*** Virology Chapter 1: General Features of Virus Structure Flashcards
What is a virus?
chemical complexes of RNA or DNA protected by protein
Are viruses living or non-living?
non-living
Why are viruses obligate intracellular parasites?
require host cells to reproduce
What does each type of virus infect?
a specific unicellular species OR cell type in a multicellular species
Are viruses cells? Why?
no
cannot synthesize their own ATP or amino acids or nucleotides
- lack abilities to extract energy from molecules to build nucleic acids
and proteins
- lack “instructions” to make nucleotides and amino acids
- lack ribosomes
What are the 2 possible states viruses can be in?
extracellular state
intracellular state
What is a virus in extracellular state?
inert particle (protein, nucleic acid, maybe lipid)
What is a virus in intracellular state?
replicating nucleic acids (gene expression, viral proteins)
Viruses vs. Cells
What is the hereditary material?
virus: DNA or RNA, can be single or double-stranded
cells: DNA, always double-stranded
Viruses vs. Cells
Is there a plasma membrane?
virus: no, but may have an envelope
cell: yes
Viruses vs. Cells
Can it carry out transcription independently?
virus: no – requires ATP and nucleotides from host cell
cell: yes
Viruses vs. Cells
Can it carry out translation independently?
virus: no
cell: yes
Viruses vs. Cells
What are the metabolic capabiities?
virus: none (not detectable)
cell: extensive – ATP synthesis, oxidation of reduced C, amino acid and nucleotide biosynthesis, etc.
What percentage of your genome has its origins in virus sequences?
~ 50%
What is the central dogma of molecular biology?
flow of genetic information within a biological system – applies to all living cells, and viruses to (sometimes with a twist)
DNA → RNA → protein
RNA → RNA → protein
RNA → DNA → RNA → protein
What are the 3 types of virus structures?
naked
enveloped
complex capsids
Viruses are a diverse group of infectious agents. What are the types of genetic material viruses can be made of?
ssDNA
dsDNA
ssRNA
dsRNA
(can be linear or circular DNA or RNA)
What type of host cell is infected by viruses?
species specific, and cell/tissue specific
Can viruses infect more than one cell type, and/or more than one species?
yes
ie. infects only humans: poliovirus, HIV, smallpox
ie. infects humans and other animals: influenza, rabies, measles, coronavirus, ebola
What are the replication strategies of viruses?
- where the genome is replicated in the cell
- replication cycle (lytic, persistent, latent)
- on or more ‘rounds’ of mRNA expression
- etc.
What are the 2 general categories of virus structure?
enveloped and non-enveloped (naked)
What is the genome of a naked virus enclosed by?
only the capsid (protein shell – aka nucleocapsid)
What is a capsid made of?
capsomeres (multiple copies of protein subunits) – it could be made from one type of protein or several different proteins
What do capsomeres do?
might bind directly to a structure on a host cell, or there may be attachment proteins that project out of the capsid (ie. spikes)
Where is the gene that encodes the protein structure that is called the
capsomere?
in virus’ genome
What is the structure of an enveloped virus?
- capsid
- lipid bilayer that surrounds capsid (can be derived from plasma membrane or any other membrane structure)
- some have a matrix layer of protein just below the envelope
What does the envelope envelope?
glycoproteins encoded by the virus (proteins modified with carbohydrate) and embedded in the lipid bilayer, that are required for attachment to the host cell
Where would the envelope structure come from?
stolen from host cell
Why do some viruses have envelopes, and others don’t?
related to viruses replication cycle
enveloped:
- virus buds through the membrane and acquires the envelope without killing host cell immediately
- virus replication occurs for prolonged periods of times (as long as years)
naked:
- cell lyses, and all progeny are released at once (cell is dead)
Which virus would be more fragile in the environment? For example, which type of virus would likely be infectious after 1 hour on a countertop surface?
enveloped virus
What is the ‘complex’ structure that some viruses have?
possesses a capsid that is not helical or icosahedral and may have other structures like a protein tail or complex outer wall.
Why study viruses?
- some viruses do cause disease – humans, domesticated animals (pets and livestock), wild animals, and crops
- many societal impacts
- if one understands the virus that causes a specific disease, one can develop
protocols to limit the spread and/or prevent infection altogether
What are cool enzymes? What do they do? What are some examples
reverse transcriptase and RNase H
enables pharmaceutical companies to manufacture human products in bacteria or yeast cells
insulin, Factor VIII, Factor IX, human growth hormone, erythropoietin,
interferons, interleukins, parathyroid hormone and others
What is anti-microbial therapy?
bacteriophages to treat patients with bacterial infections (topical ointments, enemas, mouthwashes) and also used as antiseptics and disinfectants
What is gene therapy?
use viruses as a delivery system to deliver a wild type copy of a gene to a cell type that is carrying a mutated copy of the gene
How are viruses used in treatment of cancer?
- some cause cancer
- some can be genetically modified to be used as anti-cancer agents that can replicate in cancer cells (ie. cancer cells are permissive) but not healthy cells (ie. healthy cells are not permissive)
Where do viruses that infect humans come from?
not clear – many hypotheses…escaped genetic elements?
problem: no fossil record.
difficult to track: high mutation rate, some integrate into host genome, some do genetic re-assortment
Where do ‘new (strains)’ viruses evolve from?
pre-existing viruses
What might describe some method(s) that viruses use to “jump” to new hosts?
- virus is excreted by an animal in its urine/feces – the urine/feces
contaminates a food source or water supply - virus is excreted by an animal in its urine/feces onto a dirt floor – the virus
is inhaled by the new host when it is aerosolized - infected animal is slaughtered – the butcher cuts their hand with a knife
and the animal’s blood enters into the wound - infected animal dies or is slaughtered – the resulting meat is consumed
by the new host without cooking
What are the two general pathways of virus evolution?
- co-evolution with host
- infection of multiple host species
What is an advantage, disadvantage, and example of co-evolution with host (pathway of virus evolution)?
- advantage for virus: prosperous host = prosperous virus
- disadvantage for virus: host becomes extinct, so does virus
- example: smallpox (in humans)
What is an advantage, disadvantage, and example of infection of multiple of host species (pathway of virus evolution)?
- advantage: if one host species is compromised, virus can replicate in another
- disadvantage: cannot optimize for any one situation
- example: influenza, rabies
Fitness in Co-evolution
What happens if a mutation is not good for the virus?
it might lose its ability to infect a cell, which is needed to produce more virus
Fitness in Co-evolution
What happens if a mutation is neutral to the virus?
no obvious effect on the virus
Fitness in Co-evolution
What happens if a mutation is beneficial to the virus?
might gain an ability to infect a new host, or infects its host more efficiently (higher virulence)
What is the downside to highly virulent viruses?
- will kill the host too soon
- if it is too exposed, the host’s immune system will kill it
What type of relationship do viruses and hosts tend to co-evolve toward?
symbiotic
What are coronaviruses?
enveloped virus with a helical capsid, and a single-stranded RNA genome
What is the length of the coronavirus genome?
between 27-32 kb (largest genome of RNA viruses)
What are the 3 virus proteins?
- spike (S) protein: binding and entry
- membrane (M) protein: assembly, shape
- envelope (E) protein: ? assembly
