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Chapter 19: Viruses (Concept 19.1: A virus consists of a nucleic acid…
Chapter 19: Viruses
Concept 19.1
: A virus consists of a nucleic acid surrounded by a protein coat
A Borrowed Life
viruses inject their genetic information into a cell, hijack it, recruit cellular machinery to manufacture many new viruses
viruses are not exactly living because they can't carry out metabolic activities outside a host cell, but rather exist in between life-forms and chemicals, "a kind of borrowed life"
rather exist in between life-forms and chemicals, "a kind of borrowed life"
Discovery
tobacco mosaic disease:stunts growth of tobacco plants, mottled, mosaic coloration
Mayer searched for the bacteria that would be causing it, couldn't find it with a filter, rubbed the filtrate sap on a plant and it still became infected
agent could not be cultivated on a petri dish, concluded it was something smaller and simpler than bacteria,
Stanley crystallized the virus, viruses were seen under an electron microscope
virus
: an infectious particle consisting of little more than genes packaged in a protein coat, lack structures and metabolic machinery found in a cell
Structure
Viral Genomes
Capsids and Envelopes
viral
envelopes
: accessory structures, helps them infect hosts, membraneous envelope surrounding capsids
derived form membranes of host cell, host cell phospholipids and membrane proteins
proteins and glycoproteins(proteins with carbohydrates covalently attached) of viral origin
bacteriophages
: phages, virus that infects bacteria, elongated icosahedral, heads enclosing their DNA, protein tail piece with fibers to attach to a bacterial cell
capsid
: protein shell enclosing the viral genome, rod-shaped, polyhedral, or more complex
built from protein subunits called capsomeres
Concept 19.2
: Viruses replicate only in host cells
Replicative Cycles of Phages
Lytic Cycle: phage replicative cycle that culminates in death of the host cell
lytic- last stage of infection, bacterium lyses(breaks open) and releases the phages that were produced within the cell
virulent phage: replicates only by a lytic cycle
Lysogenic Cycle: lysogeny, coexist with host cell, allows replication of the phage genome without destroying the host
temperate phages
: phages capable of using both methods of replicating within a bacterium, lambda, tail is only one short fiber
prophage
: phage DNA integrated into the bacterial chromosome, viral proteins break both circular DNA molecules and join them to each other
one prophage gene codes for a protein that prevents transcription of most of other prophage genes, each daughter cell inherits a prophage
Bacterial Defenses Against Phages
natural selection favors bacterial mutants with surface proteins that are recognized as receptors by a particular type of phage
when phage DNA enters a bacterium, the DNA is identified as foreign and is cut up by restriction enzymes
restriction enzymes
: cellular enzymes, restrict a phage's ability to replicate within the bacterium
CRISPR- Cas system: present in bacteria and archaea, sequences are clustered regularly interspaced short palindromic repeats, specific nucleases that interact with the system can identify and cut phage DNA,
Replicative Cycles of Animal Viruses
Viral Envelopes: membraneous outer layer, virus uses its envelope to enter the host cell
glycoproteins
: protrude from outer surface of envelope, bind to specific receptor molecules on surface of host cell, derived from host cell's plasma membrane
ribosmoes on endoplasmic reticulum of host cell make the protein parts
cellular enzymes in in ER and golgi apparatus then add the sugars
Viral Genetic Material
retroviruses
: most complicated replicative cycles
reverse transcriptase: transcribes an RNA template into DNA (RNA to DNA is unusual, retro means "backward")
HIV is a retrovirus, causes AIDS
provirus
: after viral DNA is made it enters the cell's nucleus and integrates in to the DNA of a chromosome, never leaves host genome, permanent resident of the cell
General Features of Viral Replicative Cycles
host range
: the limited number of host species that a virus can infect, "lock-and-key" fit
viral genome enters the host, the proteins it encodes can commander the host,, the host is reprogrammed to copy the viral genome and manufacture viral proteins
Evolution of Viruses
likely that viruses are not descendants of precellular forms of life, but evolved after the first cells appeared, scientists favor hypothesis that they originated from naked bits of cellular nucleic acids that moved from one cell to another
mimivirus, pithovirus
Concept 19.3
: Viruses are formidable pathogens in animals and plants
Viral Diseases in Plants
same basic structure and mode of replication as animal viruses, have RNA genome, helical capsid or icosahedral capsid
spread by two major routes:
horizontal transmission
- a plant is infected from an external source of the virus
more susceptible if outer protective layer of cells, epidermis, is damaged, by injury, herbivores, wind
vertical transmission
: a plant inherits a viral infection from a parent, occur in asexual propagation(cuttings) or in sexual reproduction through infected seeds
common signs: bleached or brown spots on leaves and fruits, stunted growth, damaged flowers or roots
plasmodesmata
: cytoplasmic connections that penetrate the walls between adjacent plant cells, a virus replicates and its genomes and associated proteins spread from plant cell to plant cell
Prions: as Infectious Agents
prion
: an infectious protein, cause degenerative brain diseases in various animal species
can be transmitted in food, not genetic
prion act very slowly with an incubation of year years before symptoms develop, also not destroyed by heat like cooking
misfolded form of a protein normally present in brain cells, prion gets into a cell and somehow converts normal protein molecules to the misfolded prion version. then they form an aggregate chain of prions
prion aggregation interferes with normal cellular functions and causes disease symptoms
Emerging Viruses: viruses that suddenly become apparent
epidemic
: a widespread outbreak
HIV, AIDS, encephalitis, West Nile, Ebola, Zika,
pandemic
: global epidemic
three processes contribute to the emergence of viral diseases:
dissemination of a viral disease from a small, isolated human population, AIDS went unnoticed for years before it became widespread
spread of existing viruses from other animals, the animal is unaffected by the virus and act as a natural reservoir
mutation of existing viruses
RNA viruses have unusually high rate of mutation because viral RNA polymerases do not proofread or correct errors in replicating their RNA genomes, creates new strains of the virus
Viral Diseases in Animals
different routes:
cause cells to produce toxins that lead to disease symptoms
have molecular components that are toxic, such as envelope proteins
viruses may damage or kill cells by causing the release of hydrolytic enzymes from viruses
vaccine
: a harmless derivative of a pathogen that stimulates the immune system to mount defenses against the harmful pathogen
antibiotics
: powerless against viruses, they kill bacteria by inhibiting enzymes specific to bacteria but have no effect on eukaryotic or virally encoded enzymes