Microbiology Exam 1 Topic 5 Review

What makes a virus host specific?

Determined mainly by whether a virus can attach to a cell It needs special receptor sites on the surface of host cells that match up with specific attachment structures on the envelope or capsid. It is also determined by whether appropriate host enzymes and other proteins the virus needs to replicate are available in the host cell Finally, it depends on whether the replicated virus can be released by the cell to spread infection to other cells.

How do you calculate pfu/ml?

Each plaque represents one phage and by counting plaques and multiplying by the dilution factor the initial stock concentration can be determined

Three basic shapes of a capsid:

Helical capsids are composed of capsomeres that bond together in a spiral fashion to form a tube around the nucleic acid. Polyhedral capsids are roughly spherical with many triangular faces. Complex capsid is the term used to describe the rest of viral shapes that do not readily fit into the helical or polyhedral categories.

What is A bacteriophage?

A bacteriophage is a virus that infects, replicates and usually destroys bacterial cells. It is significant because it can potentially be used as an alternative to antibiotics.

An animal virus does not have the means to inject its genetic information inside the host cell. Rather the entire virus gets inside.

True

If the prion protein gets into a mammalian system (human, cow, sheep) then it can travel to the brain and induce normal brain proteins to fold into the prion form. Prion disease leads to slow neurological degeneration and eventual death

True

Synthesis is simply the making of new viral components - genetic information and proteins. For DNA viruses, it is pretty straightforward because the host has all the machinery it needs (DNA polymerase, RNA polymerase, and ribosomes.) For a RNA virus, it is much more difficult because the host cell does not have all the machinery it needs. It has to supplement the host cell machinery either with an ______ that it carries with it or in the case of a ________ sense RNA virus, it could direct the host cell to make an ______ that it needs. Two examples of these enzymes are an RNA dependent ___ __________which is able to make RNA from a RNA template and _______ _____________ which is able to make DNA from an RNA template.

enzyme Positive enzyme RNA polymerase Reverse Transcriptase

With an animal virus, you need to consider if it is naked or enveloped. These two viral types will do the Adsorption step a bit differently. A _____ virus will attach via the capsomeres that make up the capsid while an _________ virus will attach via the spikes on the _______.

naked enveloped envelope

Is a virus a cell?

no

Examples of extensive and narrow host range:

polioviruses only cause infection in humans (narrow range) rabies has been shown to affect the central nervous system of many warm blooded animals (extensive range) Therefore the host range of the rabies virus is more extensive than that of the poliovirus

What is the purpose of the capsid?

protects nucleic acids from degradation can play a key role in the attachment of some viruses give shape to the virion

Define Nucleocapsid:

refers to the viral genome together with the capsid the capsid of a virus with the enclosed nucleic acid

Some DNA phages, called temperate phages, undergo adsorption and penetration but don't _________. The viral genome inserts into bacterial genome and becomes an inactive ________ and so the cell is not lysed. The prophage is retained and copied during normal cell division resulting in the transfer of temperate phage genome to all host cell progeny - lysogeny. _________ can occur resulting in activation of lysogenic prophage followed by _____ ___________ and cell lysis.

replicate prophage Induction viral replication

Define Enveloped viruses:

composed of a nucleocapsid and an envelope

Define a "naked" or "non-enveloped" virus:

composed of a nucleocapsid and no envelope

Release is getting the new virus out of the host cell. If it is a _____ virus, it will lyse the host cell. If it is an _________ virus, it will push against the cell membrane of the host cell and bud out, taking part of the membrane with it and become enveloped.

naked enveloped

What two things do all viruses have?

1. An outer protein coating called a capsid made up of capsomeres 2. a Genome or Nucleic Acid Core which is made up of DNA or RNA (both can be either single-stranded or double-stranded)

Name the five general stages of the replication cycle in viruses:

1.attachment [AKA adsorption]of the virion to the host; 2.entry of the virion or it's genome into the host; 3.synthesis of new nucleic acids and viral proteins by the host cells enzymes and ribosomes; 4. assembly or maturation of the new virions inside the host cell; 5. release of the new virions from the host.

Negative sense RNA:

3' to 5' Negative (-) sense RNA does not make sense to the host cell ribosomes and they cannot make a functional protein from it. So it must bring along its own enzyme. The RNA polymerase it inserted is able to make +RNA from the -RNA. This means it can now use that +RNA as mRNA and the host ribosomes can make protein from it. It can also use the +RNA as a template to make more -RNA. Now it has viral proteins and viral RNA which you can put together to make a full virus.

Positive sense RNA:

5' to 3' Positive (+) sense RNA "makes sense" to the host ribosomes. This means that the ribosomes can read the codons and make functional proteins from it.

temperate virus definition:

A Temperate virus is not destructive initially. It integrates its genetic info into the host cell in what is called the lysogenic cycle and is replicated when the host cell replicates. They can become destructive and initiate a lytic cycle via the process of induction

Define virulent virus:

A Virulent virus is destructive. It replicates in large numbers and destroys the bacteria they infect using the Lytic cycle.

Define lysogenic:

A process in which a virus incorporates its genetic material into the genome of its host. This allows the virus to lie dormant within the host until it enters the lytic stage, in which the virus reproduces.

Some viruses contain an envelop; where did the envelop come from?

An envelope is essentially stolen from the host cell. Some viruses mature inside the host cell and bud (push against the inside of the membrane) to get out. In the process, they take that membrane with them and become enveloped or covered by it.

The abundance of viral nucleic acid, enzymes, and other proteins have been synthesized resulting in ________ of components into complete virions.

Assembly or maturation

What can trigger induction?

Can happen spontaneously Or in response to external stimuli [Provirus can sense that living conditions are no longer optimal and exits to find a new home]

animal viruses have to do an extra step that bacteriophage did not have to do. They need to free their genetic information from the protein ______.

Coat or capsid

___ animal viruses are able to use the host cell machinery (enzymes, nucleotides, amino acids, ribosomes, ATP) ___ animal viruses are not able to use all parts of the host cell machinery

DNA RNA

Compare and contrast DNA virus to RNA virus:

DNA viruses are pretty straight forward. They infect, insert their genetic info into the host cell, and the host cell knows what to do with it. It has all the necessary enzymes and proteins to replicate the genetic info and make proteins to create new viruses. RNA viruses are a bit more tricky. At most, the host cell can make proteins from the RNA (if it is positive sense) and sometimes can't even do that (if it is negative sense.) The host cannot copy the genetic information and make more RNA from RNA. An RNA virus must therefore provide the host with an RNA-dependent RNA polymerase (an enzyme that can make RNA from an RNA template) or it must provide the host with the directions to make it (via a positive sense RNA template.)

Disadvantages??

Environmental conditions that destroy membranes will also hurt enveloped viruses High temp, freeze, thaw, high or low pH, detergents, chemical disinfectants such as chlorine, hydrogen peroxide, and alcohol Naked viruses are more resistant to being killed by these conditions

When a virion contains enzymes, what is the use of the enzymes and where in the virion are they found?

Enzymes are found inside the capsid of a virus They are needed to supplement the host cell's replication machinery if the virus has RNA as its genetic material

Define a "host range":

Host range of a virus describes the "who?" Who does a virus infect?

_________ is when the prophage becomes active and initiates a traditional lytic cycle

Induction

What is the viruses envelope made up of?

It is made up of a mixture of normal host plasma membrane materials (phospholipids and proteins) as well as special viral proteins (spikes or glycoproteins) that the virus directs the host cell to produce

What is a prion?

Prion = Proteinaceous infectious particle

An enveloped virus attaches via ______ to the membrane ________ and then entering the cell via membrane ______. Because the plasma membrane on the new host cell is the same as the plasma membrane on the old host cell, the two like membranes will fuse.

Spikes Proteins Fusion

What is the viruses advantage of being enveloped?

The envelope is taken from the host and therefore it camouflages the uniquely viral proteins of the capsid and helps the virus evade detection by the host immune system. Helps to fuse with other new like cell membranes to perpetuate the infection

Define prophage:

The latent form of a bacteriophage in which the viral genes are incorporated into the bacterial chromosomes without causing disruption of the bacterial cell.

what is the purpose of a plaque assay?

The plaque assay is a virological assay developed to count and measure infectivity of bacteriophages. Later, it was applied to count mammalian viruses as well. Is the most widely used technique for virus isolation and purification, and to determine viral titers.

How do phages attach to the bacterial cell?

They do not have any means of motility, and need to bump into their host cell in the correct orientation (tail fibers down.) Phage can attach to all parts of the bacterial cell. Sometimes they attach to the flagella or pili since these are hollow structures and they can insert their DNA and it will eventually make it to the cytoplasm.

How are viruses classified?

Type of nucleic acid [do they have DNA or RNA? are they single or double stranded? Positive sense or negative sense?] Presence of an envelope Shape Size

Define viral specificity:

Viral specificity of a virus describes the "what?" Once on or inside of a host, what cell types can a virus infect?

What is another name for a virus?

Virion

Is a virus alive?

Viruses are considered non-living; however..... When they invade susceptible host cells, viruses display some properties of living organisms and so appear to be on border between living and nonliving

Can you kill a virus?

Viruses can only reproduce using the mechanisms of other cells that they invade, so if you kill the infected cell, then that "kills" the virus that is now inside it and all the viruses that the cell would have made. You basically inactivate viruses by preventing their replication

How do you perform a plaque assay?

You start by making serial dilutions of your sample and adding them to an agar plate with a susceptible bacterial host to grow them on. You incubate them and the phage will infect a bacterial cell. Once they have completed the replication cycle and released, they'll infect another bacterial cell. This creates holes in the bacterial lawn where you can see the agar. We call these holes plaques.

Define provirus:

a form of a virus that is integrated into the genetic material of a host cell and by replicating with it can be transmitted from one cell generation to the next without causing lysis

What 2 things might viruses contain?

an Envelope and/or Enzymes

define parasite:

an organism that lives in or on another organism (its host) and benefits by deriving nutrients at the host's expense

How do the stages of replication differ in bacteriophages vs animal viruses?

bacteriophage- lysogenic=prophage; animal virus-latent=provirus. Also, when the bacteriophage enters a bacterium, only the DNA/RNA is released into the cell (capsid remains outside). However, when viruses enter animal cells, the capsid goes into the cell creating the extra step of uncoating.

Definition of obligate:

by necessity; must have a certain condition

Why are viruses considered Obligate intracellular parasites?

cannot reproduce outside their host cell, therefore, reproduction is entirely reliant on intracellular resources

Most naked viruses enter cell by ___________ in which virions are captured by pitlike regions on cell surface

endocytosis

What are spikes on the enveloped virus made up of?

glycoproteins that extend from the viral envelope and may aid in attachment to the host cell

Examples of viral specificity:

papillomaviruses (warts) infect only skin cells cytomegaloviruses can attack many organs including the liver, lung, digestive tract, etc. Some viruses can also cross the placenta and attack fetal tissue especially that of the central nervous system If a virus has a broad viral specificity it can attack many tissue types causing many symptoms and making it hard to diagnose