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Infectious agents Eukaryotic- parasites, protists, fungi Prokaryotes Nonliving- viruses, viroids, prions Viruses apparen PowerPoint Presentation
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Infectious agents Eukaryotic- parasites, protists, fungi Prokaryotes Nonliving- viruses, viroids, prions Viruses apparently infect every living thing (bacterio)phages- bacteria plant and animal viruses. All viruses have a protein capsid Capsid shape is characteristic of the virus

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Presentation Transcript
slide1

Infectious agents

Eukaryotic- parasites, protists, fungi

Prokaryotes

Nonliving- viruses, viroids, prions

Viruses apparently infect every living thing

(bacterio)phages- bacteria

plant and animal viruses

slide2

All viruses have a protein capsid

Capsid shape is characteristic of the virus

Viruses are naked

Or enveloped

Much smaller

than cells

slide4

General features of viruses, continued

All have nucleic acid (DNA or RNA, but not

both)

genome is very small

gene sequences sometimes overlap

Must have genes to:

make the capsid

replicate itself (special polymerases)

get into and out of the cell

Viruses use the host replication processes

slide5

What do viruses do to cells?

Some viruses kill the host cell (lytic; virulent)

Some viruses are released from living cells

(not lytic, but still productive)

Some viruses become integrated into the DNA

of the host

(temperate: latent infection. Infection

is lysogenic)

slide8

Bacteriophage are classified according to

host range

attachment site

genome (DNA or RNA)

shape

effect on host cell

slide9

Temperate phages can either lyse their host

or replicate with it

Bacteriophage lambda () is especially well

analyzed

Why lytic? Why lysogenic?

Lytic if bacteria are actively dividing; otherwise

lysogenic

How are viruses maintained this way?

slide10

Virus becomes prophage (integrated into

host genome)

 has gene sequences homologous to sequences

in E. coli, and integrates at that site

Is excised in a reverse process

In lysogen, some viral genes are repressed

(prevented from being expressed)

slide11

How does phage become lytic again?

loss or inactivation of repressor

damage to bacterium

Bacteria that already are lysogenic are

immune to infection by the same type of

phage

slide13

Transduction- phage-mediated genetic

transfer

Generalized- any bacterial gene can be

transferred

Specialized- virus inserted in a specific site

so only genes adjacent to it are

transferred (i.e., temperate phages)

slide14

Why can phages infect some bacterial strains

but not others?

Phage must attach to specific receptors on host

cell

Restriction-modification- prevents the “right”

phage DNA from being degraded

slide15

Infectious agents of animals and plants

Analogous biologically to bacteriophages

Taxonomic grouping according to:

mode of replication (genome structure)

shape of capsid

presence of envelope

Both RNA and DNA genomes; wide variety

of arrangement of nucleic acid

slide17

Some other grouping may be more useful,

e.g., mode of transmission

slide18

Growing and studying viruses

Tissue culture

propagation

cytopathic effect

Quantitation

plaque assay

titration

hemagglutination

antibody titer

slide19

adenovirus

Herpes simplex

slide20

Viruses may be pathogenic or have no apparent

effect

May cause acute or persistent infections

slide22

Many viruses have genomes that cannot

be transcribed by host enzymes

They must encode their own

How are viruses released?

from dead cells (not actually lysed by virus)

budding

shed from host

In persistent infections, viruses are continuously

produced by budding

Infected person is a carrier

slide24

Retroviruses are analogous to bacteriophage

such as 

Can be integrated (randomly) into host genome

Reverse transcriptase

Retroviruses (and other, DNA viruses) can cause

tumors in susceptible animals

slide25

Viruses and tumors

When viral genome gets inserted into a host

genome, it can cause changes

Activation of host genes

Mutation of host genes

If these genes regulate the cell cycle, this

can lead to tumors (e.g., proto-oncogenes)

Papilloma, herpes, Epstein-Barr

slide26

Retroviruses and cell transformation

Viruses are integrated into host DNA and later

excised

May incorporate regulatory gene

Virus polymerase is error-prone; may mutate

gene

This gene may then be transferred to a new

host

slide27

Host range: the type of organism (or tissue)

that a virus can infect

Some can infect many species (zoonoses)

Some can acquire that wide range through

modifications

Phenotypic mixing

Mutation

slide29

Probably how

influenza virus

changes so

rapidly

slide30

Viruses infect plants, too

Wounds

Contaminated soil

Vectors

Spread through plasmodesmata in cell walls

Viruses can cause a variety of pathologies

in plants

slide32

Other, novel infectious agents

Prions- infectious proteins

Viroids- naked, small RNA molecule

replicate autonomously

single viroid can infect cell

resistant to proteases and nucleases

only infect plants (as fas as we know)

slide34

Prions are species-specific

Some apparently can cross species, e.g.,

mad cow disease