ch 19
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19.3
People completely recover from colds due to the epithelium of the respiratory tract can repair itself
Prions are smaller, less complex entities cause disease in animals
Viruses that suddenly become apparent are often referred to as emerging viruses
HIV, the AIDS virus, is an example of an emerging virus
An example of a mosquito-borne virus is called chikungunya which are packed together
Types of influenza often emerge as outbreaks of ilness
A widespread outbreak is called an epidemic
A global epidemic is a called a pandemic
Types B and C of influenza virus can only infect humans and have never causes an epidemic
Type A can infect birds, prigs, horses, and humans
Influenza A strains have causes four major flu epidemics among humans in the last 100 years
There are 16 different types of hemagglutinin, a protein that helps the flu virus attach to host cells
The first transmission of H5N1 from birds to humans was documented in Hong Kong in 1997
The many avian flu viruses carried by wild and domestic birds pose a potential long-term threat
People who have never been exposed to a particular strain before will lack immunity, and the recombinant virus recombines with viruses that circulate widely among humans
A vaccine is a harmless derivative of pathogen that stimulates the immune system to mount defenses against the harmful pathogen
19.1
A capsid is the protien shell enclosing the viral genome
The smallest viruses known have only three genes in their genome
The largest viruses have several hundred to 2,000
Bacterial genomes contain about 200 to a few thousand genes
A viral structure; viruses are made up of nucleic acid enclosed in a protein coat
Instead of being a cell, a virus is a an infectious particle consisting of nucleic acid enclosed in a protein coat, and for some viruses, surrounded membranous envelope
In the late 1800's, Martinus Beijerinck investigated the properties of the agent that causes tobacco mosaic disease (the spot disease)
In 1883, Adolf Mayer discovered that he could transmit the spot disease from plant to plant by rubbing sap extracted from diseased leaves onto healthy plants
Mayer's then suggested after an unsuccessful search for a infectious microbe in the sap, that the disease was caused by unusually small bacteria that were invisible underneath a microscope
This hypothesis was later tested by Dimitri Ivanowsky who passed sap from infected tobacco leaves through a filter to remove bacteria. Results found that the sap still produced mosaic disease - spot disease
Bacteriophages - many of the most complex capsids are found among the viruses that infect bacteria, also called phages
The first phages studied included seven that infect Escherichia coli
They were names type 1 (T1) and so forth in the order that they were discovered
T2, T4, and T6 (or the three "T-even" were very similar in structure as their capsids have elongated icosahedral heads that enclose their DNA.
Viral envelopes are derived from the membranes of the host cell are connected to the fact that some viruses have accessory structures that help them to infect their hosts
The viral envelopes contain host cell phosophilipds and membrane proteins that also contain proteins and glycoproteins of viral origin
The tiniest viruses are only 20nm in diameter which is smaller than a ribosome
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19.2
Viruses replicate only in host cells
The West Nile and equine encephalitis virus is an example of a virus with a broad host range
The measles virus is an example of a virus with a host range so narrow that it can infect only humans
A broad host range would mean that the virus would be able to infect many different types of organisms and wouldn't be limited to only animals or only humans
General features of viral replicative cycles
A viral infection begins when a virus binds to a host cell and the viral genome makes its way inside
Viral reproductive cycle
1. The virus enters the cell and is uncoated,releasing viral DNA and capsid proteins
2.Hosts enzymes replicate the viral genome
3. Meanwhile, host enzymes transcribe the viral genome into viral mRNA, which host ribosomes use to make more capsid proteins
4. Viral genomes and capsid proteins self-assemble into new virus particles which exit the cell
The mechanism of genome entry depends on the type of virus and the type of host cell
Many DNA viruses use the DNA polymerases of the host cell to synthesize new genomes along the templates provided by the viral DNA
The simplest type of viral replicative cycle ends with the exit of hundreds or thousands of viruses from the infected host cell, a process that damages or destroys the cell
Replicative Cycles of Phages
The lyctic cycle culminates in death of the host cell
A phage that replicates only by a lytic cycle is a virulent phage
Lytic cycle refers to the last stage of infection
the lysogenc cycle allows replication of the phage genome without destroying the host
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Viruses are obligate intracellular parasites that can only replicate only within a host cell
Each particular virus can infect cells of only a limited number of hosts species that are called the host range of the virus