Microbiology Ch. 6 Viruses
Group IV viral genome
(+) single-stranded RNA
Group V viral genome
(-) single-stranded RNA
Viral genes encode proteins for 3 things:
-Capsid -Envelope proteins (if need be) -Any polymerase not found in host cell
Name 3 virus origin theories.
-Cellular theory -Regressive theory -Coevolution theory
Most plant viruses enter by one of three routes:
-Contact with damaged tissues -Transmission by an animal vector -Transmission through seed
Name 3 characteristics of viral genomes.
-DNA or RNA -Single- or double-stranded (ss or ds) -Linear or circular
Since viruses are ubiquitous, a wide range of defense mechanisms have evolved in animals and plants. Name 3 defense mechanisms of plants.
-Genetic resistance: hosts continually experience mutations -Immune system: "Innate immunity"—interferons, "Adaptive immunity"—antibodies -RNA interference (RNAi): widespread among eukaryotes and archaea
The International Committee on Taxonomy of Viruses (ICTV) has devised a classification system, based on 5 criteria:
-Genome composition: DNA/RNA, single/double stranded, linear/circular -Capsid symmetry -Envelope: host derived envelope? -Size of the virion -Host range
All viruses require a host cell for reproduction. Thus, they all face the same 4 needs for host infection:
-Host recognition and attachment -Genome entry -Assembly of virions -Exit and transmission
Progeny phage can exit the host cell in 1 of 2 ways:
-Lysis (make enzyme that breaks down cell wall, host cell bursts to release progeny phage) -Slow release (filamentous phages can extrude individual progeny through cell envelope, host cells grow slowly but do not die)
Release of progeny viruses from animal host cell
-Lysis of cell -Budding: virus passes through membrane; membrane lipids surround capsid to form envelope; all enveloped viruses bud from a membrane; either plasma membrane or organelle membrane
Bacteriophages can undergo two different types of life cycles:
-Lytic cycle -Lysogenic cycle
Name 3 features that all viruses have.
-Nucleic acid -Capsid -Envelope
Icosahedral viruses
-Symmetrical virus -Are polyhedral with 20 identical triangular faces -Have a structure that exhibits rotational symmetry -In some, the capsid is enclosed in an envelope, formed from the cell membrane -The envelope contains glycoprotein spikes, which are encoded by the virus. -Between the envelope and capsid, tegument proteins may be found (aid in viral DNA replication and evasion from host immune response).
Filamentous viruses
-Symmetrical virus -The capsid consists of a long tube of protein, with the genome coiled inside -Vary in length, depending on genome size -Include bacteriophages as well as animal viruses -Show helical symmetry -The pattern of capsid monomers forms a helical tube around the genome, which usually winds helically within the tube.
In 1971, David Baltimore proposed that the primary distinctions among classes of viruses be:
-The genome composition (RNA or DNA) -The route used to express messenger RNA (mRNA)
Asymmetrical viruses
-These have complex multipart structures -Includes T4 bacteriophages, which have an icosahedral "head" and helical "neck"
Bacteria have evolved several forms of defense against bacteriophage infection. Name 3 defense mechanisms of bacteria.
1) Genetic resistance: altered receptor proteins 2) Restriction endonucleases: cleave viral DNA lacking methylation 3) CRISPR integration of phage DNA sequences: Clustered regularly interspaced short palindromic repeats, a bacterial immune system of sorts
Name the 2 cycles of the viral life cycle.
1) Lytic cycle 2) Lysogenic cycle
How does a seemingly "new" virus emerge to sicken humans? Name 2 possible explanations.
1)As a result of human consumption of wildlife; Ex: SARS coronavirus 2) As variants of endemic milder pathogens; Ex: The avian influenza strain H7N9
Small viruses commonly have a small genome, encoding under _____ genes. The genes may actually overlap in sequence.
10
Large viruses may have more than _______ genes. Indeed, mimivirus encodes over 1,000 genes.
100
So far, the genome composition and mechanisms of replication and mRNA expression define _____ fundamental groups of viral species.
7
Virus
A non-cellular particle that must infect a host cell, where it reproduces; typically subverts the cell's machinery and directs it to produce viral particles
DNA virus (animal)
Animal virus that can utilize the host replication machinery
Retroviruses (animal)
Animal virus that uses a reverse transcriptase to copy their genomic sequence into DNA for insertion in the host chromosome
RNA virus (animal)
Animal virus that uses an RNA-dependent RNA-polymerase to transcribe their mRNA
Assembly of new virions within animal host cell
Capsid and genome form within host cell; May occur in the cytoplasm or nucleus; Envelope proteins are inserted into a membrane; Cell membrane or organelle membrane
Group VII viral genome
DNA pararetroviruses
Host range
Each species of virus infects a particular group of host species.
Bacterial cell receptors are normally used for important functions for the host cell. Give an example.
Example: Certain cell-surface receptors control sugar uptake.
Viruses are part of our daily lives. Give 2 examples of some often-encountered viruses.
Examples: -Respiratory pathogens such as rhinovirus (the common cold). -Sexually transmitted viruses such as herpes simplex virus (HSV) and papillomavirus (genital warts).
Lytic cycle
In this cycle, a virus injects its genetic material into a cell; the cell makes copies of its own DNA and the newly added viral DNA, from which host and viral proteins are produced; viruses form within the cell and lyse the cell to exit and move on to the next cell.
Lysogenic cycle
In this cycle, a virus injects its genetic material into a cell; viral DNA/RNA becomes part of the host cell's DNA; the viral DNA/RNA can sit in cell DNA and wait for a suitable time to be transcripted and translated into viral proteins and eventually viruses years later. Ex: People who had chickenpox as an infant often develop shingles decades later.
Lysogenic cycle
In this cycle, the bacteriophage is quiescent (inactive). Integrates into cell chromosome, as a prophage. Can reactivate to become lytic.
Lytic cycle
In this cycle, the bacteriophage quickly replicates, killing host cell.
Viroids
RNA molecules that infect plants; Have no protein capsid; Are replicated by host RNA polymerase; Some have catalytic ability
Group VI viral genome
RNA retroviruses
True or False? All animal viruses make proteins with host ribosomes. Translation occurs in the cytoplasm.
True
True or False? All life-forms can be infected by viruses.
True
True or False? Animal and plant viruses solve problems similar to those faced by bacteriophages: Host attachment, genome entry and gene expression, virion assembly, and virion release.
True
True or False? Different viruses make different capsid forms. These can be divided into symmetrical and asymmetrical types.
True
True or False? In contrast to animal viruses and bacteriophages, plant viruses infect cells by mechanisms that do not involve specific membrane receptors.
True
True or False? Our concept of a virus has developed over the past century.
True
True or False? The "decision" between the two cycles is dictated by environmental cues. In general, events that threaten host cell survival trigger a lytic burst.
True
True or False? Viral ecologists consider viruses as living entities, playing roles comparable to those of cellular organisms. Example: "predator" or "parasite" .
True
True or False? Viruses are ubiquitous in all environments.
True
True or False? Viruses exist naturally within host organisms in complex ecosystems. Their persistence is significant for human health and for agricultural plants and animals.
True
true or False? However, eukaryotic cells have a more complex structure than prokaryotic cells. Therefore, animal and plant viruses have greater complexity and diversity of viral replication cycles than we see in bacteriophages.
True
True or False? Some viruses are good, others are bad.
True; Some viruses generate serious epidemics, from dengue fever to influenza to AIDS. Others fill essential niches in the environment, particularly in marine ecosystems.
True or False? Viruses are non-living.
True; Viruses don't have a metabolism and cannot reproduce on their own without a cell.
Capsid
Viral protein coat made up of at least 2 different proteins
Regressive theory
Viruses may have come from cells that regressed (returned to a previous, simpler state).
Cellular theory
Viruses may have come from plasmids from ancient bacteria.
Coevolution theory
Viruses may have evolved with cells.
Bacteriophages
Viruses that inject their DNA into a bacterium and use the machinery of the bacterium to make copies of themselves
Virion
aka virus particle; consists of a single nucleic acid (DNA or RNA) contained within a protective protein capsid
Culturing viruses requires growth in host cells. Bacteriophages may be cultured either in _________ culture (in liquid) or as isolated plaques on a bacterial lawn (on a plate).
batch culture
Ebola virus exhibits (narrow/broad) tropism, infecting many kinds of host tissues.
broad
Contact and attachment are mediated by _____-__________ ___________, which are proteins that are specific to the host species and which bind to a specific viral component.
cell-surface receptors
Capsid
composed of repeated protein subunits; this maximizes the capacity while minimizing the required number of genes; packages the viral genome and delivers it into the host cell
Group I viral genome
double-stranded DNA
Group III viral genome
double-stranded RNA
The primary factor determining the life cycle of an animal virus is the form of its ____________.
genome
Most bacteriophages (phages) inject only their genome into a cell through the cell envelope. The phage capsid remains outside, attached to the cell surface. It is termed a "___________."
ghost
Viruses fill important niches in all ecosystems. -They limiting _______ _________ _________: without extinction of the host; in animals, viruses participate in population decline and resurgence -They select for ________ __________: prevent dominance of any one species; foster the evolution of many distinct host species
host population density; host diversity
Certain human-infecting viruses are well known to persist in the wild, such as rabies virus and West Nile virus. Their persistence requires a broad _______ ______________.
host range
Envelope
lipid bilayer that allows the virus to fuse with other cells that have similar cell membrane structure; gathered from infected cells
Entry of plant viruses into host cells usually requires ____________ transmission.
mechanical
Papillomavirus shows (narrow/broad) tropism, infecting only epithelial tissues.
narrow
In viruses, their _________ _______ can be ds/ss DNA or ss/ds RNA.
nucleic acid
Within a plant, the thick cell walls prevent a lytic burst or budding out of virions. Instead, plant viruses are transmitted to uninfected cells by _____________________ (membrane channels that connect adjacent plant cells; inner channel connects the ER).
plasmodesmata
Many viruses interact ________________-ly with hosts, by expressing host genes and by protecting hosts from other organisms.
positive
Prions
proteins that infect animals; have no nucleic acid component; have an abnormal structure that alters the conformation of other normal proteins
Animal viruses can be cultured within whole animals by ____________ _____________, which ensures that the virus strain maintains its original virulence, but this process is expensive and laborious. They can also be grown in human cell tissue culture.
serial inoculation
Group II viral genome
single-stranded DNA
Animal viruses bind specific receptor proteins on their host cell. Receptors determine the viral __________, the cells and tissues of a host that support growth of a particular virus.
tropism
Internalized virions undergo ________________, where genome is released from its capsid. This can occur in several different ways.
uncoating
Most animal viruses enter host as _______________.
virions
