Download Chapter 18 * genetics of viruses and bacteria

Viruses are extremely small.
Look at the size comparison with
a bacteria cell and a eukaryotic
cell! The smallest viruses are
only 20nm in diameter – smaller
than a ribosome!
Viruses are the simplest biological systems. A virus is an infectious particle
consisting of genes (DNA or RNA) packaged in a protein coat (capsid). Most
virologists would probably agree that viruses are not alive but lead “a kind of
borrowed life.”
In 1883 Adolf Mayer did extensive research
on the plant disease Tobacco Mosaic
Disease. He discovered that it could be
transmitted by rubbing sap from an infected
plant onto a healthy plant. He thought it
was caused by a really small bacteria.
Later it was found that the infectious agent
was a virus and that it could only reproduce
in the host cell – a characteristic of all
viruses.
Adolf Mayer
In 1935, Wendell Stanley crystallized the
TMV virus.
Viruses are NOT cells – they need host
cells in order to survive. There are 3 main
parts:
- Nucleic Acid (DNA or RNA)
- Capsid (protein coat surrounding the
genetic info)
- Envelope (glycoprotein membrane
around capsid; not all have this)
Genome of a Virus:
A Virus can have double stranded DNA,
double stranded RNA, single stranded
DNA, or single stranded RNA for their
genome.
Different viruses
have differently
shaped capsids
Remember that
capsids are the
PROTEIN part
of the virus.
Capsids are
made up of
subunits called
capsomeres.
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Viruses that infect bacteria are called
bacteriophages or just phages.
Viruses require a host to
reproduce (they are obligate
intracellular parasites). They
can reproduce one of two ways:
- Lytic Cycle
- Lysogenic Cycle
The viruses invade the host cell
and use the host cells materials
to build more viruses (to make
viral proteins).
Viruses attack specific tissues;
ex. influenza attacks respiratory
system; HIV virus attacks
certain types of white blood
cells
Host Range:
Each type of virus can only infect a limited number of host cells called its host range.
The viruses identify the cells like a “lock and key” fit between the proteins on the
outside of the virus and the receptor molecules on the host’s surface. Examples: West
Nile can infect mosquitoes, birds, horses, and humans; measles virus can infect only
humans.
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A viral infection begins when the genome of the virus enters the host cell.
Once inside, the viral genome commandeers its host, reprogramming the
cell to copy the viral genome and manufacture viral proteins.
The host provides nucleotides, enzymes, ribosomes, tRNAs, amino acids,
ATP, and other components for making viral proteins.
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 viral infection.
-Viruses that use this cycle are called virulent and kill the host
immediately by lysing the cell and releasing a large number
of new viruses
-Some bacteria have a defense against these viruses called
restriction nucleases (restriction enzymes)….these enzymes
cut up foreign DNA. Bacteria prevent their OWN DNA from
being digested by methylating it
The viral genomes and
viral proteins are
assembled into viruses
during this phase (not
the lysogenic!)
-Viruses that go through both the lysogenic and the lytic cycle are called temperate
phages. The lysogenic cycle does NOT kill the host immediately. Viruses can remain
dormant for several years.
-The viral DNA gets incorporated into the host cells genome and is then called a
prophage (its get copied with the host cells DNA during replication, so all the newly
made cells have the viral DNA component)
-The viral genome can exit the lysogenic cycle and go into the lytic cycle when signaled
to (stress, environmental factors such as chemicals or high energy radiation, etc)
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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.
The viral envelope is usually
derived from the host’s plasma
membrane, although some viruses
have envelopes that are derived
from the nuclear envelope of the
host.
The viruses that use RNA as the
genetic material are quite diverse,
especially those that infect animals.
RNA viruses mutate more easily than
DNA viruses because they lack
proofreading enzymes.
RNA viruses are very diverse; several
different classes.
Retroviruses (Class VI) – RNA gets
converted into DNA using the enzyme
reverse transcriptase; RNA polymerase
then transcribes the DNA into RNA
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Retroviruses have the most complicated
replicative cycles.
Retroviruses are single stranded RNA viruses
that can act as a template for DNA
Retroviruses carry an enzyme called reverse
transcriptase that transcribes DNA from an RNA
template. This provides RNA  DNA
information flow.
 Viruses that would most likely have reverse
transcriptase are RNA viruses that do the
lysogenic cycle.
 Viral genomes can encode reverse
transcriptase, so it can be used in host cells
even if it was not in the host cell originally
Human immunodeficiency virus (HIV), the
virus that causes AIDS (acquired
immunodeficiency syndrome), is a retrovirus.
 After HIV enters the host cell, reverse
transcriptase molecules are released into the
cytoplasm and catalyze the synthesis of viral
DNA.
The newly made viral DNA enters the cell’s
nucleus and is inserted as a permanent provirus
into a chromosome. (Provirus  viral genome
that is permanently inserted into a host genome)
Theory of How Viruses Came
About:
Researchers believe that viruses
developed from plasmids or transposons.
They arose AFTER cells and came from
fragments of DNA or RNA that could
move from one cell to another.
Plasmid → an extra piece of DNA
that is separate from chromosomes;
they can copy independently and
can be transferred from one cell to
another; common in bacteria, but
are also found in eukaryotic cells
(Ex. yeast cells)
Transposons → pieces of DNA that can move
from one place in the genome to another
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A viral infection can produce
symptoms by a number of different
routes.
 Some viruses damage or kill cells
by triggering the release of
hydrolytic enzymes from
lysosomes.
 Some viruses cause the infected
cell to produce toxins that lead to
disease symptoms.
 Others viruses have molecular
components, such as envelope
proteins, 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
polio).
Many of the temporary symptoms
associated with a viral infection, such
as fever and aches, result from the
body’s own efforts at defending itself
against infection.
Modern medicine has developed
vaccines to mount defenses against
the actual pathogen.
Cow Pox
Vaccines are dead or harmless forms of the
pathogen that are exposed to the body. The
body makes antibodies to fight it off, and then if
the person is ever exposed to the real pathogen,
your body already has the materials to fight it
off. Vaccines can prevent the onset of viral
infections in humans.
Cow Pox Virus
Edward JennerFather of Immunology
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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 nucleosides
and interfere with viral
nucleic acid synthesis
(they interfere with viral
replication).
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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 mutation of existing viruses is a major source of
new viral diseases.
 RNA viruses tend to have high mutation rates because
replication of their nucleic acid lacks proofreading.
 Second, a viral disease can spread from a small, isolated
population to become a widespread epidemic.
 HIV/AIDS is considered an emerging virus because it
was virtually unnoticed for decades before spreading
around the world and becoming widespread and
apparent in the 1980’s.
 A third source of new human viral disease is the spread of
existing viruses from other animals (bird flu, swine flu).
Viruses can also spread in plants via
plasmodesmata.
Most plant viruses stunt plant growth and decrease their yield (think crops).
Usually they are RNA viruses. They can be transmitted one of two ways:
- Horizontal Transmission (external source)  insects are major carriers of
viruses; also farming tools
- Vertical Transmission (inherited)  this can occur in asexual propagation
or in infected seeds
Viroids are infectious nucleic acids that
infect PLANTS. They are tiny molecules
of naked, circular RNA  they do NOT
have capsids. They show that a single
MOLECULE can act as an infectious agent
and spread disease. They can’t encode
their own proteins, but they can interfere
with the normal metabolism of the plant
to hamper the growth of the infected
plant.
Normal
Plant
Plant with a
Viroid
Symptoms of viroids
Prions are infectious proteins. They are
misfolded versions of normal proteins.
They can convert normal proteins to the
prion (misfolded) versions. They usually
attack proteins that are present in brain
tissue. The prion aggregation interferes
with normal cellular functions and causes
disease symptoms.
Ex. Mad Cow Disease (degenerative
brain disease) and Scrapie in sheep
Viroids and
Prions are the
simplest
infectious agents!
Viruses can convert normal cells (proto-oncogenes)
into cancer cells (oncogenes). The results in the cell
cycle being de-regulated and protein production
can be altered. Tumor formation follows shortly
after
Examples of viruses that can cause cancer:
- Retroviruses
- Adenovirus (lung cancer)
- Papovavirus (HPV)
Proto-oncogene being converted into an
oncogene
HPV virus attacking epithelial cells