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Viruses (Discovery of Viruses (In the late 1800's, researchers…
Viruses
Discovery of Viruses
In the late 1800's, researchers hypothesized that unusually small bacteria could be responsible for tobacco mosaic disease, which stunts the growth of tobacco plants and gives the leaves a mosaic coloration.
Later work suggested that the infectious agent did not share features with bacteria, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle; now known as tobacco mosaic virus.
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Evolution
Are not cells but a small infectious particle consisting of nucleic acids enclosed in a protein coat and/or a membranous envelope
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Structure
Viruses are a simple structure, even simpler than prokaryotic cells.
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Viral Envelope: A combination of viral and host cell molecules, which helps the virus infect hosts.
Viral glycoproteins on the envelope bind to receptor molecules on the host cell's surface, the envelope is usually derived from the host cell's plasma membrane. Some are formed by the host cell's nuclear membrane or Golgi apparatus.
Replication
Obligate intracellular parasites as they can only replicate, or carry out metabolic functions, within a host cell.
Viruses exist in a gray area between life-forms and chemicals, and are a sort of "borrowed life"
Replicative Cycle
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Virus uses host's enzymes, ribosomes, tRNA, amino acids, ATP, and other molecules.
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Bacteriophages (Phages)
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Replicative Cycles
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Lysogenic Cycle
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Viral DNA is incorporated into the host cell's chromosome, every time the host divides the daughter cell gets a copy of the phage DNA. Then environmental signals trigger the virus genome to switch to the lytic cycle.
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Defense Against Phages
Natural Selection favors bacterial mutants with surface protiens that cannot be recognized as receptors by a particular type of phage.
Foreign DNA can be identified, and destroyed, by restriction enzymes.
CRISPR-Cas System: Allows bacterial cells that survive infection to block attempts of reinfection by the same type of phage.
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Viral Genetic Material
Genomes
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Can have between 3 and 2,000 genes in their genome.
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Viral Diseases in Cells
Animal Cell Viruses
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Treatments
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Antiviral drugs and help, not cure, by inhibiting synthesis of viral DNA and interfering with viral assembly.
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Viruses damage or kill cells by causing the release of hydrolytic enzymes from lysosomes. Some viruses cause infected cells to produce toxins leading to the disease symptoms. Others have molecular components that are toxic; such as envelope proteins.
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Can have Single or double stranded DNA or RNA, and a Membranous envelope.
Plant Cell Viruses
Over 2,000 types of viral diseases of plants cause spots on leaves and fruits, stunted growth, and damaged flowers or roots.
Most have RNA and a helical capcid, others have a icosahedral capsid.
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Prions
Smaller, less complex entities that also cause disease.
Incorrectly folded proteins, that can be transmitted through food and act slowly and are virtually indestructible.
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