Micro Exam No. 3

What are viruses?

Acellular infectious agents

What are virologists?

Scientists that study viruses

What is virology?

The study of viruses

True or False: Viruses are a minor cause of disease.

False; major

True or False: Viruses are used as a new source of therapy

TrueT

True or False: New viruses are emerging

True

Viruses are important members of the __________ world because they move organic matter from particulate to dissolved.

Aquatic

Viruses are important in ________ because they transfer genes between bacteria.

Evolution

Viruses are important model systems in ________ _________

Molecular biology

What a virion?

A complete virus particle

What does a virion consist of?

One molecule of DNA or RNA enclosed in a coat of protein

True or False: Virions can reproduce independent of living cells and carry out cell division.

False; cannot, nor

True or False: Virions can exist extracellularly.

True

Do viruses have a simple or complex organization?

Simple organization

True or False: Viruses are obligate intracellular parasites

True

What are bacterial viruses called?

Bacteriophages

True or False: Archaeal viruses are very common

False; uncommon/rare

True or False: Most virions are prokaryotic viruses

False; eukaryotic (plants, animals, protists, and fungi)

What is the most important way to classify viruses?

By their genome strructure

When viruses are distinguished by their genome structure, what are the distinguishing factors?

DNA or RNA, single or double stranded, and circular or linear shape

What are the four ways that viruses can classified into families?

Genome structure, life cycle, morphology, and genetic relatedness

What are possible hosts for animal viruses?

Suitable animals, embryonated eggs, and tissue (cell) cultures

What type of tissue (cell) cultures can be hosts for animal viruses?

Monolayers of animal cells and plaques

What are plaques?

A localized area of cellular destruction and lysis

What are cytopathic effects?

Microscopic or macroscopic degenerative changes or abnormalities in host cells and tissues

How are hosts for bacteriophages usually cultivated?

In broth or agar cultures of suitable, young, actively growing bacteria

True or False: Broth cultures gain turbidity as viruses reproduce

False; lose turbidity

How are plaques observed?

On agar cultures

What do lack of plaques on an agar plate mean?

Confluent growth

What are the possible hosts for plant viruses?

Plant tissue cultures, plant protoplast cultures, and suitable whole plants

What might viruses in suitable whole plants cause?

Localized necrotic lesions or generalized symptoms of infection

What are virus assays used for?

To determine quantity of viruses in a sample

What are the two approaches of virus assays?

To count particles and to measure concentration of infectious units

What are direct particle counts?

Particle counts made with an electron microscope

What does the hemagglutanation assay measure?

Particle counts the determine the highest dilution of virus that causes red blood cells to clump together

What is an example of indirect particle counts?

Hemagglutination assay

How can one measure the concentration of infectious units?

By using plaque assays

What do plaque assays show?

Dilutions of virus preparation made and plated on lawn of host cells and the number of plaques counted

What are results of a plaque assay expressed as?

Plaque-forming units (PFU’s)

What is a plaque in a plaque assay?

A clear area due to lysis of cells

What is the virion size range?

10-400nm in diameter

What must be used to view most viruses?

An electron microscope

What do all virions contain?

A nucleocapsid

What is a nucleocapsid composed of?

Nucleic acid (DNA or RNA) and a protein coat (capsid)

What component do some (not all) viruses have in addition to a nucleocapsid?

Envelopes

What is the capsid?

The protein coat that surrounds and protects the genome and aids in transfer between host cells

What is the protomer/capsomer?

The protein subunits that make up the capsid

What are the mophological types of viruses?

Icosahedral, helical, envelope, and complex

What is an example of an icosahedral virus?

Polyomavirus

What is an example of an envelope virus?

Herpresvirus

What is an example of a helical virus?

Tubulovirus

What is an example of a complex virus?

T-even coliphage

What are helical capsids shaped like?

Hollow tubes with protein walls

What are icosahedral capsids shaped like?

A regular polyhedron with 20 equilateral triangular faces and 12 vertices

What are the capsomers of the icosahedral capsid structure shaped like?

Ring or knob-shaped units made of five or six protomers

What are pentamers (pentons)?

Five subunit capsomers in the icosahedral capsid structure

What are hexamers (hexons)?

Six subunit capsomers in the icosahedral capsid structure

What is the viral envelope?

The membrane structures surrounding some viruses with lipids and carbohydrates usually derived from host membranes

What are peplomers (spikes)?

Proteins within the viral envelope (virus specific)

True or False: Viral enzymes are observed within all viruses and are associated with or are within capsid

False; some

True or False: All viruses fit into the category of having helical or icosahedral capsids

False; some viruses do not

What are poxviruses?

The largest complex animal virus

What do large bacteriophages have?

Binal symmetry

Why are large bacteriophages complex?

They have icosahedral heads and helical tails

What are the tail fibers and sheaths used for in large bacteriophages?

Binding

What are the pins used for in large bacteriophages?

Injecting the genome

True or False: Nucleic acids in viral genomes can be either DNA or RNA

True

True or False: Viral DNA can be single OR double stranded

True

True or False: Viral RNA can be single OR double stranded

True

What are two different types of single stranded RNA in viral genomes?

Positive and negative

True or False: In the viral genome, transcription can turn RNA into DNA

True

True or False: In the viral genome, reverse transcription can turn DNA back into RNA

True

What are plus/positive single stranded RNA (ssRNA) viruses?

When the sequence of nucleotides in genomic RNA is the same as the sequence of nucleotides in viral mRNA

What are minus/negative single stranded RNA (ssRNA) viruses?

When the sequence of nucleotides in genomic RNA is complementary to viral mRNA

What are segmented genomes?

When virions contain more than one unique RNA

What is a segmented diploid?

When there are two copies of the segmented genome

What are the five steps of viral multiplication?

Adsorption, penetration, replication, maturation, and release

How long is the viral multiplication process?

From minutes to days long

What does the adsorption step of viral multiplication consist of?

Binding the virus to the cell surface

What does the penetration step of viral multiplication consist of?

The virus or viral genome enters the cell

What does the replication step of viral multiplication consist of?

The viral genome is copied and viral proteins are made using the host cell machinery

What does the maturation (assembly) step of viral multiplication consist of?

Newly made viral genomes and proteins are assembled into complete virus particles (virions)

What does the release step of viral multiplication consist of?

The new viruses leave the cell by bursting the cell (lysis) or budding off without immediately killing it

What is direct penetration?

When only the viral genetic material in injected into the cell

What is the lytic cycle?

The phage life cycle that culminates with host cell bursting, releasing virions

True or False: Bacteriophages are mostly single stranded DNA viruses

False; double stranded

What are virulent phages?

Phages that lyse their host during the reproductive cycle

What is the purpose of receptor sites during viral manipulation?

They are virus specific surface structures on the host cell where viruses attach for adsorption and penetration

What can make up receptor sites for viral manipulation?

Protein, lipopolysaccharides (LPS), and techoic acids (in gram + bacteria)

What are scaffolding proteins?

Proteins that aid in the construction of procapsids/complex viruses

What causes the release of phage particles?

Lysis of host brought about by several T4 proteins

What does the T4 protein endolysin do?

Attack peptidoglycan

What does the t$ protein holin do?

Produce lesions in the cell membrane

What is lysogeny?

A nonlytic (non deadly) relationship between a temperate bacteriophage and its host

What does lysogeny usually involve?

Integration of phage genome into host DNA

What is a prophage?

The integrated phage genome

What are lysogens/lysogenic bacteria?

Infected bacterial hosts

What are temperate phages?

Phages that are able to establish lysogeny

What is the first step of the lytic cycle?

Phage binds to bacterial cell

What is second step of the lytic cycle?

Phage injects its DNA into the cytoplasm