Chapter 13 Micro

Chapter 13 viruses,viroids, prions

• Viruses: – are acellular, obligate intracellular parasites– require a host cell to multiply – Not assigned to any kingdom

• Virion is another name for a complete virus particle

• Viral structure– Small size = 20 – 1000 nm, need EM to see,

go thru filters that retain bacteria

Viral structure pg 389

• Core – contains nucleic acids – DNA or RNA (contains genetic info) but not both

• Capsid – outer protein coat that protects the core and maintains shape– Composed of identical protein subunits =

capsomeres

• Envelop – membrane around capsid = enveloped virus– Not all viruses have an envelop = nonenveloped or

naked virus

Viru

s A

rchi

tect

ure

Minimally, a virus is a

proteinaceous carrier of nucleic

acid.

Many viruses are more

complicated than that, such as

having a lipid envelope

surrounding the protein

capsid.

Viral structure

• Envelop continued– May have spikes to aid in attachment to host

Partially derived from host cell plasma membrane during viral replication

– cell

• Shapes – Polyhedral = many sided

• Icosahedral – has 20 triangular faces– Enveloped – herpes simplex– Nonenveloped – Adenovirus, poliovirus

Viral structure

• Shapes continued– Helical

• Nonenveloped = rabies, Ebola• Enveloped = influenza

– Complex • Bacteriophages with polyhedral head and a

helical tail• Pox viruses with several coats around the nucleic

acids but no clear capsid pix pg 391

Not-Complex Virions

Complex (Tailed) Phage VirionNote that this head actually is elongated

top to bottom rather than isometric.

Host range = range of host cells in which a virus can multiply

• Animal viruses – contain either DNA or RNA– Chickenpox, smallpox – humans– Rabies - dogs

• Plant viruses – RNA– Tobacco mosaic– Usually polyhedral or helical– Can produce both internal and external effects and

have an economic impact on agriculture– Some plant diseases are caused by viroids = short

pieces of RNA

Bacteriophages = bacterial viruses

• Some have tail-like fibers thru wh/ they inject their nucleic acid into the bacterial cell

• Multiplication of bacteriophages– All viruses carry only the genetic info needed

for replication of their nucleic acids and synthesis of their protein coats

– They require energy production and ribosomes from the host cell

Multiplication or bacteriophages

• Phages can cause lytic (virulent) infections or lysogenic (temperate) infections in E. coli– Lytic infections – caused by T-even phages

cause lysis and death of the host cell– Lysogenic infection – caused by phage

lambda don’t cause death or lysis of host cell• Phage can spontaneously become lytic

Lytic cycle of T-even phage

• Adsorption – Tail fibers adheres to specific sites on

bacterial cell wall

• Penetration– Enzymes released from the phage tail

dissolve a hole in cell wall– Phage DNA is injected into cell thru hollow tail– Capsid remains outside the bacterial cell – no

uncoating needed

Lytic cycle of T-even phage

• Biosynthesis– Viral DNA core directs synthesis of viral parts

by the bacteria– Maturation = phage DNA and capsids which

were formed separately are assembled into virions (individual virus particles)

– Release of virus causes lysis of bacterial cell

Viru

lent

Pha

ge L

ife C

ycle

Lysogenic cycle – phage lambda

• Phage lambda attaches to (adsorption) and penetrates into bacterial cell but are not replicated and released immediately

• Phage DNA forms a circle – Can now go thru lytic or lysogenic cycle

• Prophage = phage DNA incorporated into bacterial DNA and remains latent or inactive

Lysogenic cycle – phage lambda

• Bacteria divides producing more cells with viral DNA

• Rare – prophage can leave the hosts DNA and initiate the lytic cycle– Phage may take some of the hosts DNA w/ it and

infect a new cell taking with it new genes– The new host cell may exhibit some properties of the

old host cell– Ex. Toxin production – Corynebacterium diphtheria

can only produce disease when it carries a lysogenic phage because the phage carries the gene coding for the toxin

Tem

pera

te P

hage

Life

Cyc

le(s

)

Replication of animal viruses

• Attachment or adsorption– Virus attaches to host cell plasma membrane– Specific attachment – viral spikes

complementary to host cell attachment sites

• Penetration– Virus enters host cell by:

• Pinocytosis • Fusion – viral envelop fuses with plasma

membrane and releases capsid into the host cell’s cytoplasm

Replication of animal viruses

• Uncoating – Viral capsid dissolved by host enzymes

releasing viral nucleic acids

• Manufacture of viral parts – Under the direction of the viral genome but carried out using the host’s machinery, proteins, energy

Replication of animal viruses

• Biosynthesis of DNA virus– Viral DNA is synthesized in host’s cell nucleus– Capsid proteins are synthesized in cell’s

cytoplasm– Capsid protein enter the nucleus and combine

with DNA– New viruses new viruses bud thru cell

membrane and the cell membrane becomes part of the virus envelop

Replication of animal viruses

• Biosynthesis of RNA virus – multiplication takes place in the cytoplasm

• Assembly (maturation) virus parts are assemble to produce mature virions

• Release – mature virus leaves host cell– Lysis of host cell if nonenveloped (naked)

virus– Budding out if enveloped virus pg 410

Virus culture methods

• Animal viruses are cultured by 3 methodsliving animals, embryonated eggs, cell cultures

• Living animals– Some viruses can only be cultured in living animals –

mice, rabbits, guinea pigs– Studies of immune system response to viral infections

– animals are observed for signs of disease or killed so the tissue can be examined for the virus

– Human AIDS virus can’t be grown in animals so vaccines are tested on humans but it takes years to determine the effectiveness

Virus culture methods

• In embryonated eggs– Convenient and inexpensive, once was the

most widely used method– Viral growth = death of embryo, embryo cell

damage, lesions to egg membranes– Harvest egg contents for viable viruses– If allergic to eggs need to know if vaccine was

prepared in eggs

Virus culture methods

• In cell cultures – preferred growth for virus– When viruses are grown in labs, use continuous

diploid cell lines– Cells are trypsinized (to break apart), suspended in a

growth media, growth is in a monolayer of cells on the flask after incubation

– Virus cause cell deterioration = cytopathic effect (CPE)

– Problem with cell cultures is microbial contamination

Virus culture methods

• Bacteriophages – can be grown in bacterial suspensions of liquid media or agar

• Plaque method uses solid media for detection and counting of viruses– Melted agar containing bacteriophage and host

bacterial cell is poured into a Petri dish that contains a hardened layer of agar growth media

– Each virus infects a bacterium, multiplies, and releases hundreds of new viruses wh/ destroy the surrounding bacteria and produce clearings or plaques

• Each plaque represents one virus, reported as PFU

Viruses and cancer

• 10% of cancer is caused by viruses– Viruses that cause tumors in animals = oncogenic

viruses or oncoviruses– An oncogene is a cancer causing gene

• Tumor cells undergo transformation – now have properties different from non tumor forming cells– After being transformed by viruses, most tm cells

contain a virus specific antigen on their cell surface = tumor specific transplantation antigen (TSTA) or an antigen in their nucleus = T antigen

Viruses and cancer

• Tm cells undergo transformation cont.– Transformed cells – less round than normal

cells, have chromosomal abnormalities = unusual # of chromosomes or fragmented chromosomes

• Oncogenic viruses– DNA viruses

• Human papillomavirus – cervical cancer• Hepatitis B causes liver cancer

Viruses and cancer

• Oncogenic viruses cont.– RNA viruses: only viruses from the family Retroviridae

cause cancer• T-cell leukemia and lymphoma

• Prions – pure protein molecules that can catalyze the conversion of normal proteins into more prions– No nucleic acids– Infection develops slowly and is usually fatal

• Ex. Mad cow disease

Prions

• Prions cont.– 9 animal diseases that are neurological

diseases called spongiform encephalopathies – large vacuoles form in the brain ex. Mad cow disease

– Human disease are – kuru, Creutzfeldt - Jacob disease (CJD)

• Both diseases progressively destroy ms coordination and brain fx

– CJD has been transmitted w/ transplanted nerve tissue and contaminated surgical instruments

Viral infections

• Persistent or chronic viral infection = occurs gradually over a long period of time. Usually fatal

• Latent viral infections = virus stays in the host for long periods of time without causing disease– Stress or other causes can trigger re-

appearance of disease• Ex. Herpes simplex –cold sores• Chickenpox virus - shingles