Chapter 19: Viruses (Concept 19.2 Viruses replicate only in host cells…
Chapter 19: Viruses
Concept 19.1 A Virus consists of a Nucleic Acid surrounded by a protein coat
The Discovery of Viruses: Scientific Inquiry
In 1883, Adolf Mayer, discovered that he could transmit disease from plant to plant by rubbing sap extracted from diseased leaves onto healthy plants.
Mayer suggested that the disease was caused by unusually small bacteria; unsuccessful search for infection.
After filtration, Ivanowsky concluded the sap still produced mosaic disease.
1935, TMV: Tobacco Mosaic Virus discovered
Beijerinck concluded that the infectious agent in the filter sap could replicate.
Fact: the pathogen replicated only within the host it infected.
Beijerinck was the first scientist credited with the concept of a virus.
Agent of mosaic disease could not be cultivated on nutrient media in test tubes or petri dishes.
What causes tobacco mosaic disease?
1) Extracted sap from tobacco plant with tobacco mosaic disease
2) Passed sap through a porcelain filter known to trap bacteria
3) Rubbed filtered sap on healthy tobacco plants
3) Healthy plants became infected
Results: Healthy plants became infected when filtered sap was rubbed onto them. Their sap, extracted and filtered, could then act as a source of infection to another group of plants.
Conclusion: The infectious agent was not a bacterium b/c it could pass through a bacterium-trapping filter. The pathogen must have been replicating inside the plants b/c the ability to cause disease undiluted after several transfers from plant-to-plant.
Structure of Viruses
Tiniest Viruses smaller than a ribosome: only 20
Unlike simplest of cells, viruses could be crystallized.
The structure of viruses reveals that it consists of nucleic acid enclosed in a protein coat, and some are surrounded by a membranous envelope.
A virus is called DNA virus or an RNA virus based on the kind of nucleic acid that makes up its genome.
Genome may be single linear or circular molecule of nucleic acid, or multiple molecules of nucleic acid.
The smallest virus known have only 3 genes in their genomes, while the largest have a 100 to 2,000.
Capsids and Envelopes
Protein shell enclosing the viral genome.
may be rod-shaped, polyhedral, or more complex in shape.
built from a long number of protein subunits called
, but the number of different
of proteins in a capsid is small.
TMV has a rigid, rod-shaped capsid made from over 1,000 molecules of a single type of protein arranged in a helix; known as
icosahedron: protein molecules arranged in a polyhedral capsid with 20 triangular facets; referred to as
Host cell phospholipids
Proteins and glycoproteins of vital origin (glycoproteins proteins and carbs covalently attached).
Most complex capsids found among viruses that infect bacteria; can be known as
First phages included 7 that infect
T1-T7; The three "T-even" phages (T2, T4 & T6) turned out similar in structure
Capsids were elongated icosahedral heads enclosing their DNA.
Attached to the head is a protein tail piece with fibers attached to a bacterial cell.
Concept 19.3 Viruses & prions are formidable pathogens in animals and plants
Viral Diseases in Animals
the immune system is a critical part of the body's natural defenses
are harmless derivatives of a pathogen that stimulates the immune system to defend against another harmful pathogen.
Ex: Smallpox virus
Antibiotics cannot kill viruses; cannot inhibit enzymes into a virus specific site.
Ex: Acyclovir can only impede a herpesvirus from replicating but cannot cure it.
West Nile Virus (1999)
Ebola Virus (1976)
Chikungunya (mosquito-borne virus) - causes acute illness, fever, rashes, and persistent joint pain.
Zika Virus (mosquito-borne) - outbreak occurred in spring 2015 in Brazil; causes microcephaly in newborn babies
Zika & Ebola declared as international health emergencies.
are widespread outbreaks
Ex: The Flu; also known as H1N1.
are global epidemics
Viral Diseases in Plants
There are over 2,000 types of viral diseases of plants that estimate $15 billion worldwide to destruct.
Common Signs of viral infection: bleached or brown spots on leaves/fruits, stunted growth, & damaged flowers or roots.
Same basic structure and mode replication as animal viruses.
1) Horizontal Transmission: a plant is infected from an external source of the virus.
insects may transmit viruses from plant to plant.
plants inherits a viral infection from a parent.
can occur in asexual propagation
can occur in sexual reproduction via infected seeds.
cytoplasmic connections that penetrate the walls between adjacent plant cells.
Prions: Proteins as Infectious Agents
proteins that cause a number of degenerative brain diseases in various animal species.
misfolded versions of normal brain proteins
it may induce normal protein to assume the abnormal shape
chain reactions may continue until high levels of prions cause cell malfunction or brain degeneration
Ex of prions: Scrapie in sheep; mad cow disease
may be transmitted in food.
Spread by ritual cannibalism, a widespread practice among South Fore natives.
Stanley Prusiner recently proposed that prions are involved in neurodegenerative diseases such as Alzheimer's & Parkinson's disease.
Two characteristics: 1) Prions act very slowly (incubation period of at least 10 years)
2)Prions are virtually indestructible (no known cure)
Concept 19.2 Viruses replicate only in host cells
they can replicate only in a host cell
viruses in isolation are packaged sets of genes in transit form one host cell to another
A virus can infect cells of only a limited number of host species
Viruses only bind to host cells by "lock-and-key" fit
bind only to viral surface proteins & specific receptors on outer cell.
Ex: West Nile Virus can infect mosquitoes, birds,horses and humans.
Ex: Cold Viruses only infect Upper Respiratory Tract
General Features of Viral Replicative Cycles
1) The Virus enters the cell, uncoated, releasing viral DNA and capsid proteins.
2) Host 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 & capsid proteins self-assemble into new virus particles and exit the cell.
Replicative Cycles of Phages
The Lytic Cycle
phage replicative cycle that culminates in death of the host cell.
refers to the last stage of infection; bacterium breaks open & releases the phages that were produced within the cell.
replicates only by a lytic cycle
kills the host cell
The Lysogenic Cycle
Allows replication of the phage genome without destroying the host.
are capable of using both modes of replicating with a bacterium
Ex: Lambda : widely used in biological research
Lambda DNA molecule forms a cycle: lytic or lysogenic
Lytic: lambda turn the host cell in a lambda producing factory, and cell lyses and releases its virus progeny.
Lysogenic: lambda DNA molecule is incorporating into a specific site on the
chromosome by viral proteins that break circular DNA molecules and join them together.
known as a
, viral DNA integrated into bacterial chromosome in this fashion.
Bacterial Defenses Against Phages
1) Natural selection favors bacterial mutants w/ surface proteins no longer recognized as receptors by a particular type of phage.
2) When phage DNA enter a bacterium, the DNA is identified as foreign & cut up by cellular enzymes, called
, restricting the phage's ability to replicate.
A bacterium's DNA is methylated for protection to prevent attack by its own restriction enzymes.
Replicative Cycles of Animal Viruses
An animal virus uses its membranous outer layer to enter the host cell.
Outer Surface contain glycoproteins that bind to specific receptors of host cell's surface.
2) The capsid and viral genome enter the cell. Digestion of the capsid by cellular enzymes releases the viral genome.
3) The viral genome functions as a template for synthesis of complementary RNA strands by a viral RNA polymerase.
6) Vesicles transport envelope glycoproteins to plasma membrane.
5) Complementary RNA strands also function as mRNA which is translated into capsid proteins (inside cytosol) & glycoproteins for the viral envelope (inside ER & Golgi apparatus).
4) New copies of viral genome RNA are made using the complementary RNA strands as templates.
1) Glycoproteins on the viral envelope bind to specific receptor molecules on the host cell, promoting viral uptake by the cell.
8) Ea. new virus buds from the cell, its envelope studded w/ viral glycoproteins embedded in membrane derived from the host cell.
7) A Capsid assembles around each viral genome.
Viral Genetic Material
- most complicate replicative cycles in RNA animal viruses.
enzyme that transcribes an RNA template into DNA, RNA --> DNA info flow that is the opposite of usual direction.
human immunodeficiency virus is a retrovirus
contains two identical molecules of single-RNA & two molecules of reverse transcriptase.
is a viral DNA that never leaves the host's genome, remaining permanent.