Micro exam 4 Characterizing and Classifying Viruses, Viroids, and Prions

What causes most of the diseases that plague the world?

Viruses - responsible for cold, influenza, herpes, HIV, SARS, MERS, avian flu, ebola, Marburg

What is a virus?

a tiny, acellular infectious agents of DNA or RNA

means poison in Latin

Virus growth

do not grow independently

Virus reproduction

host cell replicates the virus

Virus responsiveness

reaction to host cells seen in some viruses

Virus metabolism

use host cells metabolism

Virus cellular structure

lack cytoplasmic membrane or cellular structure

What is a virion?

complete virus particle

Capsid

protein coat surrounding a virus

Nucleoplasmid

capsid with its enclosed genome

Enveloped virus

A virus enclosed within a phospholipid bilayer membrane derived from its host cell - additional around the nucleocapsid

Envelope proteins

some are from the virus, others are coded for by the host cell

What makes the envelope important to a virus?

it allows it to enter and invade a host cell. from there it can attack the immune system without immediate detection.

Naked virus

virus without an envelope, harder to kill

Genetic material of a virus

either DNA or RNA. viral families are typically listed by the type of nucleic acid: dsDNA, ssDNA, ssRNA, dsRNA

dsDNA, ssDNA

double stranded DNA

single stranded DNA

ssRNA, dsRNA

single stranded RNA --> +/-

double stranded RNA (unusual)

Host ranges

all organisms are susceptible to viruses. depends on capsid or envelope structure

Generalists

can infect many types of cells

Tissue Tropism

inside a host the virus may only infect certain tissues. due to affinity of viral surface proteins for complementary receptors on host cells

Virus size

diameter of 20nm-500nm. only the very largest can be seen with a light microscope

Helical capsid

helical symmetry

Icosahedral capsid

3D, 20 sided figure with 12 evenly spaced corners

Complex capsid

bacteriophages may have a protein coat composed of both helical and polyhedral capsids

Classifying Viruses

1. filterable vs unfilterable (old)

2. shapes

3. sizes

4. presence of an envelope

5. type of nucleic acid

Herpesviridae

HHV1 - coldsores, HHV2 - genital herpes

chicken pox, E-B, roseola

Papillomaviridae

genital warts, cervical cancer

HPV vaccine super important

Picornaviridae

polio, enterovirus D68 - AFM

Lytic Cycle

a viral reproductive cycle in which copies of a virus are made within a host cell, which then bursts open, releasing new viruses

attachment, entry, synthesis, assembly, release

Lytic Cycle - Attachment

virions nonmotile and connect with host cells randomly. dependent on chemical attraction and precise fit between tail fibers and host receptors

Lytic Cycle - Entry

phage releases lysozyme to break thru cell wall by dissolving it. tail sheath of virus contracts and forces a tube into bacterial cell. after entry, viral enzymes degrade the bacterial DNA

Lytic Cycle - Synthesis

bacterial chromosome degraded, so it stops making its own molecules and begins synthesizing new viruses directed by the viral genome

Lytic Cycle - Assembly

capsid proteins begin to assemble inside host cell. tails and tails fibers begin to assemble and attach to head. capsids form around viral genomes or the genomes are pumped into the newly formed capsid

Lytic Cycle - Release

lysozymes weaken cell wall and bacterium of new virions can burst free from the cell. for T4 bacteriophage the whole process takes about 25 min and up 200 new virions released

Burst time

how long it takes to complete the process

Burst size

how many new virions are released

Lysogenic Cycle

a viral reproductive cycle in which the viral DNA is added to the host cell's DNA and is copied along with the host cell's DNA

attachment, entry, prophage entry, lysogeny, induction, synthesis, assembly, release

Lysogenic Cycle - Attachment

same as in lytic cycle, but the host cells DNA is not destroyed

Lysogenic Cycle - Entry

viral DNA remains silent as a prophage

Lysogenic Cycle - Prophage Entry

prophage incorporates itself in the host cell's DNA

Lysogenic Cycle - Lysogeny

every time the bacterial chromosome replicates, the viral DNA is copied along with it. lysogenic phages can cause the phenotype of the bacterium to change from harmless into pathogenic

Lysogenic Cycle - Induction

at some point the prophage may be excised from the host DNA, then reenters the lytic phase

Inductive Agents

physical and chemical agents that damage bacterial DNA

Lysogenic Cycle - Synthesis, Assembly, Release

occur just as in lytic cycle

Replication of Animal Viruses

share same 5 stages as lytic bacteria. there are some changes because many animal viruses are enveloped

Replication of Animal Viruses - Attachment

animal viruses do not have tail fibers. instead use glycoprotein spikes to attach to host cells

Replication of Animal Viruses - Entry and Uncoating

three methods for viruses to enter animal cells

1. direct penetration

2. membrane fusion

3. endocytosis

Direct Penetration

done by some naked viruses. receptors on host cells match with spike proteins on the viruses

Membrane Fusion

phospholipid of the viral envelope fuses with host cell, allowing virus in

Endocytosis

when the virus trigger receptors on the cell surface to engulf the entire virion

Virus Synthesis

host cell manufactures viral nucleic acids and proteins

Synthesis - dsDNA viruses

similar to replication of normal cellular DNA and translation of proteins, replication usually happens in the nucleus

Synthesis - ssDNA viruses

animal cells do not use ssDNA - when an ssDNA virus enters a cell, the host will synthesize a complementary strand of DNA to viral genome. then replication and protein synthesis will proceed

Synthesis - +ssRNA

ribosomes of the host cell directly translate proteins using the codons of these types of viruses. essentially the +ssRNA viruses acts as a mRNA recognized by the cell.

EX: polioviruse

Synthesis - +ssRNA Retroviruses

do not use their genome as mRNA, instead use reverse transcriptase to create a new strand of cDNA the cell will use. the newly made DNA serves as a template to make more of the retrovirus and as the template for genome of the virus.

EX: HIV

Synthesis - -ssRNA

host ribosomes cannot translate a -RNA strand so these viruses carry RNA dependent RNA transcriptase in their capsids. the enzyme then creates +RNA strands which can serve as mRNA for creating more of the virus' genome and proteins.

EX: influenze

Synthesis - dsRNA

the +RNA strand can act directly as mRNA, the -RNA strand can be transcribed in +RNA and then translated.

EX: certain rotaviruses have this genome which causes gastroenteritis

Assembly and Release

-DNA viruses are assembled in the nucleus and released into the cytoplasm before release

-RNA viruses are assembled and released directly from the cytoplasm

What determines the number of viruses produced>?

the type of virus and the health of the host cell

Budding

many enveloped viruses incorporate phospholipid membranes from their host cells as they are released. often the host cells remain alive for some time during the process and do not lyse like the bacteriophages cause

Persistent Infection

a long, steady release of virions in budding fashion

Naked Viruses

are either expelled in vesicles via exocytosis or through cell lysis

Latent Viruses

some viruses can remain inactive in the host for many years before becoming active

Proviruses

viruses capable of latency - most times never becomes part of the host cell's DNA. if incorporated into the host, it is permanent and will be passed on to all descendants of that cell

What percentage of cancers are caused by viruses?

20-25%

Most common cancer caused by a virus?

cervical cancer caused by HPV

Protooncogenes

genes in a host cell involved in normal cell division

Two Hit Hypothesis

a virus inserts a promoter that converts a protooncogene into an oncogene. often this first hit doesn't cause cancer, but if a second hit damages the downstream repressor gene, then the oncogene disrupts cell division and causes cancer

Prions

infectious protein particles that do not have a genome

Spongiform Encephalopathies

diseases caused by prions - normal PrP protein's structure becomes altered and begins to affect other PrP proteins around it. as more of the altered PrP proteins aggregate it causes neurons to die and leave holes in the brain

Prion Disease Examples

BSE - Mad Cow Disease

CWE - in deer and elk

vCJD - in humans

How can humans become infected by prions?

ingestion of injected tissue, transplants of infected tissue or contact between infected tissues and mucous membranes. they are not removed through normal autoclaving or decontamination processes.