Chapter 8 – Bacterial and Viral Genetic Systems

1. Chapter 8 – Bacterial and Viral Genetic Systems

2. Bacteria • Prototrophic • Wild-type • Can grow on minimal media • Contains minimal nutrients – carbon, nitrogen, phosphorous, vitamins, ions • Auxotrophic • Can not produce an essential enzyme or manufacture essential molecules • Will only grow on media that contains the “missing” substance • Complete media

3. Culturing bacteria • Suspension culture • Liquid media • Bacteria dies off when nutrients are used up or waste buildup becomes toxic • Bacteria grow singularly – no colonies

4. Culturing bacteria • Petri dishes • Growth media in agar • Isolate individual colonies • Each colony originates from a single bacterium

5. Replica plating • Gives “carbon copies” of petri dish colonies • Use sterilized velvet to make a stamp • Some bacteria from each colony is transferred to velvet, and then transferred to new dishes

6. Bacterial genome • Most consist of a single, circular chromosome • Some have several chromosomes, and a few have linear chromosomes • Very little “extra” DNA between genes • Plasmids • Small, circular, extra-chromosomal DNA • Usually non-essential • Replicate independent of chromosomal DNA • Have their own origin of replication

7. F factor episome • Episome • A plasmid that can replicate independently AND also has the ability to incorporate into chromosomes

8. Gene transfer in bacteria - Conjugation • One bacteria directly transfers DNA to another bacterium • Cytoplasmic connection forms, and either entire plasmid or part of the chromosome is transferred from donor to recipient • Crossing over may occur between homolgous regions • Creates recombinant DNA • Extra DNA is degraded

9. Gene transfer in bacteria - Transformation • Bacteria takes up DNA from surrounding environment • Recombination may occur

10. Gene transfer in bacteria - Transduction • Viral particle introduced DNA from a bacterium into a new bacterium

11. Conjugation • Fertility factor/F factor contains ori and genes needed for conjugation

12. Conjugation • Hfr bacteria • F+ cell that has F factor incorporated into chromosome • As F factor enters recipient, some chromsome enters – amount depends on time length of contact • Donor DNA made into double-stranded • Crossing over can occur between homologous regions • Any DNA not incorporated is degraded • Recipient is not usually converted to F+ since the F factor is nicked in the middle

13. Conjugation • F′ bacteria • F factor excises out of a chromosome in a Hfr cell • May remove part of chromosome as well • F′ plasmid now contains F factor and some genes from chromosome • Enters F- bacteria • Produces merozygotes – partially diploid

14. Transformation • Uptake of DNA and incorporation into chromosome or plasmid • Competent – cells able to take up DNA • CaCl2, heat shock, electrical fields • Makes membrane more permeable to DNA • DNA does not have to have bacterial origin • Transformants – bacteria that have incorporated foreign DNA

15. E. Coli has model organism • Many strains are avirulent • Small and rapid reproduction • Easy to culture • Genome is single chromosome - haploid • Small genome • 4.5 million bp/4,000 genes • Wild-type are prototrophic

16. Viral genetics • DNA or RNA (single or double stranded) as genetic material • Can not reproduce on their own • Bacteriophages – viral particles that infect bacteria

17. Bacteriophage – lytic cycle • Virulent phages • Viral DNA is injected into host cell where it replicated, transcribed, and translated into more phages • Host cell bursts open to release viral particles

18. Bacteriophages – lysogenic cycle • Temperate phages • Phage DNA is incorporated into host genome – prophage • Passed onto all progeny cells • Can be transcribed and translated • Can exit from host genome to enter lytic cycle

19. Transduction • Generalized • Any gene is transferred • During lytic cycle, bacterial DNA is degraded • Some may enter viral protein coat instead of viral genetic material • Transducing phages • Can become incorporated into new host’s genome

20. Transduction • Specialized • Few genes are transferred/genes near certain sites of chromosome • During lysogenic cycle, prophage enters at specific sites of host’s genome • When prophage excises, it may do so imperfectly and bring some hot DNA with it • Then introduced to new host

21. RNA viruses • Positive strand RNA viruses • Single strand directly codes for viral proteins • Negative strand RNA viruses • Must make complementary RNA strand, which then codes for proteins • Retroviruses • Incorporate into host genome • Must make DNA from RNA • Reverse transcriptase • Makes cDNA from DNA or RNA template • Enters host genome as a provirus • Can be transcribed and translated • Some retroviruses contain oncogenes • Cause tumors