2.6: Introduction to Viruses Flashcards
(29 cards)
what characterizes viruses? (what they are and structure)
- acellular infectious particles
- obligate intracellular pathogens -reproduce only inside of living cells, lack independent metabolism so they must use
- composed of at least 2 parts:
-nucleic acid genome (DNA or RNA) and a protein coat (capsid), together = Nucleocapsid - some viruses have an envelope - layer of lipid surrounding the nucleocapsid.
describe Viral genomes
- DNA or RNA not both
- can be single stranded or double stranded
- circular or linear
- can be in several pieces -segmented, not called chromosomes bc that implies many genes , here only 1 gene per piece.
-ex: influenza if there’s 8 pieces of ssRNA, there’s 8 genes, each gene is on its own strand of RNA.
what is the smallest and largest genome sizes for viruses
- smallest ~ 3.6 kb for some ssRNA viruses (3 genes)
- largest ~ 150 kbp for some dsDNA viruses (> 100 genes)
kbp = kilo base pairs, (pairs bc double stranded)
what’s a capsid? whats it made of? what does it allow for?
- protein coat that surrounds the genome
- made of identical polypeptides - called protomers
- allows tranfer of viral genome between host cells
what’s the only component of viruses needed to infect host cells
the nucleic acid genome
(capsid and envelope disappear)
what are the 4 shapes of viruses (capsids dictate the viruses shape)
- Helical capsids
- icosahedral capsids
- binal capsids
- nucleocytoplasmic large DNA viruses
describe helical capsids and an example
- protomers form a spiral cylinder
- nucleic acid genome coiled inside
- ex: tobacco mosaic virus capsid made of ~2100 identical protomers (Identical only needed 1 gene to make capsid)
describe icosahedral capsids and an example
- regular geometric shape with 20 triangular faces
- exhibit symmetry (rigidity allows for this)
- protomers aggregate to form capsomeres
- ex. human papillomaviruses form their capsids from pentamers (clusters of 5 protomers)
(pentamers = type of capsomeres)
describe binal capsids and an example
- Geometric head with an attached helical tail
- ex: T4 bacteriophage of E. coli
-Genome is carried in a polyhedral head, helical tail is used to inject DNA into host cell
(pentamers = type of capsomeres)
what was the first discovered virus
the tobacco mosaic virus
describe nucleocytoplasmic large DNA viruses and an example
- viruses with complex multi-layered structure
- ex: Mimivirus (infects amoebae)
- 0.75 um in diameter, 1200 kbp DNA (smaller than average bacteria but larger genome)
describe what the virion envelope is, what it consists of, and example
- a lipid bilayer surrounding the nucleocapsid, acquired from the membrane of last host cell
- consists of host lipids and viral proteins (spikes)
- ex: influenza virus -flexible helical capsid, surrounded by an envelope
- two major spikes: hemagglutanin (H) and neuraminidase (N)
what two major spikes (viral proteins) does influenza have?
- hemagglutanin (H)
- neuraminidase (N)
what does naming codes for influenza come from like H1N1
It’s spike proteins (hemagglutanin and neuraminidase)
which domains of life do viruses infect?
all domains, every living cell is subject to at least 1 virus
what are bacteriophages (phage) and example?
- viruses that specifially infect bacteria, can’t infect human cell
- T4 -Infects* E. coli*
what are animal viruses and an example
- infect and multiply only inside of animal cells
- ex. Human papillomavirus -infects human epithelial cells
- causes benign tumors (warts)
most viruses are specific to ….
they must attach to….
but some viruses…
- a single host species
- viruses must attach to specific receptors on the host cell surface
- infect more than one species
why does HIV only affect humans?
- HIV binds to CD4 (receptors) which are only on the surface of some human immune system cells like T-helper cells
- only humans have this receptor
what’s an example of a virus that must attach to a specific receptor on the host cell surface
HIV binds to CD4
What’s an example of a virus that infects more than one species?
- Influenza attaches to a glycoprotein found on surface of several animal cells
- infects humans, pigs, chickens, seals etc.
what are the 5 stages of the virus ‘life cycle’
-
Adsorption -attachment to the host cell
-Involves specific receptors on the host cell surface
-ex: LPS, outer membrane proteins or glycoproteins - Penetration (how it gets into cell) and uncoating -how it releases genome into cell, removing capsid.
-
synthesis of viral nucleic acids and protein
-viral genes are expressed and viral proteins are synthesizes (by host’s ribosomes)
-viral genome is replicated (by host’s replication machinery) -
Assembly of new virions
-viral proteins are assembled into capsids, and then genomes are packaged into nucleocapsids.
-viruses do not reproduce by division! - Release of new virions -through lytic infection or budding.
how do bacteriophages enter the host cell
usually inject their nucleic acid into the cell, leaving the capsid outside the cell as a “ghost”.
what are the two ways animal viruses enter the host cell
- endocytosis -binding to specific receptors triggering normal endocytic activity (engulfing virus) -most common.
- fusion with the plasma membrane -only for enveloped virus, lipid bilayer of viral envelope fuses with host cell membrane, releasing nucleocapside into cell.
