Understanding Microbial Genetics: From DNA Replication to Gene Expression

1. Chapter 8 Microbial Genetics

2. Terminology • Genetics Study of what genes are, how they carry information, how information is expressed, and how genes are replicated • Gene Segment of DNA that encodes a functional product, usually a protein

3. Terminology • Genome All of the genetic material in a cell • Genomics Molecular study of genomes • Genotype Genes of an organism • Phenotype Expression of the genes

4. E. coli Figure 8.1a

5. Flow of Genetic Information Figure 8.2

6. DNA • Polymer of nucleotides: adenine, thymine, cytosine, guanine • Double helix associated with proteins • "Backbone" is deoxyribose-phosphate • Strands held together by hydrogen bonds between AT and CG • Strands are antiparallel Figure 8.4

7. DNA Figure 8.3

8. DNA Figure 8.5

9. DNA • DNA is copied by DNA polymerase • In the 5 3 direction • Initiated by an RNA primer • Leading strand synthesized continuously • Lagging strand synthesized discontinuously • Okazaki fragments • RNA primers are removed and Okazaki fragments joined by a DNA polymerase and DNA ligase

10. DNA Figure 8.6

11. DNA • DNA replication is semiconservative Figure 8.7

12. Transcription • DNA is transcribed to make RNA (mRNA, tRNA, and rRNA) • Transcription begins when RNA polymerase binds to the promotor sequence • Transcription proceeds in the 5 3 direction • Transcription stops when it reaches theterminator sequence

13. Figure 8.8

14. RNA processing in Eukaryotes Figure 8.12

15. Translation • mRNA is translated in codons (3 nucleotides) • Translation of mRNA begins at the start codon: AUG • Translation ends at a STOP codon: UAA, UAG, UGA Figure 8.2

16. Translation Figure 8.9

17. Translation Figure 8.10.1

18. Translation Figure 8.10.2

19. Translation Figure 8.10.3

20. Translation Figure 8.10.4

21. Translation Figure 8.10.5

22. Translation Figure 8.10.6

23. Translation Figure 8.10.7

24. Translation Figure 8.10.8

25. Translation Figure 8.11

26. Regulation of Bacterial Gene Expression • Constitutive enzymes are expressed at a fixed rate • Other enzymes are expressed only as needed • Repressible enzymes • Inducible enzymes

27. Repression Figure 8.13

28. Operon Figure 8.14.1

29. Regulation of Gene Expression Figure 8.14.2

30. Regulation of Gene Expression Figure 8.14.3

31. Regulation of Gene Expression Figure 8.14.4

32. Regulation of Gene Expression Figure 8.14.5

33. Regulation of Gene Expression Figure 8.15

34. Mutation • Change in the genetic material • Mutations may be neutral, beneficial, or harmful • Mutagen: Agent that causes mutations • Spontaneous mutations: Occur in the absence of a mutagen

35. Mutation • Change in one base • Result in change in amino acid • Base substitution (point mutation) • Missense mutation Figure 8.17a, b

36. Mutation • Results in a nonsense codon • Nonsense mutation Figure 8.17a, c

37. Mutation • Insertion or deletion of one or more nucleotide pairs • Frameshift mutation Figure 8.17a, d

38. Mutation • Ionizing radiation (X rays and gamma rays) causes the formation of ions that can react with nucleotides and the deoxyribose-phosphate backbone. • Nucleotide excision repairs mutations

39. Mutation • UV radiation causes thymine dimers • Light-repair separates thymine dimers Figure 8.20

40. The Frequency of Mutation • Spontaneous mutation rate = 1 in 109 replicated base pairs or 1 in 106 replicated genes • Mutagens increase to 10–5 or 10–3 per replicated gene

41. Selection • Positive (direct) selection detects mutant cells because they grow or appear different. • Negative (indirect) selection detects mutant cells because they do not grow.

42. Replica Plating Figure 8.21

43. The Ames Test for Chemical Carcinogens Figure 8.22

44. Genetic Transfer and Recombination • Occurs during reproduction, between generations of cells • Transfer of genes between cells of the same generation • Vertical gene transfer • Horizontal gene transfer

45. Transformation Figure 8.24

46. Recombination Figure 8.25

47. Conjugation Figure 8.27a

48. Conjugation Figure 8.27b

49. Conjugation Figure 8.27c

50. Genetic Recombination • Exchange of genes between two DNA molecules • Crossing over occurs when two chromosomes break and rejoin Figure 8.23