Viruses Flashcards
A virus is an obligate intracellular parasite meaning
Only able to reproduce within the cell
Outside cell; has no activity
Therefore is not a cell or living organism; has no ability to do anything on its own
Structure of all viruses
Nucleic acid genome packed in a protein shell
Once in a cell, the virus
Virus genome directs the production of new copies of the genome and of the protein packing needed to produce more viruses
What form does viruses genomes come in?
ALL forms (double or single stranded, DNA or RNA, linear or circular)
If a given virus has an unequal amount of thymine to adenine in its genome, what can be said about the virus?
It is not double stranded
Limiting factor of all viral genomes: (2)
Size
-Much smaller than the host they infect
Protein coat
-Rigid: unable to accommodate a larger genome
Adaptations found in viral genomes to accommodate their limitations
Carries few genes, relies on host-encoded proteins for translation, transcription and replication
Encode more than one protein in a given length of genome, more than one reading frame so genes can overlap
The protein coat surrounding the genome in a virus is called the ___ which is made from ___ and used to:
Capsid
Repeating pattern of a few protein building blocks
Classify viruses
Helical capsids
Rod shaped
Polyhedral capsids
Multiple-sided geometric figures with regular surfaces
E. Coli
Genome is located within the ____
___ and ___ attach to the surface of the host cell
____ contracts using ATP to inject the genome into the host
Capsid head
Tail fibers and base plate
Sheath
Why does a bacteriophage inject its DNA while animal viruses do not?
Because bacteriophage must penetrate a cell wall while animal viruses can enter the cell as a whole
Viruses have ____ which surrounds the capsid, which is:
Envelope
Membrane on the exterior of the virus made from the membrane of the host cell
Enveloped versus acquire their covering by
Budding through the host cell membrane
Naked Viruses
Enveloped viruses fuse with plasma membrane of host and leave the de-enveloped capsid inside the host cell
What kinds of viruses are always naked?
All phages and plant viruses because they affect cells with cell walls
Is viral infection a specific process?
Yes the virus must bind to a receptor on the cell membrane, which explains why only certain cells are exposed to viruses
How do viruses reproduce since they are incapable of doing anything on their own?
They inject their genome into the host, modifying it to produce more viruses
Attachment / Absorption
First step in redirecting host genome to reproduce the virus
Penetration / Eclipse
2nd step in redirecting host genome to reproduce virus
Injection of the viral genome into the host cell
After attachment and penetration the phage follows one of two paths:
Lytic cycle or the Lysogenic cycle
Hydrolase
Hydrolytic enzyme that degrades the entire host genome
One of the first enzymes transcribed from the virus upon entering the host genome
Hydrolase is an example of an early gene, one that:
Is expressed immediately after infections and which includes any special enzymes requires to express viral genes
Lysozyme, an example of a late gene, functions to
Destroy the bacterial cell wall, and the host bacterium bursts, releasing more viruses to repeat the cycle
Lytic cycle of phages:
- Attachment of phage and injection of phage chromosome
- ____________ by phage-specific enzyme
- ____________ using bacterial materials and phage enzymes
- ____________ to produce phage structural components
- ___________ progeny phage particles
- Release of progeny phage by __________
Lytic cycle of phages:
- Attachment of phage and injection of phage chromosome
- Breakdown of bacterial chromosome by phage-specific enzyme
- Replication of phage chromosome using bacterial materials and phage enzymes
- Expression of phage genes to produce phage structural components
- Assembly of progeny phage particles
- Release of progeny phage by lysis of bacterial wall
Lytic cycle is effective in giving virus a way to rapidly increase in number but it is problematic in the sense that
Some viruses are clever and instead go into the
All host cells are destroyed
Lysogenic cycle
Lysogenic Cycle: Upon infection the phage genome is incorporated into the bacterial genome and is to referred to as the ___ and the host is now referred to as the ___
Prophage
Lysogen
In the lysogenic cycle, when the host cell reproduces:
Eventually, the prophage becomes
So does the prophage
Activated and removes itself via excision to enter the lytic cycle
One potential consequence of the lysogenic cycle
When the prophage excises itself from the host, it may take some of the host genome with it possibly inducing traits in other cells the prophage infects that were not previously there
Transduction
When a prophage takes a part of a host cells DNA and infects another host cell, expressing a trait that was not previously encoded for in the new host cell
Lysogenic Cycle
- Phage binds to bacterium
- Phage __________
- Phage DNA integrates and becomes _________
- Chromosome with integrated prophage ________
- In rare cases, the prophage may ________________
Lysogenic Cycle
- Phage binds to bacterium
- Phage enters the host cell
- Phage DNA integrates and becomes a non-infective prophage
- Chromosome with integrated prophage replicates
- In rare cases, the prophage may separate and the cells will be induced to lyse
Animal cell infection: Getting the genome in the cell
- Virus binds to proteins on the animal cell’s ________
- Virus undergoes penetration into the cell via _______
- Once inside, the viral genome is _____ (_______)
OR
The virus ________
Animal cell infection: Getting the genome in the cell
- Virus binds to proteins on the animal cells plasma membrane
- Virus undergoes penetration into the cell via endocytosis
- Once inside, the viral genome is uncoated (released from the capsid)
OR
The virus fuses with the plasma membrane and injects virus into the cytoplasm
After the virus enters its genome into the animal cell, it:
Enters the:
Lytic cycle / productive cycle / lysogenic cycle
Lytic cycle in animal cells is
Same as in phages
Productive cycle in animal cells
Similar to lytic cycle but does not destroy the cell. Instead the virus exits via budding through the host cell membrane, and the membrane can reseal as the virus level
In the animal virus, the lysogenic cycle’s inactive form of the viral genome is called the
Provirus (analogous to the prophage)
(+) RNA virus genome
Must ENCODE RNA-dependent RNA poly but don’t have to carry it
As soon as it is in the cell, the host translates it (acts directly as mRNA)
Infective virus
Injecting the virus into the host will result in the production of the virus
RNA virus genome
(+) RNA viruses cause ___, ___ and __
common cold, rubella and polio
(-) RNA viruses genome
must CARRY RNA-dependent RNA poly
- RNA is complementary to the RNA that encodes viral proteins meaning it is the template for mRNA production
Entering cell: create (+) strand first
(-) RNA viruses produce ___, ___, ___ and ___
Rabies, measles, mumps and influenza
Retroviruses genome
Must ENCODE reverse transcriptase (aka RNA-dependent DNA polymerase)
Integrate as proviruses, so they must be turned into DNA from RNA to integrate into the host genome for replication
Retroviruses cause ___, ___ and ___
HIV, AIDS and HTLV (human t-cell leukemia virus)
Double stranded DNA viruses genome
ENCODE enzymes required for dNTP synthesis and DNA replication
Large genomes with genes for enzymes involved in DNA synthesis and replication so that it can reproduce without having to wait for the cell to do so
Subviral particles
Infection agents that are smaller and simpler than viruses
Two categories of sub viral particles:
Prions and viroids
Prions: what are they and how do they replicate?
Mis-folded version of a protein that already exists that are self replicating (violate central dogma)
When the normal protein comes in contact with the prion:
The prion acts as a template and the normal protein alters into an infectious form
Prions are responsible for transmissible spongiform encephalopathies (TSEs) which are responsible for:
Degeneration of the nervous system
Prion diseases are transmitted via: (2)
Ingesting tissue containing the abnormally folded proteins
OR
Genetically via mutations in the gene encoding for the prion protein
Viroids structure:
Short, circular single stranded RNA with extensive self complementarity (can base pair with itself to create some double stranded regions)
Viroids lack capsids and do NOT
Code for proteins
Some viroids are ____ while others ____
Catalytic enzymes while others silence gene expression
Viroids share similarity to replication of
RNA viruses
Most diseases caused by viroids are found in ____
The only one found in humans is
Plants
Hepatitis B
