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Chapter 19 (Viruses and prions are formidable pathogens in animals and…
Chapter 19
Viruses and prions are formidable pathogens in animals and plants
(19.3)
an epidemic is a widespread outbreak of a disease, can become a pandemic, a global epidemic
outbreaks of emerging viral diseases in humans are usually not new, but rather are caused by existing viruses that expand their host territory
symptoms of viral diseases may be caused by direct viral harm to cells or by the body's immune response
Vaccines: stimulate the immune system to defend the host against specific viruses
viruses enter plant cells through damaged cell walls (horizontal transmission) or are inherited from a parent (vertical transmission)
Prions
slow acting, virtually indestructible infectious proteins that cause brain diseases in mammals
A virus consists of a nucleic acid surrounded by a protein coat
(19.1)
viruses: an infectious particle consisting of little more than genes packaged in a protein coat
examples of viruses:
Hepatitis A
Hepatitis B
Influenza virus
Rabies
Polio
Viruses can be considered to live a "borrowed life" as viruses can only reproduce and carry out metabolic activities in the host cell
structure of viruses:
capsid: the protein shell inclosing the viral genome
adenoviruses have an icosahedral capsid with a protein spike at each vertex (corner). The capsid, made up of capsomers, contains double-stranded DNA.
tobacco mosaic virus has a helical capsid surrounding a single helical RNA molecule, with the overall shape of a rigid rod
viral envelope: a membrane, derived from membranes of the host cell
Influenza virus have an outer envelope, studded with glycoprotein spikes, that contains eight double-helical RNA-protein complexes, each associated with a viral polymerase
The discovery of viruses: scientific inquiry
experiment:
in the late 1800's, Martinus Beijerinck, of the Technical School in Delft, the Netherlands, investigated the properties of the agent that causes tobacco mosaic disease
passed zap through a porcelain filter known to trap bacteria
rubbed filter on healthy Tobacco plants
He extracted sap from tobacco plant with tobacco mosaic disease
healthy plants became infected
Results:
When the filtered sap was rubbed on healthy plants, they became infected. Their zap, extracted and filtered, could then act as a source of infection for another group of plants
main idea:
Researchers discovered viruses in the late 1800s by studying a plant disease, tobacco mosaic disease
Conclusion:
The infectious agent was apparently not a bacterium because it could pass through a bacterium-trapping filter. This pathogen must have been replicating in plants because its ability to cause disease was undiluted after several transfers from plant to plant
phages: a virus that affects bacteria
bacteriophages have a complex capsid consisting of an icosahedral head and a tail apparatus.
Viruses replicate only in host cells
(19.2)
Phages
virulent phage (a phage that replicates only by a lytic cycle)
Lytic cycle
production of new phages
lysis of host cell causes release of progeny pages
Destruction of host DNA
temperate phage (phages capable of using both modes of replicating within a bacterium)
Lysogenic cycle
is replicated and passed on to daughter cells and
can be induced to leave the chromosome and initiate a lytic cycle
Genome integrates into bacterial chromosomes as prophage
Bacteria have various ways of defending themselves against phage infections, including the CRISPR- Cas system
The RNA transcript is processed into short RNA strands. Here we focus on RNA that is complementary to the DNA of the invading page
Each short RNA strand binds to a Cas protein, forming a complex
infection by phage triggers transcription of the CRISPR region of the bacterial DNA. This region consists of DNA from phages that previously infected the cell, separated by repeats
Complementary RNA binds to DNA from the invading phage. The Cas protein then cuts the phage DNA
After being cut, the entire phage DNA molecule is degraded and can no longer be replicated
host range: each particular virus can infect cells of only a limited number of host species called the host range
A simplified viral replicative cycle
Host enzymes replicate the viral genome
Meanwhile, host enzymes transcribe the viral genome into viral mRNA, which host ribosomes use to make more capsid proteins
the viral enters the cell and is uncoated, releasing viral DNA and capsid proteins
Viral genomes and capsid proteins self-assemble into new virus particles, which exit the cell
viruses use enzymes, ribosomes, and small molecules of host cells to synthesize progeny viruses during replication
each type of virus has a characteristic host range, affected by whether the cell-surface proteins are present that viral surface proteins can bind to
Many animal viruses have an envelope. Retroviruses (such as HIV) use the enzyme reverse transcriptase to copy their RNA genome into DNA, which can be integrated into the host genome as provirus
The double-stranded DNA is incorporated as a provirus into te cell's DNA
Proviral genes are transcribed into RNA molecules, which serve as genomes for progeny viruses, and as mRNAs for translation into viral protein
Reverse transcriptase catalyzes the synthesis of a second DNA strand complementary to the first
The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER)
Reverse transcriptase catalyzes the synthesis of a DNA strand complementary to the viral RNA
Vesicles transport the glycoproteins to the cell's plasma membrane
The virus fuses with the cell's plasma membrane. The capsid proteins are removed, releasing the viral proteins and RNA
Capsids are assembled around viral genomes and reverse transcriptase molecules
the envelope glycoproteins enable the virus to bind to specific receptors on certain white blood cells
New viruses, with viral envelope glycoproteins, bud from the host cell