Bacterial and Viral GeneticsCh. 17.1, 17.2

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

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

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)

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

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…

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

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

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 transformationHeat shock mix on ice then quickly heat up; sudden temp change opens membrane to DNA fragmentsElectroporation electric pulse opens up membrane to DNA fragments

Alternative Forms of Exchange2) 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

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

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

2 Basic Structures1) Helical capsid forms rod-

like spiral around nucleic acid

2) 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

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

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)

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

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.

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

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!

Homework• Suggested Homework:– Test Your Knowledge

• Actual Homework:– Discuss the Concept #1– Design the Experiment