Inßuenza virus - Suffolk County Community · PDF fileViruses andfPrions (Chapter 13) Lecture Materials for Amy Warenda Czura, Ph.D. Suffolk CountytCommunityvCollege Eastern Campus

Viruses and Prions

(Chapter 13)

Lecture Materials

for

Amy Warenda Czura, Ph.D.

Suffolk County Community College

Eastern Campus

Primary Source for figures and content:

Tortora, G.J. Microbiology An Introduction 8th, 9th, 10th ed. San Francisco: Pearson

Benjamin Cummings, 2004, 2007, 2010.

Virus: Latin for “poison”

-discovered as “contagious fluid”

-obligate intracellular pathogen

-contains few enzymes of its own

-must get most enzymes and all biomolecule

building blocks and energy from host cell

Characteristics of a virus:

1. contains a single type of nucleic acid: either

DNA or RNA but not both

2. has a protein coat (capsid) surrounding the

nucleic acid, some also have a lipid

envelope around the capsid

3. multiply inside living cells by using the

synthesizing machinery of the host cell

4. cause the synthesis of specialized viral

structures that can transfer the viral nucleic

acid to other cells

5. have a specific host range

Host range = the spectrum of host cells types

the virus can infect

-viruses are usually specific to a single species

(or even strain) of host

-a virus has molecules on its surface that

specifically adhere to some molecule on

the host cell surface, each virus is

specialized to attach to and infect one type

of cell

-in multicellular hosts viruses usually infect

only certain specific cell types in that

species

e.g. HIV: human T helper cells

-host range is determined by the virus

requirements for attachment and entry into

the cell and the availability of of host

factors necessary for viral replication

Viral Size

-must be smaller than the cells they infect:

20-1,000nm in length

-smaller than bacteria (E. coli 1000 x 3000nm)

-too small to be seen by light microscopy

Amy Warenda Czura, Ph.D. 1 SCCC BIO244 Chapter 13 Lecture Notes

Viral Structure

Virion = infectious viral particle:

completely assembled with a protein coat

surrounding the nucleic acid

1. Nucleic Acid:

- either RNA or DNA, but not both

- single or double stranded

- linear or circular

- if RNA, it can be plus/sense strand (has

codons) or minus/antisense (need to make

complement sense strand for translation)

-2,000 to 250,000 nucleotides

(E. coli ! 4 million, human ! 3 billion)

2. Capsid = protein coat

-composed of subunits called capsomeres

-some capsids have protein-carbohydrate

pointed projections called pentons

-if pentons are present they are

used for attachment to the

host cell

3. Envelope (not all viruses)

-some viruses have an envelope around the

capsid consisting of lipids, proteins and

carbohydrates (cell membrane like)

-with envelope = enveloped virus

-if a virus does not have an envelope it is

called a non-enveloped virus

-the envelope may be coded for by the virus or

taken from the host cell plasma membrane

-some envelopes have

carbohydrate-protein

complexes called

spikes which are

used for attachment

to the host cell

interesting note:

-Coronavirus (cold) and influenza virus (flu)

have high mutation rate in spike genes

-by changing the spikes, they can evade the

host immune system

-you get infected by colds and the flu over and

over since each one with slightly different

spikes looks completely new to your

immune system

Morphology

The capsid architecture can be distinct and

sometimes identifies a particular virus

1. Helical

-cylindrical capsid

-made up of a helical

structure of capsomeres

with the nucleic acid

wound up inside

e.g. Rabies virus, Ebola virus

http://images.encarta.msn.com/xrefmedia/sharemed/targets/images/pho/

t045/T045376A.jpg


2. Polyhedral

-most are icosahedrons:

20 equilateral triangle

faces & 12 corners

-may have pentons

e.g. Adenovirus,

Polio virus

Poliovirus

3. Enveloped Viruses

-appear spherical due to

the lipid envelope, but

contain a shaped capsid:

Enveloped helical

e.g. influenza virus

Enveloped polyhedral

e.g. Herpes Simplex Virus

-may have spikes

http://www.bact.wisc.edu/themicrobialworld/hsv1struc.jpg

http://www.zephyr.dti.ne.jp/~john8tam/main/Library/inf

luenza_site/influenza_virus.jpg

Herpes simplex virus

Influenza virus

4. Complex Viruses

-unique shape

e.g. bacteriophage: capsid & accessory

structures

e.g. pox virus: no clear capsid, just several

protein layers around the nucleic acid

Taxonomy

-a viral “species” is a group of viruses sharing

the same genetic information and the same

ecological niche (host range)

-species names are not used; usually viruses

are just given a Genus name (ends in

‘virus’) and a common name

Viruses are grouped into families (names end

in ‘-viridae’) based on:

1) Nucleic acid type

2) Strategy for replication

3) Morphology

Viruses are more commonly identified by their

common name – learning the family

groups has little relevance to disease ID.

e.g.

Family: Herpesviridae

Genus: Simplexvirus

Herpes Simplex Virus 2 (HSV2)

(Also known as HHV-2: human

herpesvirus 2)


Chart of families – Table 13.2

Note all the various possibilities:

-Single stranded DNA, non-enveloped

-Double stranded DNA, non-enveloped

-Double stranded DNA, enveloped

-Single stranded RNA, plus strand,

non-enveloped

-Single stranded RNA, plus strand,

enveloped

-Single stranded RNA, minus strand,

enveloped

-Single stranded RNA, minus strand,

non-enveloped

-Multiple strand RNA, minus strand,

enveloped

-Double strand RNA, non-enveloped

-Double strand RNA, enveloped

Viruses are more commonly identified by their

common name – learning the family

groups has little relevance to disease ID.

enveloped

non-

enveloped

enveloped

Double

strand

RNA

enveloped

Cultivation of viruses for study:

-viruses must be grown in living cells, usually

their specific host

-viruses cannot be grown in culture media

alone (obligate intracellular parasites)

Three ways to grow animal viruses in lab:

1. Animal Models

-infect live animals with the virus

positive aspects:

-provides the potential to study the

complete infection and disease process

caused by the virus in a living animal and

the immune response to the infection which

allows for study of drug treatments and

preventative vaccines

negative aspects:

-only if the virus will infect an animal:

many human viruses only infect humans

thus no animal model exists


e.g. There are no exact models for HIV/AIDS:

it infects and causes disease only in

humans

HIV and AIDS can be studied only in

existing human patients

All drug treatments and vaccines also have

to be tested using humans

2. Embryonated Eggs

-fertilized egg, usually chicken, with a

growing embryo

-used as a "container" to grow virus for

vaccine production

or molecular

studies

positive aspects:

-most viruses

will grow

abundantly in

some part of

the egg

negative aspects:

-no ability to study infection or disease

processes or immune response or drug

therapies or vaccines: wrong cells & not

a live functioning animal

3. Cell Culture

-grow the virus in living cells in a Petri dish

-must culture the cells before growing the

virus in them

A. Primary cell lines :

-direct tissue sample grown in culture

positive aspects:

-use the cell type the virus normally

infects: can study normal infection

process

negative aspects:

-primary cells die off quickly: new cells

must be harvested from donors

-cannot study disease process or

immune response: no live animal

B. Continuous cell lines:

-immortalized, transformed cancer cells

that grow and divide forever

positive aspects:

-cells can be grown infinitely in lab: can

grown large quantities of both cells

and virus forever

negative aspects:

-cancer cells are not normal: infection

process being observed may not be

normal

-cannot study disease process or

immune reactions

no contact inhibition or monolayer, no death

Viral Identification

-viruses are so small they can only be seen by

electron microscopy

-some are so distinct they can be recognized

on sight

-others are identified by:

-symptoms of the disease

-cytopathic effects

-serological methods

(ELISA, Western blot)

-sequencing of DNA or RNA

(RFLP/DNA fingerprinting, PCR)

Viral Multiplication

-replication must occur in a host cell

-the viral genome codes for viral structural

components and a few viral enzymes

needed for processing the viral nucleic acid

-everything else is supplied by the host:

ribosomes, tRNA, nucleotides, amino

acids, energy, etc.


Bacteriophages

-viruses that infect a specific bacteria

-serves as a well studied example of a virus

life cycle (easy to grow in lab in bacteria)

Two possible types of infections cycle:

1. Lytic cycle

-ends with the lysis and death of the host

bacterial cell

2. Lysogenic cycle

-host cell remains alive, but carries the

virus in its genome

The Lytic Cycle

1. Attachment

-the phage contacts a bacterium and uses the

tail fibers to attach to proteins on the

bacterial cell wall

(all viruses have an attachment site on their

surface which binds to a receptor site on

their host cell

in this case the attachment site = tail fibers

receptor site = specific cell wall proteins)

2. Penetration/Entry

-the phage injects its DNA into the bacterium:

-the phage tail releases lysozyme to break

down the bacterial cell wall

-the sheath contracts to drive the tail core

through the weakened cell wall and

plasma membrane

-the DNA is injected into the bacterium

through the tail core

3. Biosynthesis

-synthesis of the viral nucleic acid and protein

occur:

-the virus degrades the host DNA and/or

disrupts host protein synthesis

-the virus then directs viral nucleic acid

replication and transcription and

translation of viral genes

-this results in a pool of viral genomes and

capsid parts

4. Maturation

-the bacteriophage DNA and capsid

spontaneously assemble into complete

virions

5. Release

-virions leave the bacteria:

-lysozyme encoded by viral genes

causes the cell wall to break down

-the bacteria lyses releasing the virions

Cycle then repeats with new phages

(Phage therapy: using bacteriophage to treat

bacterial infections - experimental)


The Lysogenic Cycle

-the lysogenic phage infects the cell, but

remains inactive in a stage called lysogeny

1. the phage attaches to the host cell and

injects its DNA

2. the phage genome circularizes

-at this point, the phage could begin a normal

lytic cycle or it can begin the lysogenic

cycle / lysogeny

3. the phage DNA integrates into the

bacterial chromosome

-the phage DNA is now called a prophage

-it synthesizes viral repressor proteins to keep

the rest of its genome inactive (suppress

virion production)

4. replication of the bacterial chromosome

replicates the prophage as well and thus all

progeny cells will also be infected with the

lysogenic virus

5. occasionally, the prophage will recombine

back out of the bacterial chromosome and

initiate a lytic cycle

Three Results of Lysogeny:

1. The host cell is “immune” to reinfection by

the same phage (but not different types)

2. Phage conversion: the host cell may

exhibit new properties due to the

integration of prophage DNA

e.g. cholera toxin (diarrhea) of Vibrio

cholerae is due to a phage gene

3. Makes specialized transduction possible:

-viral genome can

move one of the

host’s genes to a

new bacterium

when it goes

lytic from first

cell ! lysogenic

in second cell

e.g. gene for

galactose

metabolism

2. Penetration:

A. Non-enveloped viruses are endocytosed into a vesicle

B. Enveloped viruses enter by fusion

3. Uncoating: enzymes

degrade capsid releasing

nucleic acid

5. Maturation and Release:

capsule and nucleic acid

assemble into virion

A. Enveloped: buds out of

cell taking membrane, cell

survives

B. Non-enveloped: ruptures

out of cell membrane, cell

dies.

1. Attachment: virus

attachment sites bind

host receptor proteins

on plasma membrane 2A

2B

1

3

5A

5B

4A

Multiplication Of Animal Viruses

4B

4. Biosynthesis

A. DNA viruses: DNA

replicated in nucleus, protein

made in cytoplasm, virion

assembly in nucleus

B. RNA viruses: RNA and

protein made in cytoplasm,

virions assemble in cytoplasm

C. Retroviruses: reverse

transcribe dsRNA genome to

dsDNA, DNA integrated into

host genome as provirus,

provirus remains latent or is

expressed to create virions in the

cytoplasm

4C

A. Non-enveloped:

viral enzymes for

escape from vesicle

B. Enveloped: host

cytoplasm enzymes


Multiplication of Animal Viruses

1. Attachment

-virus binds to proteins or

glycoproteins on the

host cell plasma membrane

by its attachment sites:

-pentons or spikes or capsid

proteins: no tail fibers

2. Penetration / Entry

-non-enveloped viruses are endocytosed by

the host cell into a vesicle (endosome)

-enveloped viruses can enter by

fusion: the envelope and cell

membrane fuse and the

capsid is released into the

cytoplasm

3. Uncoating

uncoating = separation of the

viral nucleic acid from its

protein coat

-Non-enveloped: viral or host enzymes

digest the capsid, viral enzymes allow the

genetic material to escape the vesicle

-Enveloped: done by host enzymes in the host

cytoplasm (proteases)

4. Biosynthesis

A. Biosynthesis of DNA viruses

-viral DNA is replicated in the

host nucleus

-viral proteins are made in the cytoplasm

-viral proteins migrate to the

nucleus to join the DNA and

assemble into virions

-the virions are then transported

through the host endoplasmic reticulum for

release

B. Biosynthesis of RNA viruses

-both the viral RNA and proteins

are synthesized in the

cytoplasm

-virions assemble in the cytoplasm

C. Biosynthesis of Retroviruses

-retroviruses have a ds RNA genome and

make the enzyme reverse transcriptase

-they synthesize a ds DNA copy of their RNA

using reverse transcriptase and incorporate

the DNA into the host cell genome as a

provirus

-the provirus can then remain latent in the

genome (protected from host immune

system), or the genes can be expressed to

create virions (e.g. HIV: can remain latent

20+ years before it starts replicating)

5. Maturation and Release

-the viral capsid assembles spontaneously

around the viral nucleic acid

A. if a virus is enveloped,

viral envelope

proteins will be

deposited in the host

cell membrane and

the virion will bud

out of the host cell

taking an envelope

of membrane with it

-the host cell can survive if not too many

virions are released at once

B. if the virus is not enveloped it usually

ruptures out of the

membrane causing

lysis of the host cell


Latency vs. Active Infection

for integrating proviruses:

-some viral infections spend long periods as

latent infections and activate a replication

cycle only on rare occasions

e.g. Herpes

-many integrating viruses activate a persistent

/ chronic replication cycle and will

continue to produce virions until either the

host immune system neutralizes it or until

the virus kills the host

e.g. HIV

Viruses and Cancer

-infectious cancer was first observed in

chickens and mice (early 1900s)

-Chicken leukemia was passed to healthy

chickens by cell-free filtrate from sick

chickens

-Mouse mammary gland tumors were

transmitted from mother to offspring in milk

A viral cause of cancer in humans was hard to

recognize because:

1. most virions infect cells without inducing

cancer (cold, flu, mumps, measles, etc.)

2. cancer often develops long after the viral

infection (hard to link cause and effect)

3. most cancers are not contagious (prostate,

colon, breast, brain tumor, etc.)

Cell Transformation

normal cells ! tumor cells

-viruses that integrate into the host genome

(retroviruses and some DNA viruses)

change the genetic material of the host

which has the potential to cause cancer

-segments of the DNA where cancer causing

alterations occur are called oncogenes

-oncogene = some kind of cell regulatory or

growth control gene

-an oncogenic virus / oncovirus is capable of

inducing tumors in animals

-when the virus integrates, an oncogene is

expressed resulting in a transformed cell

-transformed cells grow uncontrolled: they do

not respect contact inhibition and lead to

tumor formation

-oncogenic viruses are either DNA viruses or

retroviruses that integrate into a

chromosome. They carry a whole

oncogene (some kind of regulatory or

growth gene derived from the animal host)

or carry just a promoter that turns on an

oncogene in the host cell

e.g. Human Papilloma Virus

Cervical cancer

Hepatitis B Virus

Liver cancer

(Oncolytic viruses: viruses that will naturally

infect and lyse tumor cells. Currently

being studied as possible cancer treatment)

Prions (Proteinaceous infectious particle)

-infectious protein PrPSc

-it is a protein molecule that is misfolded and

can cause misfolding of normal proteins

-results in spongiform encephalopathies

e.g. Mad Cow disease, Sheep scrapie

Creutzfeldt-Jacob Disease, Kuru

Gerstmann-Straussler-Scheinker

Fatal Familial Insomnia

-prion protein in the

brain converts

normal proteins

PrPC into prion

proteins PrPSc

-prion proteins cause

plaques and holes

in neural tissue

resulting in

progressive loss

of function and

eventual death


Documents

Inßuenza virus - Suffolk County Community · PDF fileViruses andfPrions (Chapter 13) Lecture Materials for Amy Warenda Czura, Ph.D. Suffolk CountytCommunityvCollege Eastern Campus