Chapter 13 Flashcards
What did Dimitri Iwanowsky and Martinus Beijerinck do
Found a mosaic disease of tobacco plants that was caused by an unusual agent.
What was unusual about the agent found by Iwanowsky and Beijerinck?
The agent was too small to be seen with a light microscope
It passed through filters that retained most known bacteria
It could be grown only in media that contained living cells.
What did Beijerinck called these unusual agents
Filterable virus
In simplest terms, what are viruses
Viruses can be viewed as genetic information either DNA or RNA contained within a protective protein coat.
What are viruses incapable of
They are inert particles incapable of metabolism, replication or motility on their own.
What happens when a viral genome enters a cell
When a viral genome enters a host cell, it can hijack that cell’s replication machinery, inducing the cell to produce more viral particles.
Viruses can be grouped into two general types
- Some infect eukaryotes
- Other infect prokaryotes
Bacteriophages
Viruses that infect bacteria
What makes viruses difficult to study
They are obligate intracellular parasites
What can viruses only be visualize under
An electron microscope
What do viruses require
Viruses require live organisms as hosts.
What is a reason bacteriophages are important
Because they serve as a vehicle for horizontal gene transfer in bacteria.
They kill bacteria thereby reducing bacteria populations in nature
Phage therapy
The use of bacteriophages to treat bacterial infections is explored as being an alternative to antibiotics
At a minimum, what do virions consist of
A virion consists of nucleic acid surrounded by a protein coat.
Viruses only contain a single type of nucleic acid either RNA or DNA, never both.
What are the genomes of a virus like
The genome may be linear or circular and either single stranded or double stranded.
Capsid
Protein coat of virus
Protects the viral nucleic acid from enzymes and toxic chemicals in the environment
What is the capsid composed of
It is composed of precisely arranged identical protein subunits called capsomeres.
Nucleocapsid
Viral nucleic acid and its protein coat.
What specific protein components do phages have that allow the virion to attach to specific receptor sites on host cells
Phages have tail fibers for attachment
What specific protein components do animal viruses have that allow the virion to attach to specific receptor sites on host cells
Many animal viruses have protein structures called spikes on their surface.
Spike
Or attachment proteins
Structures on the outside of the virion that bind to host cell receptors.
Envelope of virus
Some viruses have an outer lipid bilayer called an envelope
What is another name for non-enveloped viruses
Naked viruses
Non-enveloped viruses
Type of viruses that does not have a lipid envelope.
Enveloped viruses
Viruses that have a lipid bilayer surrounding their nucleocapsid
They also have a viral matrix protein that links the envelope to the nucleocapsid.
Where do enveloped viruses obtain their outer layer from
The host cell
Why are enveloped viruses more susceptible to soaps, detergents and disinfectants
Because these chemicals damage the envelope, thereby removing the spikes and making the viruses no infectious.
Phages are typically what
Typically non enveloped
What are the three shapes viruses can be
- Icosahedral
- Helical
- Complex
Icosahedral viruses
Appear spherical when viewed with an electron microscope, but there surface is actually 20 flat triangles arranged in a manner somewhat similar to that of a soccer ball.
Helical viruses
Appear cylindrical when viewed wi5 an electron microscope; their capsomeres are arranged in a helix, somewhat similar to a spiral staircase.
Some are short and rigid while others are long and filamentous
Names of virus families are derived from a variety of sources, but they all end in suffix
Viridae
Arboviruses
Arthropod borne RNA virus, carried by vectors such as mosquitos.
The general strategies for phage replication result in two possible outcomes
- Productive infection
- Latent state
Productive infection
Viral infection in which more viral particles are produced
Latent state
The state of a phage when it’s DNA is integrated into the genome of the host.
In which the viral genome remains silent within the cell but is replicated along with the host cell genome.
Another name for lytic phages
Virulent phages
Lytic phage
Bacteriophages that lyst their host
How do lyric phages exit the host
Lytic phages exit the host at the end of the infection by lysing the cell, an action that kills the cell.
Example of lytic phage
T4
T4
A double stranded DNA phage that infects E.coli
Steps of T4 replication cycle.
- Attachment
- Genome entry
- Synthesis of phage proteins and genome
- Assembly (maturation)
- Release
Attachment step
Phage collides with host cell by chance
On contact, the phage attaches to a receptor on the host cell surface or an appendage like pilus.
Genome entry
The phage injects it’s genome into the cell.
T4 does this by degrading a small part of the bacterial cell wall, using an enzyme located in the tip of its tail.
The tail then contracts so that the phage particle appears to squat on the cell surface.
This action injects the phage dna through the hosts cell wall and cytoplasmic membrane, and into the cell.
- Synthesis of phage proteins and genome
Within minutes after the phage dna is injected into a host cell, that cell’s machinery transcribes and translates some of the phage genes.
The genes are expressed in a specific sequence to control the course of infection.
First the proteins needed to start phage replication are made; one of these is a nuclease that degrades the hosts cells DNA.
The phage genome is then synthesized. Towards the end of the replication cycle, phage structural proteins are made, including those that make up the capsid and the tail.
Assembly (maturation)
Assembles the various phage components to form new phage particles.
After phage head is formed, dna is packed into it; the tail is then attached, followed by the addition of the tail spikes.
Release
Late in infection, the phage encoded lysozyme is produced.
This enzyme digests the host cell host cell wall from within, causing the cell to lyse thereby releasing phage. These particles then infect any susceptible cells in the environment.
Burst size
Number of newly formed virus particles released when the infected host cell lyses
Temperate phage
A bacteriophage that can either direct a productive infection that leads to host cell lysis or remain silent within the host as a prophage (phage dna) that replicates along with the host cell genome.
Most common example of temperate phages
Lambda a double stranded dna phage that infects E.coli.
What does lambda look like outside the cell
It has a linear chromosome, but once that genome
What does lambda look like inside the cell
Once that genome is injected into a cell, the ends join together to form a circular dna molecule.
What are the two options that can occur to temperate phages once they are injected into the cell
- Lytic infection
- Lysogenic infection
Lytic infection
Viral infection of a host cell with a subsequent production of more viral particles and lysis of the cell.
Lysogenic infection
A phage encoded enzyme called an integrase inserts the phage dna into the host cell chromosome at a specific site. The prophage replicates along with the host chromosome prior to cell division, so it will be passed on to that cells progeny.
Prophage
Latent form of a temperate phage; the phage DNA has generally been inserted into the host chromosome.
Lysogen
A bacterium that carries phage DNA ( a prophage) integrated into its genome.
Why are most prophage genes silent
Most prophage genes are silent because a phage encoded repressor protein prevents their expression.
A prophage can remain integrated in this latent state indefinitely, except when
A prophage can remain integrated in this latent state indefinitely, but it can also be excised from the host chromosome by a phage encoded enzyme. When this happens, a lytic infection begins.
What determine which type of infection occurs initially
Which type of infection occurs initially lytic or lysogenic is mostly random, but the metabolic state of the host cell has an effect.
What is essential for maintaining the lysogenic (latent) state
The repair prevents expression of the gene required for excision and is therefore essential for maintaining the lysogenic (latent) state.
