Virus and Prions

Viruses are

Acellular, extremely small, obligate trans-cellular pathogens

Bacteriophages

Viruses that infect bacteria

Animal viruses

Viruses that infect animals and humans

Virion’s are

Single, infectious virus particle

Virion structures include

Genetic material and exterior protective protein capsid

Capsids

Protein shell that packages and protects the genome

What are capsids made of?

Capsomere subunits

Helical capsids

Hollow like tube

Icosahedral capsids

3-dimensional polygons

Complex capsids

Any shape other than helical and icsahedral

Enveloped viruses

Lipid based envelope that surrounds the capsid

How do enveloped viruses arise?

Building off hosts

Non-enveloped (naked)

Viruses that lack an envelope

How do non-enveloped viruses arise

Lysing off the host cell

Animal viruses can be either enveloped of naked

True

Bacteriophages lose host cells and are always naked

True

Spikes

Protrude from the viral capsid or enveloped

Glycoproteins extensions

Helps viruses attach and gain entry to host cells

Influenza A spikes contain

Hemaglutinin and neuraminidase

Viral genome encode

Capsomeres proteins, enzymes need for replication, structural factors

Genomes can be

DNA or RNA, Single or Double stranded, single or segmented sections, circular or linear

Double stranded DNA

Viral DNA is transcribed using host RNA polymerases and mRNA is then translated into protein

Single Stranded RNA positive

ssRNA genome functions as an mRNA and is directly translated by host cell ribosomes

Single Stranded RNA negative

RNA genome is complementary to mRNA and is transcribed into mRNA by RNA dependent RNA polymerases

Single Stranded Retroviruses

A virus with an RNA genome that is converted into DNA by reverse transcriptase

What enzyme converts retroviral RNA into DNA?

Reverse transcriptase

What happens to retroviral DNA after it is made?

It’s usually inserted into the host’s DNA

What happens after retroviral DNA is inserted into the host DNA?

It’s transcribed into mRNA

How does a double-stranded RNA virus make mRNA?

It transcribes it’s RNA genome into mRNA

What enzyme do double-stranded RNA viruses require?

RNA dependent RNA polymerases

What are attenuated strains?

Viruses with genetic changes that limit infectivity

What are attenuated strains commonly used for?

Vaccines

How can beneficial mutations help viruses?

They help viruses escape the host immune system, broaden their host range, expand tropism, and increase infectivity

What is antigenic drift?

Minor changes in influenza’s HA and NA spikes caused by frequent mutations in its RNA genome

After an influenza infection, what does the immune system recognize?

The HA and NA spikes

Why are the HA and NA spikes important?

They act as antigens that stimulate the immune system to produce specific antibodies

What causes antigenic drift?

Frequent mutations in the influenza RNA genome

What is antigenic shift?

A major genetic reassortment in influenza viruses

What can antigenic shift lead to?

Increased infectivity, an expanded host range, and potentially a pandemic

Which influenza change is minor?

Antigenic drift

Which influenza shift is major?

Antigenic shift

Which influenza change is more likely to cause a pandemic?

Antigenic shift

How are viruses grouped?

Type of nucleic acid present, capsid symmetry, presence or absence of envelopes, and genome architecture

Host Range

Collection of species that a viruses can infect

Tropism

Dictates where an infection occurs

Broad tropism

The ability of a virus to infect a wide variety of cells or hosts

Narrow tropism

The ability of a virus to infect only one specific type of cell or host

Attachment phase of Bacterial Lytic Replication

Phage binds to bacterial cell

Penetration phase of Bacteriophage Lytic Replication

Phage injects its genome into the host; empty capsid remain outside the cell

Replication phase of Bacteriophage Lytic Replication

Protein synthesis makes phage parts and genome is replicated ; host cell DNA is broken down by bacteriophage DNAases

Assembly phase of Bacteriophage Lytic Replication

Genome packed into capsid and phage structures assembled

Release phase of Bacteriophage Lytic Replication

Bacterial cell lyses and new phages are released

Integration phase of Lysogenic Replication

Phage DNA integrates into host genome, forming a prophase

Cell Division phase of Lysogenic Replication

Host and phage genomes are copied before cell division

Lytic cycle phase of Lysogenic replication

Phage may enter lytic cycle if the host is stressed

Attachment phase of animal virus replication

Naked viruses attach to host cell membranes through capsid proteins

Penetration phase of animal virus replication

Enveloped viruses enter through endocytosis or membrane fission while naked viruses enter by endocytosis

Uncoating phase of animal virus replication

Capsid is digested by enzymes in the endocytic vesicle or cytoplasm or nucleus

Replication phase of animal virus replication

Genome is replicated and viral proteins are made

Assembly phase of animal virus replication

New virons are formed

Release phase of animal virus replication

Enveloped viruses are released by budding and naked viruses rupture the host cell during release

Acute infection

Virus infects host cell and new virions are made

Persistent infections

Virus have replication strategies that allows them to avoid immune system clearance which can be chronic or latent

Provirus

Integration of viral genome into the host cell

Latent infections

Distinguished by flareups with intermittent periods of dormancy

What happens during flareups?

Virions are shed

Oncogenic virus

Viruses that can lead to cancer

What is plaque assay?

A method used to measure bacteriophages by growing bacteria on a Petri dish and observing areas of cell lysis

How do bacteriophages create plaques?

They infect a cell, lyse it, then infect adjacent cells, creating a clear zone

What does a clear zone on a bacterial lawn indicate?

Lysed cells caused by viral infection

What are Plaque Forming Units?

The quantity of bacteriophages in an initial volume of a sample

What is a viral titar?

The quantity of virus present in a given volume of a sample

Why are animal viruses difficult to cultivate?

They require living host cells for growth

How are animal viruses commonly grown?

Using tissue culture techniques

why are embryonated eggs used in virology

They are useful for growing viruses

What does specificity mean in a diagnostic test?

The test detects only the virus of interest, minimizes false positives

What does sensitivity mean in a diagnostic test?

The test can detect very low levels of a target, minimizing false negatives

What role do purified antibodies play in viral diagnostics?

They bind to viral antigens

How are agglutination tests performed?

Purified antibodies attached to tiny latex beads are mixed in with a sample

What happens if viral antigen is present in an agglutination test?

Antibodies bind the antigen and the beads agglutinate

What indicates a positive agglutination test?

Visible clumping of beads

How do latex agglutination tests detect antibodies?

Viral antigens attached to latex beads are mixed with a sample

What occurs when patient antibodies are present

They bind the viral antigens, causing bead agglutination

What does ELISA stand for

Enzyme-Linked Immunosorbent Assay

What can ELISA detect?

Either antigens or antibodies in a sample

What happens first in an ELISA test?

The target adheres to a surfaces

What indicates binding in an ELISA test?

A color change

What types of sample is required for ELISA and agglutination tests?

A liquid sample

Why must antigens be well characterized

To ensure accurate detection

How can antigenic shift affect testing?

It may make a virus no longer detectable

Why can antibody-based tests miss early infections?

Detectable antibodies take time to develop

Why is it helpful to use multiple detection methods

It improves diagnostic accuracy

What is detected in genetic testing of viruses?

Viral nucleic acids

How does genetic detection compare to many traditional methods?

It is more sensitive and rapid

What is the 1st step in detecting viral genetic material?

Collect a clinical sample

What happens after the sample is collected?

DNA and RNA are extracted

What methods can detect specific viral nucleic acid segments

Fluorescent labeled probes, sequencing, and PCR

What does PCR stand for?

Polymerase Chain Reaction

What is postexposure prophylaxis

A prepared mixture of injectable antibodies used shortly after suspected exposure

How does postexposure prophylaxis work?

It prevents viruses from binding to and entering host cells