Biomedical Sciences: Virus Characteristics (Lecture 6; Exam 2)

What is a virus?

An infectious obligate intraceullular parasite comprising genetic material (DNA or RNA) surrounded by a protein coat and/or a membrane.

• Capable of directing its own reproduction and spread by “taking over” the machinery inside of the host cell it invades.

Plant Viruses (Examples)

• Tobacco Mosaic Virus

• Potato Virus Y

• Rice Dwarf Virus

Amoeba Virus (Examples)

• Mimivirus

• Pithovirus

Human Virus (Examples)

• HIV
  Rabies Virus

• Variola Virus

• HPV-16

Can viruses live on their own?

No.

Viruses lack the complex energy-generating and biosynthetic systems necessary for independent existence.

Genetic Material in the Virions

Is either DNA or RNA, but NOT both.

Progeny Virion

Assembled during the infectious cycle; is the vehicle for transmission of the viral genome to the next host cell or organism.

• (Its disassembly initiates the next infectious cycle)

How does the viral genome direct synthesis of viral components?

By cellular systems within an appropriate host cell.

What organelles do viruses have?

None. Viruses lack organelles.

Viral Genomes

All viral genomes are obligate molecular parasites that only function after they replicate in a cell.

• Do not independently fulfill the characteristics of life.

• Cannot generate their own energy.

• Cannot make their own proteins.

Are viruses cells?

No.

Viruses replicate; they do NOT grow.

Viruses and mRNA

All viruses must make mRNA that can be translated by host ribosomes.

Virus Replication: Bacteriophage

1. Attachment

2. Genome entry

3. Synthesis

4. Assembly

5. Release

What components do viruses have?

Have a naked capsid or an envelope morphology.

Viruses exist in two phases. What are they?

1. Metabolically inert

2. Infected cell

Viral Self Assembly

Viral components must self-assemble. They must assemble using pre-formed components.

Are viruses filterable agents?

Yes.

What is the genetic material in virions?

Either DNA or RNA, but NOT BOTH.

How do viruses direct the synthesis of viral components?

By cellular systems within an appropriate host cell.

What is the vehicle for transmission of the viral genome?

A progeny virion assembled during the infectious cycle.

All viral genomes are'“

Obligate, molecular parasites that can only function after they replicate in a cell.

• Do NOT independently fulfill the characteristics of life.

• Cannot generate their own energy,

• Cannot make their own proteins.

Can viruses replicate?

NO. They do NOT grow.

Are viruses cells?

NO.

What must viruses make that can be translated by host ribosomes?

mRNA.

Viral components are _____. They do not replicate by division.

Assembled.

Viral Taxonomy: Family

Classified by genome nucleic acid, presence or lack of envelope, and shape of caspid.

• Always ends in -viridae. (ex. Adenoviridae)

Family Taxonomy:

Always end in -virus. (ex. Mastadenovirus)

• Many genera may exist in a single family.

• Single genus may contain many viral “types” or strains which may not differ significantly.

Viruses that cause human disease are sometimes referred to in “groups”.

What are examples of viruses referred to in groups?

• Herpesviruses

• Paramyxoviruses

• Poxviruses

Informal Virus Classification: Genomic Nucleic Acid

• DNA Viruses

• RNA Viruses

Genomic Nucleic Acid: DNA Viruses

• dsDNA

• ssDNA

• Partially dsDNA

Genomic Nucleic Acid: RNA Viruses

• dsRNA

• ssRNA (+)

• ssRNA (-)

• ssRNA (+) via DNA

What is the outer coating made up for enveloped viruses?

Primarily lipids (and proteins).

What is the primary coating for non-enveloped viruses?

Protein.

Types of Caspids:

• Icosahedral

• Helical

Parts of a Virus:

• Virion

• Genome

• Capsid

• Nucleocapsid: (Caspid + Genome)

• Envelope (on some; derived from host plasma membrane)

• Surface (Envelope or Capsid); contains glycoprotein spikes.

What is the Baltimore Classification?

Based on pathways by which families of viruses produce mRNA.

• Group I: dsDNA

• Group II: ssDNA

• Group III: dsRNA

• Group: IV: ssRNA (+)

• Group V: ssRNA (-)

• Group VI: retrovirues

• Group VII: hepadnaviruses

Which are stronger, non-enveloped or enveloped virsuses?

Non-enveloped due to their protein capsid.

Which viruses are usually more susceptible to heat, desiccation, detergents and pH?

Enveloped viruses.

How are enveloped viruses released from the cell?

Budding. (non-lytic)

How do enveloped viruses penetrate the cell?

Membrane fusion.

Examples of enveloped viruses?

• HIV

• Herpesviruses

• Influenza

What is used for attachment in enveloped virsues?

Spikes; Viral attachment proteins (VAPS)

What surrounds Non-enveloped viruses?

Capsid.

What do non-enveloped viruses use for attachment?

Glycoprotein spikes.

Through what process are non-enveloped viruses released from a cell?

Lysis. (Cell popping)

Examples of Non-envelope viruses:

• Rhinovirus

• Norovirus

• Papillomaviruses

What is tropism?

Target cells that a virus is able to infect.

What is a permissive cell?

A host cell that allows a virus to enter, replicate its genetic material, and produce new viral particles.

What is a semi-permissive cell?

May support SOME but not all viral replication steps leading to ineffecient replication.

What are non-permissive cells?

Do NOT allow the replication of a particular strain of virus.

Why do non-permissive cells not allow for replication?

• Lack a receptor, necessary enzyme pathway or transcriptional activator.

• Express an antiviral mechanism that inhibits replication.

Five basic steps of viral replication:

1. Attachment

2. Penetration and uncoating

3. Synthesis of viral proteins and replication of the genome

4. Assembly

5. Release

Viral Attachment protein:

A protein or glycoprotein that attaches to cellular receptor.

• Target of neutralizing antibodies!

How do viruses initiate entry?

Receptor;

Initiates entry by membrane fusion or receptor-mediated endocytosis.

How do enveloped viruses enter host cells?

Through fusion OR endocytosis.

How do non-enveloped viruses enter host cells?

Endocytosis only.

How are enveloped viruses released from a cell?

• Budding

• Cell lysis

• Exocytosis

How are non-enveloped viruses released from the cell?

• Cell Lysis

• Exocytosis

Does NOT use budding.

An entire virus particle is called ____.

A virion.

Parts of a virus:

• Virion

• Genome

• Capsid

• Nucleocapsid = Capsid + Genome

• Envelope (only on enveloped viruses)

Where is the envelope derived from?

From the host plasma membrane.

What does the surface of viruses contain, both enveloped and non-enveloped?

Glycoprotein spikes.

Why are glycoprotein spikes important?

For attachment.

All viruses must produce ____ in order to produce proteins and replicate.

mRNA.