Viruses: Bacteriophages Flashcards
Basic description of viruses
- Obligate Intracellular parasites
- Infectious agents, but not alive
- Inert outside of host cell: only because it is not growing
- Once inside host cell: viral genome hijacks host cell’s replication machinery, directs activities of cell
T/F If a dead virus is returned to a host body it will begin to grow again
False, nothing will happen, but if an inert virus finds another host it will continue to grow
What is the range of size for a virus and how is it compared to other microbes
The smallest of all microbes. Range from the smallest being ~10nm with 10 genes to the largest being ~500 nm
T/F you can observe a virus using a regular light microscope
False: can only view an electron microscope
What is a virion
A single viral particle
What does a virion contain
A nucleocapsid
Genetic information + a protective coat (capsid made up of proteins)
What kind of genetic information is found in a virion
DNA or RNA, never both
Naked Virus
- Nucleocapsid
- Lacks an envelope
- Not easily destroyed, more resistant to disinfectants
Enveloped Viruses
-Nucleocapsid + Envelope
Envelope= a lipid layer
-easily destroyed by disinfectants because the alcohol will interfere with the lipid layer more than the protein coat in naked viruses
Spikes
Allow the virus to attach to the specific host cells, essential to enter the host
considered glycoproteins, not always present
animal viruses=spikes
tail fibers=phage
Attachment of naked vs. enveloped virus
an envelope virus will always use spikes or envelope to attach
naked virus will use its capsid
Viral nucleic acid/genome (3 points)
- either DNA OR RNA (never both)
- Linear/circular/fragmented - fragmented is specific to viruses
- Double or single-stranded- DNA is not always double stranded and RNA is not always single stranded
Capsid
-A protein coat: composed of identical subunits called capsomers
How does the virus get its shape
By the arrangement of the capsomere subunits
Do viruses contain enzymes, ribosomes or have the ability to generate ATP?
NO- they don’t have ribosomes of their own, some completely lack enzymes while some have a few, and don’t have any mechanism for energy production
What is the range of hosts that viruses can affect? How do they choose what host to bind to?
There is a great range , they look for specific receptors on the surface of the cell that allows for the binding of the virus
Host specificity
viruses will exploit certain receptor molecules that are already present excessively within our cells, which is why specific infections happen in different parts of our body
T/F Viruses are easier to kill once they are intracellular
False, you need to catch a virus before it attaches to the host cell because they are very difficult to kill once they are intracellular
What are the three shapes of viruses and what is important about each
Icosahedral- specific number of sides
Helical- looks like a tunnel
Complex- contains a phage, uses tail fibers for attachment (spikes)
Naming viruses: family
end in suffix- viridae
Naming viruses: genus
ends in -virus
Ex: retovirus, Enterovirus
Naming viruses: species
name is often the name of the disease
ex: poliovirus, influenza virus
viruses are commonly referred to only by their species names
Bacteriophages: Productive Infections
- Virus goes into the host and actively multiplies
- Increases the number of virulent prodigy
Bacteriophages: Latent Infection
- Virus goes into the cell and remains dormant
- As a human cell multiplies, a latent cell will multiply its DNA or RNA but will NOT create more prodigies
- Can remain dormant as long as our immune system can surpass them
T/F Most viral infections are active
TRUE
What is a bacteriophage
A virus that only infects bacteria
What is the fate of the infected host if there is a productive infection
A: Release of virions, host cell lyses and dies
B: Release of virions, host cell does not die and continues to multiply with continuous release of virions
What are the three general types of bacteriophage life cycles based on their relationship with host
- Lytic phage (virulent)- Productive infection ending with lysis
- Temperate phages- (Latent/Lysogenic)- latent infection
- Filamentous phages (Extrusion)- productive, but host cell survives
Lytic Phage Infection- Basics
- Lytic/virulent phage exits the host
- Host cell is lysed
- It is considered productive infetion since new phage particles are formed
What phage is the model for lytic phage infections
T4 phage (dsDNA)
What is the 5 step process of the lytic phage lifecycle
- Attachment
- Genome entry
- Synthesis
- Assembly
- Release
Genome entry (3)
- Only the nucleic acid is injected into the cell, the rest of the phage is outside of the cell where it gets disintegrated
- T4 Phage carries enzyme; lysozyme which degrades a little bit of cell wall and allows the nucleic acids to enter
- Tail contracts and injects genome through cell wall and membrane
Attachment
- Phage exploits bacterial receptors on cell wall
- tail fibers are used
T/F phages can only attach to the cell wall of bacteria
False, some attach to the flagella or the pilli
Synthesis of protein and genome (step 3)
- Early proteins and late proteins are synthesized
What is the role of an early protein during the synthesis phase?
Translated within minutes
- They hijack the host & they take control of the host cell and manipulate it
- Proteins modifies host’s RNA polymerase (so that it does not recognize its own promoters and will now recognize the phage promoters)
What is the role of a late protein during the synthesis phase?
produced towards the end of the cycle, are the structural components to make the structure of the new phage (capsid and tail)
In what step does this occur? The phage genome is transcribed and phage proteins are produced, and the phage DNA is replicated while other virion components are made and the host DNA is degraded
Synthesis phase
T/F In the assembly phase, protein modifies the host RNA polymerase so that it will now recognize the phage promoters and not the host promoters
False, this occurs in the synthesis phase
Assembly (step 4)
also called Maturation
- phage components are assembled to form mature viruses
- some proteins randomly assemble, others require protein scaffolds
What are protein scaffolds
components of the new phage that assemble together through protein-protein interactions
Release (step 5)
- Lysozyme produced in late infection digests the cell wall
- cell will LYSE, releasing phage
What is burst size and what is typical for a T4 phage
T4 ~200
-it is the number of phage particles that are released from each infected host
Life cycle #2: Temperate Phages
Basics
Have the option of following two patterns: Lytic or Lysogenic Infection
What is the phage model for the temperate phage
Phage Lambda
What is the basic definition of a lysogenic infection
The virus remains latent in the host cell by incorporating phage DNA into the hosts DNA
(lysogenic always means temperate)
What are the steps of a lysogenic pathway (7 steps)
- attaches to the bacterium
- Injected linear phage DNA circularizes and enters lytic or lysogenic cycle
- Prophage is integrated into the bacterial chromosome
- Phage remains latent in the prophage stage
- Cell divides
- Prophage DNA is excised and carries an independent lifecycle (becoming active)
- Phage DNA enters lytic cycle at the synthesis step
What does the term prophage mean
Incorporation of phage DNA into host cell genome (lysogeny)
T/F an infected cell carrying a prophage is called a lysogen
True
Once the phage DNA comes out of the dormant stage, it is still considered a prophage
FALSE, once it is excised and becomes active it is no longer considered a prophage
Until what step is the prophage dormant in the temperate phage life cycle
Until after it is excised and no long a part of the bacterial chromosome
How does the lambda phage integrate the phage DNA into the bacterial chromosome
Through site specific recombination (remains latent)
How does the replication of phage DNA occur in the host genome
- Replicates with host chromosome
- The host cell is able to multiply unaffected by the harbored phage
What maintains the latent lysogenic state
A repressor protein that keeps the phage DNA in the latent stage and in the prophage form
How is the phage excised
by phage-encoded enzyme
What is the trigger for the phage to come out of latency
Damage to the host cell DNA will cause the cell to return to lytic infection
T/F DNA excised from chromosome only about once per 1,000 divisions of lysogen
False, 10,000
Why is the prophage DNA excised if DNA is damaged?
the repair system will turn on and activate a protease that will in turn destroy the repressor that is keeping the prophage latent
What does the term phage induction mean?
Causing the phage to become active again by allowing the phage to escape a damaged host
What is the difference between the lytic and lysogenic cycle in terms of host DNA
lytic: host DNA is degraded
lysogenic: Host DNA is saved and prophage DNA is incorporated within it.
What are two advantages of Temperate Phage Infections
- Partial Immunity
2. Lysogenic Conversion
Describe Partial Immunity
- Lysogen is immune to superinfection
- Infected bacterial cell becomes immune, has a higher chance of survival
What is superinfection
Infection by the same phage
Describe Lysogenic Conversion
- bacteria that harbor a phage actually gain new properties
- Change in phenotype of lysogen from prophage
- Ex; Aquire the ability to produce toxins
- Carry the ability to transfer DNA
Explain the importance of toxin production by viruses in lysogenic conversion
Bacterial phages carry the genes that are responsible for toxin production, will donate this DNA to bacteria then if these bacteria infect humans it will cause the disease
T/F Toxins can be produced even when bacteria are not infected by a phage
False, Bacteria only produce toxins when they are infected by a phage
Match with Medical Importance
- Corynebacterium diphthariae
- Clostridium botulinum
- Escherichia coli O157:H7
- Stretococcus pyogenes
- Salmonella enterica
- Vibrio cholerae
- Causes diphtheria
- Causes botulism
- Causes hemolytic uremic syndrome
- Causes Scarlet fever
- Causes Food poisoning
- Causes cholera
Life Cycle 3: Filamentous Phage
Basic
- ss-DNA phages that look like long filament
- Productive infection
- Host cells not killed by lysis, but grow more slowly
- Released through extrusion
What type of infection does the filamentous phage cause
Productive infection- multiplies b large numbers and increases its prodigy
What is the model phage for the Filamentous Phage
M13
How do filamentous phages attach?
On the F pili of E. Coli
T/F All bacteria can be infected through the filamentous phage
False, only bacteria that have a pili
What type of DNA is used in the filamentous phage
single stranded phage DNA, enters through a hollow tube
What is the phage replication method used
DNA polymerase of the host synthesizes complementary DNA strand for M13
One DNA strand used as a template for synthesis of mRNA
Another new strand used for copies of the genome
What occurs with the protein molecules from the original M13 (2 points)
- When phage DNA replicates, phage capsomeres are synthesized and embedded into the host cell membrane
- others form pores
Describe the Release mechanism of the filamentous phage
- Phage DNA is excreted through pores, coat proteins coat the DNA and form nucleocapsids
- Phage DNA attaches to the proteins that are incorporated into the membrane, which pushes against the membrane and is released through pores
- Mature phage DNA leaves surrounded by capsid proteins
True/False Bacteria cells lyse when filamentous phages are released
False, they exit through pores
