Microbial Models: Viruses and BacteriaAP Biology1/08/03

Viruses and Bacteria

Size comparison

Discovery of virusesScience as a process

Tobacco Mosaic Virus*Mayer 1883Disease is contagious; smallmaybe bacteria*Ivanowsky 1893Filtered and found pathogen tobe very small bacteria or toxin*Beirjerinck 1897Found it could reproduce only inthe host; couldn’t be cultivated;not inactivated by alcohol*Stanley 1935Crystalized the infectious agent

size demonstration

Basic structure of a virus

Some form ofnucleic acid(DNA or RNA)Enclosed in a protein coat. (capsid)

Viral envelopes-membranes that cloak their capsids. Often derived from hostcell membrane.

Bacterio-phages

Viral ‘life’ cycleViruses are obligate intracellular parasites

-they can reproduce only within a host cell

Therefore, viruses are basically packages of genetic material that move from host to host.

Host range: the potential hosts that a given virus can infect.-Host specificity like a ‘lock-and-key’ system-depend on proteins on the outside of the virus and

the receptors on the host cell

Something to ponder: should viruses be considered to be alive?

(…are computer viruses or chain letters alive?)

The basics of viral reproduction1) Entry into the host cell

-injection-membrane fusion

2) Replication and Translation of the genetic material-using the host cells genetic machinery

3) Assembly and release of the new viral particles-lysis of host cell-budding from the host cell

Symptoms from a viral infection:-Host response to the viral infection (immune response)-Prolific cell death-Proteins produced by viral genetic material (e.g. diptheria)-Cancer resulting from disruption of cell growth

control mechanisms (oncogenes)

A generalized viral reproduction cycle

Lytic and Lysogenic viral cycles: focusing on phages

Lytic cycle: reproductive cycle that results in the death of thehost cell as it breaks open (lyses), releasing the new viral particles.

-lysis may be brought on by the release of lysozyme,from the newly assemble viral particles,that weakens the bacterial cell wall.

Lysogenic cycle: replicates the viral genome withoutdestroying the host cell.

Prophage: viral DNA that is incorporated into thegenetic material of the host cell.

‘Virulent’ viruses utilize this reproductive cycle.

Temperate viruses utilize both modes of reproduction

Lytic

The lytic cycle

Protection for the bacterialcells:- receptor variation- restriction nucleases-lysogenic evolution ofviruses (?)

Lytic and Lysogenic

Protein represses most of the other phage genome

Environmental trigger

Lysogenic

Animal viruses

Most vertebrate viruses have tissue specificity- cold virus : upper respiratory tract- HIV : lymphocytes-WNV : brain tissue

Impact of the virus may depend on the type of tissue and the possibility of cell renewal

- cold virus destroys epithelium that can be repaired-poliovirus attacks nerve cells, and therefore thedamage is permanent.

Vaccines: Jenner and his faith in the milk maid-Stimulate the immune system to set up thedefenses against the actual pathogen.

Defenses:

Disruption of the genetic translation mechanisms- AZT for HIV

Reproductive cycle of animal virusesViral envelopes

Equipped with an outer membrane, outside of the capsid.- lipid bilayer and glycoproteins-often derived from the host cell

Helps the viral particle to enter the host cell and also helpsto disguise the viral particles to limit recognition bythe host immune system.

Genetic material: Animal viruses may contain DNA or RNAProvirus: DNA that is integrated into the host cells DNA

-HerpesRNA – broad variety of RNA genomes in animal viruses

-mRNA -retroviruses utilize reverse transcription (e.g. HIV)

RNA -> DNA -> Provirus -> RNA

Enveloped virus

reproductive cycle

HIV – enveloped

retrovirus

Retrovirus

Outbreaks: Emerging virusesThree main components to ‘new’ viruses appearing

1) Mutation of existing diseases-RNA viruses have high mutation rates; lack

proof-reading steps.-individuals may not have immunity to new strains

-flu virus

2) Spread from a different host species- hantavirus outbreak in 1993 spread from rodents

to humans-resulted from very high populations of rodents

3) Dissemination of a virus from an isolated population-international travel and tourism-AIDS

Bacteria

Short generation time facilitates evolutionary adaptation.

Binary fission-Asexual reproduction-under optimal conditions E. colican reproduce every 20 minutes

Spontaneous mutation rate: 1 * 107 per cell division,

therefore: approx. 2000 mutations per gene per day

Genetic recombination in bacteriaCombining theDNA from twoindividuals intothe genome ofa single individual.

After 24 hrs, the # of cells that cansynthesize bothArg and Trp excedes the rate of mutation….

Must be recombination.

Bacterial genomealteration

Transformation-alteration of bacterial

DNA by uptake of naked, foreign,DNA from the surroundingenvironment.

Transduction-DNA transfer via phages

*Generalized-random pieces of host DNA gets transfered

*Specialized-prophage exits chromosome and carried pieces of host DNA with it

PlasmidsSmall self-replicating DNA molecule

Can undergo reversible incorporation into the cell’sChromosome…plays an important role in recombination

Episome: genetic element that can replicate either as a plasmidor as part of the bacterial chromosome.

(lambda phage also an episome…similarities anddifferences between plasmids and episomes?)

F-plasmid R-plasmid‘fertility’ ‘resistance’

Conjugation and plasmids

Hfr can ‘mate’ with F- cell, sending DNA fragment. Crossing over can then occur between frament and the original DNA

Transposons-a piece of DNA that can move from one location toanother; moves genes into new areas (target sites)

Insertion sequence: simplest transposon-one gene that codes for transposase-transposase recognizes the inverted repeates and cuts

The DNA at that site, and at the target site.

Insertion sequence

Composite transposonsContain additional genes, such as those for antibiotic resistance

May help bacteria adapt to new environments of harsh conditionspackaging these genes on an R plasmid would be especially favorable.

(also found in eukaryotic cells…chap 19)

Control of gene expression

the lac operon model