembles a strand of RNA • Covalently bonds RNA nucleotides (adenine, guanine, cytosine, uracil) according to the nucleotide sequence of the exposed gene

7. Three Types of RNA • Messenger RNA (mRNA) • Carries protein-building codes from DNA to ribosomes • Ribosomal RNA (rRNA) • Forms ribosomes (where polypeptide chains are assembled) • Transfer RNA (tRNA) • Delivers amino acids to ribosomes

8. RNA and DNA Compared

9. RNA Base Pairing

10. phosphate group base (uracil) sugar (ribose) Fig. 13.2, p.198

11. Gene Transcription

12. Fig. 13.3, p.198

13. Fig. 13.3, p.198

14. newly forming RNA transcript gene region RNA polymerase, the enzyme that catalyzes transcription DNA template winding up DNA template unwinding Fig. 13.3, p.198

15. Fig. 13.3, p.198

16. direction of transcription growing RNA transcript Fig. 13.3, p.198

17. Fig. 13.3, p.198

18. Animation: Gene transcription details CLICK HERE TO PLAY

19. RNA Modification: Alternative Splicing • Before mRNA leaves the nucleus: • Introns are removed • Some exons are removed along with introns; remaining exons are spliced together in different combinations • Poly-A tail is added to 3’ end of new mRNA

20. The Poly-A Tail • The longer its poly-A tail, the more time an mRNA transcript (and its protein-building message) will remain intact in the cytoplasm

21. Post-Translational RNA Modification

22. unit of transcription in DNA strand exon intron exon intron exon transcription into pre-mRNA cap poly-A tail 5' 3' snipped out snipped out mature mRNA transcript Fig. 13.4, p.199

23. unit of transcription in DNA strand exon intron exon intron exon transcription into pre-mRNA cap poly-A tail 5' 3' snipped out snipped out mature mRNA transcript Stepped Art Fig. 13-4, p.199

24. Key Concepts:TRANSCRIPTION • During transcription, the two strands of the DNA double helix are unwound in a gene region • Exposed bases of one strand become the template for assembling a single strand of RNA (a transcript) • Messenger RNA is the only type of RNA that carries DNA’s protein-building instructions

25. Animation: Uracil-thymine comparison CLICK HERE TO PLAY

26. Animation: Pre-mRNA transcript processing CLICK HERE TO PLAY

27. 13.2 The Genetic Code • Messenger RNA (mRNA) carries DNA’s protein-building information to ribosomes for translation • mRNA’s genetic message is written in codons • Sets of three nucleotides along mRNA strand

28. Codons • Codons specify different amino acids • A few codon signals stop translation • Sixty-four codons constitute a highly conserved genetic code

29. Genetic Code: RNA Triplets

30. Animation: Genetic code CLICK HERE TO PLAY

31. From DNA to Polypeptide

32. DNA mRNA mRNA codons threonine proline glutamate glutamate lysine amino acids Fig. 13.5, p.200

33. Variation in Genetic Code • Variant codons occur among prokaryotes, prokaryote-derived organelles (such as mitochondria), and some ancient lineages of single-celled eukaryotes

34. Key Concepts:CODE WORDS IN THE TRANSCRIPTS • The nucleotide sequence in RNA is read three bases at a time • Sixty-four base triplets that correspond to specific amino acids represent the genetic code, which has been highly conserved over time

35. 13.3 tRNA and rRNA Function in Translation • Transfer RNA (tRNA) • Anticodon binds to mRNA codon • Also binds amino acid specified by codon • Different tRNAs carry different amino acids • tRNAs deliver free amino acids to ribosomes during protein synthesis

36. tRNA

37. anticodon amino acid attachment site Fig. 13.7, p.201

38. rRNA • Ribosomal RNA (rRNA) and proteins make up the two subunits of ribosomes

39. large subunit small subunit intact ribosome Fig. 13.6, p.201

40. Animation: Structure of a tRNA CLICK HERE TO PLAY

41. Animation: Structure of a ribosome CLICK HERE TO PLAY

42. 13.4 Three Stages of Translation • mRNA-transcript information directs synthesis of a polypeptide chain during translation • Translation proceeds in three stages • Initiation • Elongation • Termination

43. Initiation • One initiator tRNA, two ribosomal subunits, and one mRNA come together as an initiation complex

44. Initiation

45. Elongation • tRNAs deliver amino acids to the ribosome in the order specified by mRNA codons • Ribosomal rRNA catalyzes the formation of a peptide bond between amino acids

46. Elongation

47. Elongation

48. Elongation

49. Termination • Translation ends when RNA polymerase encounters a STOP codon in mRNA • New polypeptide chain and mRNA are released • Ribosome subunits separate from each other

50. Termination