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Microbial Models: Viruses and Bacteria AP Biology 1/08/03 . Discovery of viruses Science as a process Tobacco Mosaic Virus *Mayer 1883 Disease is contagious; small maybe bacteria *Ivanowsky 1893 Filtered and found pathogen to be very small bacteria or toxin *Beirjerinck 1897

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Viruses and bacteria

Discovery of viruses

Science as a process

Tobacco Mosaic Virus

*Mayer 1883

Disease is contagious; small

maybe bacteria

*Ivanowsky 1893

Filtered and found pathogen to

be very small bacteria or toxin

*Beirjerinck 1897

Found it could reproduce only in

the host; couldn’t be cultivated;

not inactivated by alcohol

*Stanley 1935

Crystalized the infectious agent

Viruses and Bacteria

Size comparison

size demonstration

Basic structure of a virus



Basic structure of a virus

Some form of

nucleic acid

(DNA or RNA)

Enclosed in a protein coat. (capsid)

Viral envelopes

-membranes that cloak their capsids. Often derived from host

cell membrane.

Viral life cycle
Viral ‘life’ cycle

Viruses 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.

Something to ponder: should viruses be

considered to be alive?

(…are computer viruses or chain letters alive?)

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

The basics of viral reproduction
The basics of viral reproduction

1) Entry into the host cell


-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
A generalized viral reproduction cycle

Lytic and lysogenic viral cycles focusing on phages
Lytic and Lysogenic viral cycles: focusing on phages

Lytic cycle: reproductive cycle that results in the death of the

host 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.

‘Virulent’ viruses utilize this reproductive cycle.

Lysogenic cycle: replicates the viral genome without

destroying the host cell.

Prophage: viral DNA that is incorporated into the

genetic material of the host cell.

Temperate viruses utilize both modes of reproduction

The lytic cycle
The lytic cycle

  • Protection for the bacterial

  • cells:

  • receptor variation

  • restriction nucleases

  • lysogenic evolution of

  • viruses (?)

Lytic and lysogenic
Lytic and Lysogenic



Protein represses

most of the other

phage genome

Animal viruses

Most vertebrate viruses have tissue specificity

- cold virus : upper respiratory tract

- HIV : lymphocytes

-WNV : brain tissue

Animal viruses

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 the

damage is permanent.


Vaccines: Jenner and his faith in the milk maid

-Stimulate the immune system to set up the

defenses against the actual pathogen.

Disruption of the genetic translation mechanisms

- AZT for HIV

Reproductive cycle of animal viruses
Reproductive cycle of animal viruses

Viral 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 helps

to disguise the viral particles to limit recognition by

the host immune system.

Genetic material: Animal viruses may contain DNA or RNA

Provirus: DNA that is integrated into the host cells DNA


RNA – broad variety of RNA genomes in animal viruses


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

RNA -> DNA -> Provirus -> RNA

Enveloped virus reproductive cycle
Enveloped virus reproductive cycle

Hiv enveloped retrovirus
HIV – enveloped retrovirus

Outbreaks emerging viruses
Outbreaks: Emerging viruses

Three 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



Short generation time facilitates evolutionary


Binary fission

-Asexual reproduction

-under optimal conditions E. coli

can reproduce every 20 minutes

Spontaneous mutation rate: 1 * 107 per cell division,

therefore: approx. 2000 mutations per gene per day

Genetic recombination in bacteria
Genetic recombination in bacteria

Combining the

DNA from two

individuals into

the genome of

a single individual.

After 24 hrs, the #

of cells that can

synthesize both

Arg and Trp

excedes the rate

of mutation….

Must be recombination.

Bacterial genome alteration
Bacterial genomealteration


-alteration of bacterial

DNA by uptake of naked, foreign,

DNA from the surrounding



-DNA transfer via phages


-random pieces of host DNA gets transfered


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


Small self-replicating DNA molecule

Can undergo reversible incorporation into the cell’s

Chromosome…plays an important role in recombination

Episome: genetic element that can replicate either as a plasmid

or as part of the bacterial chromosome.

(lambda phage also an episome…similarities and

differences between plasmids and episomes?)

F-plasmid R-plasmid

‘fertility’ ‘resistance’

Conjugation and plasmids
Conjugation and plasmids

Hfr can ‘mate’ with F- cell, sending DNA fragment.

Crossing over can then occur between frament and the original DNA


-a piece of DNA that can move from one location to

another; 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
Insertion sequence

Composite transposons
Composite transposons

Contain additional genes, such as those for antibiotic resistance

May help bacteria adapt to new environments of harsh conditions

packaging these genes on an R plasmid would be especially


(also found in eukaryotic cells…chap 19)

Control of gene expression the lac operon model
Control of gene expressionthe lac operon model