Microbiology Chapter 13 EXAM

True or false: you can have single DNA, double DNA, single RNA, and double RNA. Multiple in one virus.

FALSE - you can have any of the 4 listed but not more than one within a virus.

Where does the viral envelope come from?

The envelope comes from the host’s membrane. Enveloped viruses are more weak in their environment (soap) but stronger within the body.

How are viruses classified?

• type of nucleic acid - single or double DNA/RNA

• presence of an envelope - naked vs envelope around capsid

• shape - viruses form different shapes → helical, polyhedral, complex

• size - can vary

What committee is responsible for classification?

The international committee on taxonomy of viruses (ICTV) is responsible for classification, naming, & recognizing the family / genus names.

True or false: viruses are acellular

TRUE

Explain why viruses are considered “obligate intracellular parasites”

They can never live on their own outside of a host body. They must be inside of a host cell to grow and replicate.

Draw a typical animal virus and label the components. Explain the function of each component.

• Viral genome (nucleic acid core): DNA or RNA (single or double)

• Helical capsid: protein coat surrounding the nucleic acid that protects it & aids in the delivery of host cells

• Envelope: phospholipid bilayer membrane surrounding the outside of the capsid; derived from host membrane

• Glycoproteins: spike-like structures embedded in the viral envelope, crucial for attachment to the host cell

What is a capsomere and where are they found?

Capsomere = protein subunits (single or multiple types of proteins) that make up the capsid of a virus

Function: protection around genome, attachment of virus to host, structural integrity

What is a complete virus particle called?

Virion → includes a nucleic acid, capsid, and sometimes an envelope

When would a virus be considered “naked”?

Naked viruses are very resistant in the environment/outside the body. Die once in the body.

Naked virus = lacks an envelope and only consists of a nucleic acid core & capsid

• released through exocytosis (when host cell dies)

Briefly describe the variations in viral shapes and sizes.

• Helical = consists of a ribbon-like protein that forms a spiral around the nucleic acid (rod-shaped)

• Polyhedral = many-sided, one of the most common is the icosahedron; smaller and round

• Complex = have an elaborate capsid, varied shapes; bacteriophage

What are the parts of a bacteriophage?

The head, tail, base plate, and tail fibers

What type of hosts can be infected by viruses?

They can infect any living hosts (humans, animals, bacteria, protozoa, archaea, fungi).

• viruses are highly specific, targeting particular host cells due to affinity between viral attachment & host cell surface proteins

• generalist viruses → can infect multiple hosts

• diverse host range

• viral interactions → larger viruses can be infected by smaller viruses

True or false: the envelope of a virus is more susceptible to detergents and drying

TRUE

What is the size of a virus?

They can range from 20 nm to 900 nm

• must use an electron microscope to view them

Which family is coronavirus from?

Coronaviridae

Herpesvirus

• creeping

• can be latent for years

• ex: herpes, chickenpox

Poxvirus

• smallpox killed more than 500 million

• smallpox first virus eradicated → 1980

Picornaviruses

• very small

• naked

• polyhedral RNA viruses

Retroviruses

• contain the enzyme reverse transcriptase

• HIV

Paramyxovirus

• near mucus membranes

Rhabdoviruses

• rabies can infect mammals

Filoviruses

• direct contact → blood, semen, needles

• Ebola

True or false: the glycoprotein spike acts as a lock and key

TRUE

Describe the types of RNA viruses and some of the differences between them and DNA viruses, Name a few examples of each type.

• + sense = acts like mRNA; - sense = template for mRNA

• positive-sense RNA = can serve as mRNA for protein synthesis (coronavirus)

• negative-sense RNA = template; requires transcription into positive-sense RNA by RNA polymerase before protein synthesis (lyssavirus → rabies + influenza)

• double-stranded RNA = each strand serves as a template for its complement

Differences = RNA mutates faster due to no proofreading & DNA has more stable genomes

Bacteriophage replication lytic cycle

Lytic replication = usually results in death & lysis of host cell

Always Play By My Rules

1. Attachment = virus attaches to host cell surface

2. Penetration = entry of virions into host cell (at least the genome)

3. Biosynthesis = making new nucleic acids, capsid proteins, and other viral components

4. Maturation = assembly of newly synthesized viral components into complete virions

5. Release = departure of newly made virions by killing their host cells

What type of phages undergo the lytic cycle?

virulent phages

Viral growth curve

• stage 1: infection (inoculation phase)

• stage 2: eclipse phase → virus binds & penetrates the host cell

• stage 3: burst → virions are released from lysed host cells at same time

What is the number of virions released per bacterium called?

the BURST SIZE

Lysogenic cycle

1. Attachment & Entry = similar to the lytic cycle

2. Prophage formation = viral DNA integrates into the host’s chromosome, becoming a prophage

3. Replication = prophage is replicated with the host’s DNA during cell division

4. Induction = prophage may exit the host chromosome, entering the lytic cycle

(in binary fission each daughter cell will receive a phage of DNA)

What type of phages undergo the lysogenic cycle?

temperate phages

• called a prophage during its inactive state

When viral DNA is inserted into a bacterial chromosome, it is called a __________, which is part of the ____________ cycle.

Propahage (inactive state) = integrates into host DNA & replicates along the chromosome

Lysogenic

What is an advantage of the lysogenic cycle over the lytic cycle?

The lytic cycle kills the host cell

The lysogenic cycle incorporates into the host, allowing the virus to persist without causing as much harm.

• prophage integrates into chromosome, allowing viral DNA to be replicated with the host’s DNA

• prophage can remain dormant in geome for many generations without killing the host

• lysogenic conversion = prophage can alter phenotype, turning it into a pathogen that produces other toxins

The period of time required to complete the lytic cycle is called…

the burst time

What is the connection between viruses and cancer?

1. proto-oncogenes are kept in check by repressors

2. viral insertion → 1st hit the virus may insert a promoter next to a proto-oncogene turning it into an oncogene

3. second hit & cancer development → disrupts repressor & allows cancer formation

What environmental factors are linked to cancer?

UV, radiation, carcinogens, viruses

What percentage of cancers do viruses cause?

20-25% of cancers

What are prions? What are some human diseases caused by prions?

Prions = infectious protein particles that lack nucleic acids. They replicate by inducing normal proteins, a-helices made by mammals known as cellular PrP → misfolded b-pleated sheets (prion PrP) which cause disease.

• Ex: spongiform encephalopathies → variant creutz-feldt-jacob disease & chronic wasting disease

What are viriods?

Extremely small, circular pieces of ssRNA without a capsid that cause disease in plants.

• does not code for proteins; adheres to complementary plant RNA

• plant enzyme degrades dsRNA → disease state

How is the T4 virus able to penetrate E. Coli’s cell wall?

It releases lysozyme, which weakens the cell wall.

If one virus particle infects a cell, approximately how many infectious virus particles are present during the synthesis step of the virus replication cycle?

0

Which is not a way that viruses can cause human cancer?

viruses can cause abnormalities in the host cytoplasmic membranes

What is accepted argument for why viruses should be considered living?

they possess genomes with instructions for replicating themselves

What is a difference between phage T4 and lambda phage?

Phage T4 uses lytic replication; lambda follows lysogenic replication

Virus

miniscule, acellular, infectious agent composed of nucleic acid surrounded by a protein coat (capsid).

• can be extracellular or intracellular (virus lacks a capsid & exists solely as nucleic acid)

Virion

complete, extracellular form of a virus

• includes nucleic acid, capsid, sometimes an envelope

• the virion is the infectious form of a virus outside of a host cell

Fungal virus

• exists only within cells

• no extracellular state

• spread through membrane fusion

Plant viruses

• enter through abrasions or via plant parasites

• agricultural damage

Bacterial viruses

• highly specific

• very numerous

Sars-CoV-2

• enveloped virus with a + ssRNA

• helical capsid with envelope + spike proteins

• enters through respiratory droplets

Viruses evolve quickly because..

• genetic material (ssRNA mutation)

• using host cell to replicate leads to mutation

• selective immune system pressure (new strains)

Bacteriophage vs animal virus replication

• attachment = bacteriophages attach with tail fibers; animal viruses attach using glycoproteins

• entry = bacteriophages only inject their nucleic acid; entire virion of animal virus enters cell → uncoating

• replication environment = bacteriophages replicate in prokaryotes; animal viruses in eukaryotic cells (no cell wall)

• release = bacteriophages kill host cell to release; animal viruses kill host cell or undergo budding

Neoplasia

uncontrolled cellular division in animals

Malignant tumors

cancer

Metastasis

tumor spreads

What are the 3 mechanisms of entry of animal viruses?

1. direct penetration → naked viruses inject genome into host cell

2. membrane fusion → enveloped viruses dump capsid into cell

3. endocytosis → can be naked or enveloped virus (engulf)