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