Bacterial and Viral Genetics Ch. 17.1, 17.2

1. Bacterial and Viral GeneticsCh. 17.1, 17.2

2. Who Cares About Tiny Weak Bacteria?!? • WE ALL DO! • Bacteria is good for us (E. coli) • Bacteria is bad for us (E. coli!!) • Most of life comes in single celled organisms and understanding how they evolve and alter their DNA could safe many lives • Complex cells can undergo sexual reproduction and gain a ton of variation through meiosis • Do bacteria gain variation through DNA swapping? • YES!!! BUT not the same way

3. Gene Swapping Bacteria Style • Who discovered that bacteria can gain the properties of each other? • Griffith; Killed mice is S and R type bacterium • Lederberg and Tatum; 1946 • Mutated E. coli to be make strains (auoxtrophs) that could only survive on specific growth medium • Normal E coli can grow on minimal medium (water, glucose, and salts) • Strain 1: needs AA Met and vitamin biotin; cool with AA Leu, Thr, and vitamin thaimine • Strain 2: needs AA Leu and Thr, and vitamin thaimine; cool with AA Met and vitamin biotin

4. Conjugation • Hypothesis Strain 1 and 2 can grow on a minimal medium (lacking all listed amino acids and vitamins) if they are mixed together and exchange DNA • Experiment mix strains and grow on a minimal medium; grow separate on MM as control • Result totally worked!! • Mix had hundreds of colonies; unmixed had none • Conjugation copying and exchanging of DNA sections between bacteria through a sex pilus(weak cytoplasmic bridge)

5. Have to be F’d to Conjugate • Bacteria do not inherently posses the ability to conjugate • Donor bacterium most have F factor; plasmid that codes for conjugation/sex pili • F- cells can receive DNA but not start conjugation • F+ cells can conjugate but with only F- bacteria • F+transfer F+ plasmid is copied and given to F- cell • F- cell is now a F+ cell • No other DNA is exchanged; no recombination

6. Genetic Recombination in Bacteria • How does bacterial DNA mix if the F+ plasmid is the only thing passed around? • Mixing of DNA by crossing over • Sometimes F+ plasmid will integrate into the bacteria’s DNA before the copy/exchange; • Hfr cells High Frequency Recombination cells • Recipient bacteria gets F+ and extra random genes (partial diploid cell) • Homologous DNA regions in match up and crossing-over allows integration of new genes • Is the recipient bacteria a F+ cell now? • Probably not…

7. Why am I F-??? I studied so hard… • Sex pili are delicate and often break before conjugation is complete • Connections break before whole F +plasmid/new genes can cross • Partial F+ plasmid can’t form plasmid ring • Any material not homologous to the recipient bacteria (not integrated be crossing over) is degraded by enzymes • F+ plasmid and any unused genes are destroyed

8. Gene Mapping?! Again? • So how can successful conjugation of a genes tell where they are located? • Show their order and their distance (map) by amount of time it takes for full transfer • Measure distance on gene not in cM but in minutes • The longer it takes gene A AND gene B to both be transferred, the further they must be away from each other

9. Alternative Forms of Exchange • Wait! Griffith used heated S-type bacteria to change R-type into S-type. If the S-type were killed, how did the gene transfer happen? • 2 methods: 1) Transformation DNA from destroyed cells pulled into cell and added to DNA • Only 1% of bacteria can do this; very rare skill • Natural transformation; have DNA recognizing membrane receptors • Artificial transformation Heat shock mix on ice then quickly heat up; sudden temp change opens membrane to DNA fragments Electroporation electric pulse opens up membrane to DNA fragments

10. Alternative Forms of Exchange 2) Transduction DNA transfer by a virus (bacteriophage) • Sometimes viruses will incorporate host DNA while making copies of themselves • Incomplete/altered phage copy might not function in the new host (kill it) • If this new mixed DNA virus infects another bacteria cell and has its DNA integrated into the new host, that bacteria gains both bacterial and viral DNA • Process extremely limited on the amount of DNA that can be transferred

11. Horizontal Gene Transfer • Conjugation, transformation, and transduction are all forms of exchange of DNA through none related members • In all cases, the new DNA must be incorporated through crossing-over; so there must be some similarities between the new gene and an existing gene • So different species of bacteria CAN gain new genes if there is enough similarity in their DNA; 20% E. coli is from this • Eukaryotic-Prokaryotic transfer CAN happen, but it is very rare

12. What is a Virus? • DNA or RNA, single or double stranded, segment (virion) protected by a protein coat (capsid) that is only 300-500nm long and cannot reproduce unless it infects a host cell • A few genes or hundreds; one strand or several; capsid of one type of protein or one of many types of proteins • Random mixing of DNA, RNA, proteins, and enzymes that are neither alive nor dead

13. 2 Basic Structures • Helical capsid forms rod-like spiral around nucleic acid • Polyhedral capsid made of triangle units that form icosahedral structures • Enveloped cover made from plasma membrane of previous host cell • Bacteriophages and other complex viruses head and tail regions (look like a space ship) inject DNA into host

14. Taxonomy of the Undead! • Even though not a living thing, viruses are still categorized by various methods • shape/structure • Size • DNA/RNA and how many strands • Method of replication • Host range • Infective cycle/symptoms • Specific attacks of species support a theory that viruses are a method of nature keeping dominate species in check

15. Infective Cycles • Virulent Lytic cycle • Virus infects the host cell • Uses mechanics of host to make copies of virion and capsid • Releases copies slowly or bursts from unstable host (killing it) • Host DNA may be incorporated through generalized transduction • T-even bacteriophages are like this (T2, T4, T6)

16. Infective Cycles • Temperate Lysogenic Cycle • Virus attacks host and viral DNA is added to the host DNA (provirus) • Stays dormant in host; DNA replicated and past on into daughter cells • Environmental stimulus causes virus to switch to lytic cycle after uncertain amount of time • Viral DNA is removed from the host DNA and used to start lytic cycle • May integrate host cell DNA through specialized transduction • HIV is like this

17. Viral Infections in Animals • Viruses can only infect animals cells if they have appropriate what? • Receptors! Virus use the receptors in cell membranes to attach to the cell or trick the membrane into endocytosis • Some viruses with envelopes (HIV) join by the fusing of plasma membranes • Some viruses with envelopes enter the cell as a whole and enter a latent phase (in cell cytoplasm but inactive) • Herpes virus does this and it triggered to enter the lytic cycle by stress and cold temps.

18. Viral Infections in Plants • Mostly RNA rod-like or polyhedral viruses • None have a envelope • Difficult to infect through cell wall but can be penetrate plant through damage caused by animals • Names after the type of plant they infect and visible symptoms • Tobacco mosaic virus

19. RNA Based Viruses • Your Turn!!! • Essays! Essays!  to help you practice your writing skills of the AP exam you will be getting 1 page essays (about 1 per chapter) • DO THESE! If you copy from the internet you are just wasting time • Topic: What are the basic characterizes of RNA viruses and how do these make the HIV virus so difficult to cure? • Research and write • Due Thursday! No excuses and no late work!

20. Homework • Suggested Homework: • Test Your Knowledge • Actual Homework: • Discuss the Concept #1 • Design the Experiment