Viruses Flashcards
(25 cards)
What is a virus?
an infectious particle consisting of genes (nucleic acid) packaged in a protein coat. They cannot exist independently
Are viruses cells?
No
How are viruses classified
DNA viruses or RNA viruses (single or double-stranded)
How does virus make more viruses?
- Virus enters host cell
- Viral genome is replicated by enzymes
- Copies of viral genome transcribed by host enzymes
- Viral mRNA translated by host ribosomes
- Viral proteins reassembled with viral genome
What is a capsid?
- It is the protein shell that encloses the viral genome
- built from protein subunits called capsomeres
- have a variety of structures (helical or icosahedral viruses)
What are Viral envelopes?
Are accessory structures that help them infect hosts
▪Viral envelopes (derived from membranes of host cells) surround the capsids of influenza viruses and many other viruses found in animals
- contains molecules from virus and host
Bacteriophage
Viruses that infect bacteria
▪They have an elongated capsid head that encloses their DNA
▪A protein tail piece attaches the phage to the host and injects the phage DNA inside
Where do viruses replicate?
They can only replicate inside the host cell.
Host range of viruses
Viruses have a limited number of host species that they can infect
2 types of replicative cycles of phage
Lytic cycle and lysogenic cycle
The Lytic cycle
Phage replicative cycle that culminates in the death of the host cell
▪The lytic cycle produces new phages and lyses (breaks open) the host’s cell wall, releasing the progeny viruses
▪A phage that reproduces only by the lytic cycle is called a virulent phage
The Lysogenic cycle
Replicates the phage genome without destroying the host
▪The viral DNA molecule is incorporated into the host cell’s chromosome
Phages that use both cycles
Phages that use both the lytic and lysogenic cycles are called temperate phages, e.g. lambda
What is a prophage
- integrated viral DNA
- Every time the host divides, it copies the phage DNA and passes the copies to daughter cells
- An environmental signal can trigger the virus genome to exit the bacterial chromosome and
switch to the lytic mode
What are bacterial defenses against phages ?
Natural selection favors bacterial mutants with surface proteins that cannot be recognized as receptors by a particular type of phage
▪Foreign DNA can be identified as such and cut up by cellular enzymes called restriction
enzymes
▪The bacterium’s own DNA is protected from the restriction enzymes by being methylated
▪Both bacteria and archaea can protect themselves from viral infection with the CRISPR-
Cas system. (ability to change any letter of their Dna code to protect themselves)
2 Variables that classify viruses that infect animals
▪ An RNA or DNA genome, either single-stranded or double-stranded
▪ The presence or absence of a membranous envelope
Viral envelopes (viruses that affect animals)
Many viruses that infect animals have a membranous envelope
▪ Viral glycoproteins on the envelope bind to specific receptor molecules on the
surface of a host cell
▪ The viral envelope is derived from the host cell’s plasma membrane as the viral
capsids exit
- Other viral membranes form from the host’s nuclear envelope and are then replaced by an envelope made from Golgi apparatus membrane, e.g herpes
Viral Genetic Material
▪The broadest variety of RNA genomes is found in viruses that infect animals
▪Retroviruses use reverse transcriptase to copy their RNA genome into DNA (that can then for apart of the host’s DNA=provirus)
Provirus
- Unlike a prophage, a provirus remains a permanent resident of the host cell
▪RNA polymerase transcribes the proviral DNA into RNA molecules
▪The RNA molecules function both as mRNA for synthesis of viral proteins and as
genomes for new virus particles released from the cell
Possible sources of viruses
plasmids and transposons
What are prions?
Smaller, less complex entities that can cause of disease ( affect the brain)
They are incorrectly folded proteins that act very slowly and are virtually indestructible
What are vaccine?
a harmless derivative of a pathogen that stimulates the immune system to
mount defenses against the harmful pathogen
How do antiviral drugs treat viruses?
Inhibiting synthesis of viral
DNA and by interfering with viral assembly
What are emerging viruses
Those that suddenly become apparent
causes: 1. mutation of existing viruses into new
ones that can spread more easily
2.Spread of a viral disease from a small,
isolated human population
3. Spread of existing viruses from other
animals
