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Chapter 19
Viruses (Replicative cycles of animal viruses (Viral envelopes…
Chapter 19
Viruses
A borrowed life
Compared to eukaryotic and prokaryotic cells, viruses are much smaller and simpler in structure.
(They lack structures and metabolic machinery found in a cell).
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Structure of viruses
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Viral Genomes
We usually think of genes as being made of double-stranded DNA, but many viruses defy this convention.
Their genomes may consist of double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA, depending on the type of virus.
A virus is called a DNA virus or an RNA virus, based on the kind of nucleic acid that makes up its genome.
The genome is usually organized as a single linear or circular molecule of nucleic acid, although the genomes of some viruses consist of multiple molecules of nucleic acid.
Viruses can have between three and 2,000 genes in their genome.
Capsids and Envelopes
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Some viruses have accessory structures that help them infect their host.
(An example, a membrane envelope surrounding the capsids of influenza viruses and many other viruses found in animals.
Viral envelopes (derived from membranes of host cells) surround the capsids of influenza viruses and many other viruses found in animals
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Viruses, viroids, and prions are formidable pathogens in animals and plants
Emerging Viruses
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The Ebola virus is one of several emerging viruses that cause hemorrhagic fever, an often fatal illness
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In 2009, a general outbreak (epidemic) of a flu-like illness appeared in Mexico and the United States, caused by an influenza virus named H1N1
A global epidemic is called a pandemic
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Mutations
Changes in host behavior or the environment can increase the spread of viruses responsible for emerging diseases
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It is possible that global climate change may allow mosquitoes that carry viruses to expand their range
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The lytic cycle of phage T4, a virulent phage
- Attachment: The T4 phage uses its tail fibers to bind to specific surface proteins on an E.coli cell that acts as receptors.
- Release:. The phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. The cell swells and finally bursts, releasing 100 to 200 phage particles.
- Synthesis of viral genomes and proteins. The phage DNA directs production of phage proteins and copies of the phage genome by host and viral enzymes, using components within the cell.
- Entry of phage DNA and degradation of host DNA. The steath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. The cells DNA is hydrolyzed.
- Self -assembly. Three sperate sets of proteins self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms.
Results: When the filtered sap was rubbed on healthy plants, they became infected. Their sap, extracted and filtered, could then act as a source of infection for another group of plants. Each successive group of plants developed the disease to the same extent as earlier groups.
Conclusion: The infectious agent was apparently not a bacterium because it could pass through a bacterium-rapping filter. The pathogen must have been replicating in the plants because it ability to cause disease was undiluted after several transfers from plant to plant.
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Diseases causes d by viral infections afflict humans, agriculture crops, and livestock worldwide.
Other smaller, less complex entities known as viroids and prions also cause disease in plants and animals.