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Ch. 19: Viruses, Ch. 27: Bacteria and Archeae - Coggle Diagram
Ch. 19: Viruses
Virus
Virus Structure
virus- infectious particle consisting of nucleic acid (DNA or RNA) packaged in a protein coat (capsid) and sometimes an membranous envelope.
~much simpler structure than bacteria.
cannot reproduce or carry out metabolism without host- considered not living or cells.
~cause wide variety of diseases.
Viral Genomes
~Classified as DNA viruses or RNA viruses.
many animal viruses are RNA genome
~Genome is either linear or circular.
~Have between 3 and 2,000 genes.
double stranded DNA (DS DNA)
single stranded DNA (SS DNA)
double stranded RNA (DS RNA)
single stranded RNA (SS RNA)
Capsid- protein shell that encloses the viral genome.
Capsomeres- protein subunits that make up a capsid.
Shapes
helical
icosahedral
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viral envelopes- derived from membranes of host cells- surround the capsids of some viruses.
Ex. influenza, Ebola.
~help viruses infect other hosts by looking like their own cell.
~many animal viruses have envelopes
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Bacteriophage
bacteriophage- also called phages, are viruses that infect bacteria.
~elongated capsid head that encloses their DNA.
~protein tail piece attaches the phage to the host and injects the page DNA inside host.
~few bacteriophages have an envelope or RNA genome
Viral Replication
~Viruses are obligate intracellular parasites- they can only replicate within a host cell.
host range- limited number of host species that the virus can infect.
~some have broad host ranges whole others can only infect one species.
Ex. measles only infects humans.
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Two Replication Cycles
Lytic Cycle
cycle that leads to death of the host cell
shorter, more severe symptoms
new phages lyse host cell wall to release viruses
virulent phage- phage that only reproduces by the lytic cycle.
Ex. influenza
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Lysogenic Cycle
cycle that replicates the phages genome without destroying the host
longer duration and minimal/absent symptoms
viral DNA is incorporated in the host cell chromosome- FOREVER
~every time cell divides, phage gets copied and passed.
eventually will enter the lytic cycle
~some environmental signal can trigger the lytic cycle.
temperate phages- use both the lytic and lysogenic cycles.
Ex. lambda, herpes virus.
prophage- integrated viral DNA temporarily.
~some at expressed during lysogeny, some cause hot bacteria to secrete toxins that are harmful to humans.
provirus- viral DNA that is integrated permanently.
~RNA polymerase transcribes the proviral DNA into RNA molecules
~RNA molecules function as both mRNA for synthesis of viral proteins and as genomes for new virus particle released from the cell
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Viral Genetic Material
~broadest variety of RNA genomes is found in animal viruses.
Retrovirus- uses reverse transcriptase to cope their RNA genome into DNA.
Ex. HIV (human immunodeficiency virus) is a retrovirus that causes AIDS (acquired immunodeficiency syndrome)
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Viral Diseases
~diseases by viral infections affect humans, agricultural crops, and livestock worldwide.
Viruses may damage or kill cells by:
~Causing the release of hydrolytic enzymes from lysosomes
~Produce toxins that lead to disease symptoms
~Have molecular components (envelope) that are toxic
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Emerging Viral Diseases
Emerging viruses are those that suddenly become apparent.
Ex. HIV, Covid-19
Epidemic- widespread outbreak that is local or ragional
Pandemic- global outbreak that reaches every continent.
Ebola virus- one of several emerging viruses that cause hemorrhagic fever.
~ssRNA virus
~often fatal
Ex. epidemic outbreak of Ebola occurred in 2014.
Ex. Chikungunya virus
Ex. Zika virus, COVID-19
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Causes
Three main causes of rapidly emerging viral disease:
mutation of existing viruses into new ones that can spread more easily.
spread of a viral disease from a small, isolated population.
zoonotic diseases-spread of existing viruses from other animals.*
~about 3/4 of new human diseases originate this way
Flu epidemics-caused by type A influenza virus.
~ssRNA virus
~infect wide variety of animals including birds, pigs, horses and humans.
~influenza viruses have nine RNA segments in their genome- leads to high rate of mutation.
~strains of influenza A are given standardized names based on the viral surface proteins hemagglutinin (HA) and neuraminidase (NA).
about 18 types of HA and 11 types of neuraminidases have been identified.
~H5N1 is a deadly flu strain that is different from other strains infecting people.
~difficult for people to mount effective immune response.
~not transmitted human-human.
~H1N1 deadly flu strain originally called the swine flu- 2009
~was not transmitted to humans from pigs
Unique combination of swine, avian and human influenza genes
Viral Diseases in Plants
Viral diseases of plants are known to cause spots on leaves and fruits, stunted growth, and damaged flowers or roots.
Most known plant viruses have an RNA genome
Many have a helical capsid, but some have an icosahedral capsid.
Two major routes plant viruses spread disease:
Horizontal transmission- enter through damaged cell walls
Vertical transmission- inherit the virus from a parent.
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Prions
Prion- infectious proteins that cause degenerative brain diseases in animals.
Prions are misfolded proteins that convert normal proteins into misfolded versions. They can be transmitted in food, act slowly and are virtually indestructible.
Ex. Scrapie(sheep), mad cow disease, Creutzfeldt-Jakob diseases(humans)
Might also be involved in diseases such as Alzheimer's and Parkinson's
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Evolution of Viruses
~since viruses can replicate only within cells, they probably evolved as bits of cellular nucleic acid.
plasmids, transposons, and viruses are all mobile genetic elements
-Largest virus identified is the size of a small bacterium.
~its genome encodes proteins involved in translation, DNA repair, protein folding and synthesis of polysaccharides.
~not sure whether this virus evolved before or after cells.
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