Microbial Genetics Chapter 7

1. Microbial GeneticsChapter 7

2. The Structure and Replication of Genomes • Genome – the entire genetic complement of an organism

3. The Structure of Nucleic Acids Figure 7.1d

4. The Structure of Prokaryotic Genomes • Contained in two structures • Chromosomes • Plasmids

5. Prokaryotic Chromosomes • Main portion of DNA, along with associated proteins and RNA, are packaged in 1-2 distinct chromosomes • Prokaryotic cells have a single copy of each chromosome (haploid) • Typical chromosome – circular molecule of DNA in nucleoid

6. Plasmids • Small molecules of DNA that replicate independently • Carry information required for their own replication, and often for one or more cellular traits • Not essential for normal bacterial metabolism, growth, or reproduction • Can confer survival advantages

7. Plasmids • Many types of plasmids • Fertility factors • Resistance factors • Bacteriocin factors • Virulence plasmids • Cryptic plasmids

8. The Structure of Eukaryotic Genomes • Contained in two structures • Nuclear DNA • Extranuclear DNA

9. Eukaryotic Chromosomes • Typically have more than one chromosome per cell • Chromosomes are linear and sequestered within membrane-bound nucleus • Eukaryotic cells often have two copies of each chromosome (diploid)

10. Extranuclear DNA of Eukaryotes • DNA molecules of mitochondria and chloroplasts are circular and resemble chromosomes of prokaryotes • Only codes for about 5% of RNA and proteins • Nuclear DNA codes for 95% of RNA and proteins • Some fungi and protozoa carry plasmids

11. DNA Replication • An anabolic polymerization process that requires monomers and energy • Triphosphate deoxyribonucleotides serve both functions • Key to replication is complementary structure of the two strands • Replication is semiconservative – new strands composed of one original strand and one daughter strand Animation: DNA Replication (play part 1) PLAY

12. Initial Processes in DNA Replication Figure 7.5a

13. Initial Processes in DNA Replication • DNA polymerase binds to each strand and adds nucleotides to hydroxyl group at 3′ end of nucleic acid • Replicates DNA only 5′ to 3′ • Because strands are antiparallel, new strands synthesized differently • Leading strand synthesized continuously • Lagging strand synthesized discontinuously

14. Synthesis of the Leading Strand Figure 7.5b

15. Synthesis of the Lagging Strand Animation: DNA Replication (play parts 2-4) PLAY Figure 7.5c

16. Replication of Eukaryotic DNA • Similar to bacterial replication • Some differences • Use four DNA polymerases • Thousands of replication origins

17. Gene Function • Genotype – set of genes in the genome • Phenotype – physical features and functional traits of organism

18. Transfer of Genetic Information • Transcription – information in DNA is copied as RNA nucleotide sequences • Translation – polypeptides synthesized from RNA nucleotide sequences • Central dogma of genetics • DNA transcribed to RNA • RNA translated to form polypeptides

19. Events in Transcription • Four types of RNA transcribed from DNA • RNA primers • mRNA • rRNA • tRNA • Occurs in nucleoid of prokaryotes • Three steps • Initiation • Elongation • Termination

20. Initiation of Transcription Animation: Transcription PLAY Figure 7.8a

21. Elongation of the RNA Transcript Figure 7.8b

22. Concurrent RNA Transcription Figure 7.9

23. RNA Polymerase Versus DNA Polymerase • RNA polymerase does not require helicase • RNA polymerase slower than DNA polymerase • Uracil incorporated instead of thymine • RNA polymerase proofreading function is less efficient than DNA polymerase (more errors)

24. Prokaryotic mRNA Figure 7.12

25. Transcription in Eukaryotes • RNA transcription occurs in the nucleus • Transcription also occurs in mitochondria and chloroplasts • Three types of RNA polymerases • Numerous transcription factors • mRNA processed before translation • Capping • Polyadenylation • Splicing

26. Eukaryotic mRNA Figure 7.10

27. Genetic Code PLAY Animation:Translation (play part 2 genetic code) Figure 7.11

28. tRNA Figure 7.13a-b

29. Ribosomes and rRNA Figure 7.14c

30. Stages of Translation • Three stages • Initiation • Elongation • Termination • All stages require additional protein factors • Initiation and elongation require energy (GTP) Animation: Translation PLAY

31. Initiation Figure 7.15

32. Elongation Figure 7.16.1

33. Elongation Figure 7.16.2

34. Elongation Figure 7.16.3

35. Elongation Figure 7.16.4

36. Elongation Figure 7.16.5

37. Elongation Figure 7.16.6

38. Polyribosome Figure 7.17a

39. Termination • Release factors somehow recognize stop codons and modify ribosome to activate ribozymes which sever polypeptide from final tRNA • Ribosome dissociates into subunits • Polypeptides released at termination may function alone or together

40. Regulation of Genetic Expression • 75% of genes are expressed at all times • Other genes are regulated so they are only transcribed and translated when cell needs them • Allows cell to conserve energy • Regulation of protein synthesis • Typically halt transcription • Can stop translation directly

41. The Operon Figure 7.18

42. Operons • Inducible operons – must be activated by inducers • Lactose Operon • Repressible operons – transcribed continually until deactivated by repressors • Tryptophan Operon

43. The Lactose Operon Figure 7.19a

44. The Lactose Operon Figure 7.19b

45. The Lactose Operon

46. The Tryptophan Operon Figure 7.20a

47. The Tryptophan Operon Figure 7.20b

48. The Tryptophan Operon

49. Mutations of Genes • Mutation – change in the nucleotide base sequence of a genome; rare • Almost always deleterious • Rarely lead to a protein having a novel property that improves ability of organism and its descendents to survive and reproduce Animation: Mutations and DNA Repair PLAY

50. Mutations of Genes • Types • Point mutations (most common) – one base pair is affected • Insertions, deletions, and substitutions • Frameshift mutations – nucleotide triplets after the mutation displaced • Insertions and deletions