Chapter 19  Viruses
Student Guided Notes
Overview: A Borrowed Life

Viruses are the simplest biological systems.

A virus is an infectious particle consisting of __________________ (DNA or RNA) packaged in a
_________________________________.
○ Most virologists would probably agree that viruses are not alive but lead “___________________
_________________________________________.”
Concept 19.1 A virus consists of a nucleic acid surrounded by a protein coat
Researchers discovered viruses by studying a plant disease.

The story of how viruses were discovered begins in 1883 with _____________________________
research on the cause of tobacco mosaic disease.
○ This disease stunts tobacco plant growth and mottles plant leaves.

Mayer learned that the disease was infectious when he found that he could transmit the disease by ____
___________________________________________________________________________________.
○ He concluded that the disease must be caused by an ______________________________________.

In 1935, Wendell Stanley ____________________________ the pathogen, the ____________________
___________________________________________ (TMV).
A virus is a genome enclosed in a protective coat.

Viruses are not cells, however, but infectious particles consisting of ____________________________
encased in a _________________________________ and, in some cases, a ______________________.

The tiniest viruses are only ______________ in diameter—smaller than a ________________________.

The genome of viruses may consist of ____________________________________, _______________
___________________, __________________________________, or __________________________.

The capsid is the _____________________________________________________________________.

Capsids are built of a large number of protein subunits called _____________________________.
○ Viruses have different shapes including: helical (rod shaped), and icosahedral (20 triangular
facets.)
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
A membranous envelope surrounds the capsids of flu viruses. Only SOME viruses have an envelope.
○ These viral envelopes are derived from the ____________________________ of the host cell.

The most complex capsids are found in viruses that infect bacteria, called ________________________
or _______________________.
Concept 19.2 Viruses replicate only in _________________________

Lacking metabolic enzymes, ribosomes, and other equipment for making proteins, viruses are obligate
____________________________________________________ (meaning they can _______________
___________________________________________________________.)

Each type of virus can infect and parasitize only a limited range of host cells, called its _____________.
○ Viruses recognize host cells by a “________________________” fit between proteins on the outside
of the virus and __________________________________________________ on the host’s surface.
○ Some viruses have a broad enough host range to infect several species, while others infect only a
single species. Examples: West Nile can infect mosquitoes, _______________, ________________,
and humans; _______________________ virus can infect only humans.

Most viruses of eukaryotes attack specific tissues.
○ Human cold viruses infect only the cells _______________________________________________.
○ The AIDS virus binds to only certain types of white blood cells.
Viral replicative cycles have characteristic general features.

A viral infection begins when ___________________________________________________________.

Once inside, the viral genome commandeers its host, reprogramming the cell to ___________________
___________________________________________________________________________________.

The host provides nucleotides, enzymes, ribosomes, ____________________, amino acids, _________,
and other components for making ____________________________.

The simplest type of viral replicative cycle ends with the exit of viruses from the infected host cell, a
process that usually damages or destroys the host cell.
○ This cellular damage and death cause many of the symptoms associated with __________________.
Phages replicate using lytic or lysogenic cycles.

In the lytic cycle, the reproduction of the virus causes ________________________________________.
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○ In the last stage, the bacterium lyses (breaks open) and releases the phages produced within the cell
to infect others.
○ Each of these new phages (viruses) can infect a _____________________ cell.
○ _______________________________ phages replicate only by a lytic cycle.
○ While phages have the potential to wipe out a bacterial colony in hours, bacteria have defenses
against phages.
○ Bacteria produce ______________________________________ that recognize and cut up
foreign DNA, including phage DNA. Their activity restricts the ability of the phage to infect
the bacterium.
○ Methylation of the bacteria’s own DNA prevents its destruction by restriction enzymes.

In the ________________________ cycle, the phage genome replicates without destroying the host cell.
○ ____________________________ phages use both lytic and lysogenic cycles.
Process of how a virus infects a bacterium:

Infection of an E. coli cell by a phage begins when the phage __________________________________
__________________________________________ and injects its DNA.

During a lytic cycle, the viral genes turn the host cell into a virus-producing factory, and the cell ______
___________________________________________________________________________________.

During a lysogenic cycle, the viral DNA molecule is incorporated into a specific site on the E. coli
chromosome as a ____________________________ (viral DNA that has incorporated itself into the
bacterial genome).

Every time the host divides, it copies the phage DNA and passes the copies to daughter cells.
○
The viruses thus propagate _______________________________________________________
___________________________________________________________________________.

The term lysogenic implies that ______________________________________________________
____________________________________________________________________________.
○ That happens when the viral genome exits the bacterial chromosome and initiates a __________
cycle.

The switch from the lysogenic to lytic mode may be triggered by an _____________________________
signal such as certain chemicals or ________________________________________________.
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Animal viruses are diverse in their modes of infection and replication.

One key variable is the type of nucleic acid (DNA or RNA) that serves as a virus’s genetic material.

Another variable is the presence or absence of a membranous envelope derived from the host cell
membrane.

Viruses equipped with an outer envelope use it to enter the host cell.
○ __________________________ on the envelope bind to specific receptors on the host’s membrane.

The viral envelope is derived from the host’s __________________________________, although viral
genes specify some of the molecules in the membrane.

These enveloped viruses do not necessarily _______________ the host cell.

Some viruses have envelopes that are not derived from plasma membrane, for example the envelope
of the herpesvirus is derived from the ______________________________________ of the host.

The viruses that use RNA as the genetic material are quite diverse, especially those that infect animals.

______________________________________ have the most complicated replicative cycles.
○ Retroviruses carry an enzyme called ____________________________________________ that
transcribes DNA from an RNA template. This provides _____________________ information flow.

__________________________________________________________ (HIV), the virus that causes
______________________ (acquired immunodeficiency syndrome), is a retrovirus.

After HIV enters the host cell, reverse transcriptase molecules are released into the _____________
and catalyze the synthesis of ________________________.
o The newly made viral DNA enters the cell’s nucleus and is inserted as a permanent
________________________ into a chromosome. (Provirus  viral genome that is
permanently inserted into a host genome)
Viruses may have evolved from other mobile genetic elements.

Viruses do not fit our definition of living organisms.

Candidates for the original sources of viral genomes include _________________________ and
__________________________________.
○ Plasmids, found in bacteria and in eukaryote yeast, are __________________________________
____________________________ that are separate from chromosomes, can replicate independently
of the rest of the cell, and are occasionally ______________________________________________.
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○ Transposons are __________________________________________________________________
________________________________________________________________________________.

Plasmids, transposons, and viruses are all __________________________________________________
(able to move within the genome).
Concept 19.3 Viruses, viroids, and prions are formidable pathogens in animals and plants

A viral infection can produce symptoms by a number of different routes.
○ Some viruses damage or kill cells by triggering the release of _____________________________
___________________________ from _______________________________.
○ Some viruses cause the infected cell to produce ________________ that lead to disease symptoms.
○ Others viruses have ________________________________________, such as _______________
______________________________, that are toxic.

In some cases, viral damage is easily repaired (respiratory epithelium after a cold), but in others,
infection causes permanent damage (nerve cells after ________________________).

Many of the temporary symptoms associated with a viral infection, such as fever and aches, result from
___________________________________________________________________________________.

Modern medicine has developed vaccines, ________________________________________________
____________________________________________________________________________________
___________________________________________________________________________________.

Antibiotics, which can kill bacteria by inhibiting enzymes or processes specific to bacteria, are
powerless against viruses, which have few or no enzymes of their own.
○ Most antiviral drugs resemble _______________________________ and interfere with
________________________________________________________.
New viral diseases are emerging.

In recent years, several emerging viruses have risen to prominence including: HIV, Ebola, and West
Nile .

The emergence of these new viral diseases is due to three processes:

First, the ________________________________________ is a major source of new viral diseases.
○ RNA viruses tend to have high _______________________________ rates because
replication of their nucleic acid lacks ___________________________________.
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
Second, a viral disease can spread from a ___________________________________ population
to become a widespread epidemic.
○ __________________ was virtually unnoticed for ______________________ before
spreading around the world.

A third source of new human viral disease is the spread of existing viruses from _______________.
Plant viruses are serious agricultural pests.
Plant viruses can stunt plant growth and diminish crop yields.

Plant viral diseases are spread by two major routes:

In ______________________________ transmission, a plant is infected with the virus by an
_______________________________________.
○ Insects are often carriers of viruses, transmitting disease from plant to plant.

In vertical transmission, a plant ________________________________________________________
____________________________________________________.
○ This may occur by ______________________________________________ or in sexual replication
via ____________________________________.
__________________________ and ___________________________ are the simplest infectious agents.

_________________________________, smaller and simpler than even viruses, consist of tiny
molecules of naked circular RNA that infect ______________________.
o Viroids show that a single molecule can act as an infectious agent to spread disease.

________________________ are infectious ________________________ that spread disease.
o They appear to cause several degenerative brain diseases, including ___________________ in
sheep, _____________________________ disease, and __________________________________
disease in humans.
○ According to the leading hypothesis, a prion is a ________________ form of a normal brain protein.
○ When the prion gets into a cell with the normal form of the protein, the prion can _______________
_____________________________________, creating a chain reaction that increases their numbers.
○
Prion aggregation interferes with ____________________________ and causes disease symptoms.
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