¨ virus
means poison; someone once called them "a piece of bad news wrapped in a
protein;"
¨ obligate
intracellular parasites
(can reproduce/replicate only inside a host cell)
¨ not
cells; debate over whether or not they are considered “alive” (see below)
¨ consist
of nucleic acids (DNA or RNA) in a protein coat, called a capsid (no cell membrane)
¨ they
insert themselves into a host cell & direct the host cell's metabolic machinery to
make more virus; the virus supplies information (the plan) in the form of its nucleic acid
- raw materials and driving force (ATP & reducing power) are supplied by the host
cell.
¨ all
cellular organisms can be attacked by viruses; however, viruses are very specific for the
organisms & cells they infect.
Are
Viruses Alive?
¨ Characteristics
of living things: reproduction, metabolism,
organized as cells, contain all organic molecules (lipids, enzymes, nucleic acids, carbs),
evolution & adaptation to changing environments.
¨ Viruses
have some of these char's: they can evolve, they contain some macromolecules, they direct
their own reproduction; However, they are not cells - they do not have cytoplasm, a cell
membrane, organelles, ribosomes, or a nucleus. They
have DNA or RNA, unlike prokaryotic and eukaryotic cells, which have both. In addition, they lack a metabolism of their own
(they cannot produce ATP, etc.) - raw materials and driving force (ATP & reducing
power) are supplied by the host cell.
I. HOW ARE THEY CLASSIFIED? (4
ways: size, structure, host range, life cycles)
A. Size
- range from about 1/10th to 1/3rd the size of a small bacterial cell.
B. Structure
- basic structure of a virus is a nucleic acid surrounded by a protein capsid; a membrane
envelope may also be present outside of the capsid, but this is acquired from host cell. A complete viral particle (= capsid + nucleic acid
+ envelope if it is present) is called a virion.
1. Nucleic
Acid
- Viruses can store their genetic info. in different types of nucleic acid (each virus has
only type). Viruses can have DNA or RNA. Their
nucleic acid can be double stranded (ds) or single stranded (ss); they can even have
double stranded RNA! RNA viruses can have a
(-) sense strand or a (+) sense strand of RNA.
(+) sense RNA acts like mRNA and can be translated
into proteins by the host cell’s ribosomes. (-)
sense RNA does not make sense to the host cell’s ribosomes. After the virus containing this type of RNA enters
the host cell, a complementary (+) sense strand is made from its (-) sense strand. Only (+) sense strand RNA can be read by the host
cell’s ribosomes!
2. Capsids
- protein coat that surrounds the nucleic acid; the constituent protein molecules making
up the capsids are called capsomeres; there are
3 basic shapes based on how the capsomeres are arranged.
See diagrams of these shapes!!
a.
helical
- proteins fit together as a spiral to form a rod-shaped structure.
b. polyhedral
- proteins are arranged in equilateral triangles that fit together to form a geodesic
dome-shaped structure; some appear almost spherical; you may have seen architectural
structures that have this shape.
c. complex - combination viruses with a helical portion (tail) attached to a polyhedral portion (head); ex. many bacteriophages; may also have a tail sheath (participates in injecting the viral nucleic acid into the host cell), plate, pins, & tail fibers (help virus attach to host cell).
3. Viral
Envelopes -
pieces of the host cell's cell membrane that the virus acquires as it emerges from its
host cell; the virus pushes out of the cell membrane, forming a bud that encloses the
virus - then the bud pinches off behind, resealing the cell - as a result the host cell is
not lysed. Glycoprotein spikes from the host
cell’s glycocalyx may stick out of the envelope.
Viruses that lack envelopes are called naked
viruses. Because envelopes are acquired
from host’s cell membranes, viruses may be hidden from attach by the host’s
immune system. Envelopes also help viruses
infect new cells by fusion of the envelope with the host’s cell membrane. On the other hand, enveloped viruses are damaged
easily by physical and chemical antimicrobial agents.
C. Host
Range -
defined as the spectrum of organisms a virus attacks; viruses exhibit considerable
specificity for hosts and even cells within that host; viral specificity is determined by
whether or not a virus can attach to a cell. Attachment
depends on the presence specific receptor sites
on the surface of host cell and on specific attachment structures on the viral capsid or
envelope. Examples of receptor sites
are proteins, LPS’s, glycolipids, or glycoproteins.
D. Life
Cycles
of Bacteriophages (viruses that infect bacteria
– means “bacteria eating”)
1. Replication [= Lytic Cycle] See diagram
Events:
a.
Adsorption
- the virion attaches itself to a specific receptor site on the surface of the host cell.
b.
Penetration - the viral nucleic acid penetrates the host cell
c.
Uncoating - removing the capsid & envelope; basically 2 ways it can happen:
1.)
during
penetration, the virion disassembles so that only the viral nucleic acid enters host cell
2.)
the entire virion enters the host cell & uncoating occurs later
d. Viral Synthesis (Latent Period) (also called biosynthesis) - more
viral components
(nucleic
acids & proteins for capsids) are synthesized by the host cell.
d. Maturation (Assembly) - components are assembled into new
viruses
1.) If
the virus is of the naked type, an encoded protein, lysozyme, dissolves the cell
membrane &/or cell wall of the host cell, causing the cell to lyse & releasing the
hundreds of viruses inside it.
2.) If
the virus is to be an enveloped virus, it pushes out the cell membrane, forming a bud that
encloses the virus - then the bud pinches off behind, resealing the host cell; as a result
the host cell is not lysed.
2. Lysogenic
Cycle (Lysogeny or Temperance) - Temperence involves
the capacity of certain viruses to set up long-term relationships with their host cells -
the virus remains latent for many cellular generations by becoming integrated into a host
cell's chromosome (the integrated viral DNA is called a prophage). In
this case no new viral components are synthesized & the host cell is not harmed. The virus may remain latent for long periods of
time before initiating a lytic cycle. The
problem with this type of cycle is that the viral nucleic acid that becomes integrated
into the host cell's chromosome gets replicated along with the host cell's chromosome and
is passed to daughter cells during cell division. In
the prophage state, some viral genes are expressed, which may slightly change the host
cell's phenotype (ex. only lysogenic strains of Corynebacterium
diphtheriae cause the disease diphtheria
because the disease-causing toxin is encoded in the prophage of the infecting virus). Something (ex. temperature change) may trigger prophages to go into the lytic cycle. Released virions cannot infect cells that are
carrying the same prophage - it makes the cell immune to attack by a virion of the same
phage.
¨ Family
names all end in viridae ; family names are
often converted into English (ex. Retroviridae
are called retroviruses). Genus names end in virus - species names are English words.
Ex.
Retroviridae, Lentivirus, Human Immunodeficiency Virus (HIV)
¨ Groupings
reflect only common characteristics and are not intended to represent evolutionary
relationships.
A. Cultivating Animal Viruses & Diagnosing Viral
Illnesses
¨ At
one time animal viruses had to be cultivated & counted by infecting animals.
¨ In
the 1930's it was discovered that embryonated
chicken eggs could be used to culture animal viruses; embryonated eggs are inoculated
with dilutions of a virus sample to determine the highest dilution that kills the embryo;
this procedure was more economical & efficient than using adult animals.
¨ In
the 1950's cell culture & tissue culture methods were developed. This solved the problem of viral specificity. Ex. Before cell cultures it was impossible to
culture viruses in mice or chicken eggs that only infected humans (ex. HIV); continuous cell lines are usually derived from cancerous tissue &
grow indefinitely in culture; regular cell lines grow increasingly slowly after 20-30
subcultures & eventually lose their ability to support viral replication; the most
famous c.c.l. is the HeLa cell line (named
after Helen Lack, the donor - from cervical cancer).
¨ Important
Note: Physicians rely on symptoms to diagnose most viral
illnesses. Culturing viruses takes too long
& antibodies in the blood can usually be detected only after patient has recovered.
¨ Viral
infections sometimes affect human cells in ways that can be seen under the microscope. For ex. the measles virus causes the membranes of
neighboring cells to fuse, creating giant, multinucleated cells. Some virus-infected cells can be id. because they
contain inclusion bodies, collections of viral
components such as capsids and nucleic acid, waiting to be assembled into new viral
particles. For example, the rabies virus
produces inclusion bodies called negribodies in
infected nerve cells (this is what we look for in suspected cases of rabid animals - have
to look for negribodies in brain - animals have to be euthanized).
B. Replication of Animal Viruses
(Lytic cycle) - proceeds through similar
stages as bacteriophage replication.
1. Adsorption
- Proteins in cell membrane act as receptor sites for a virus; remember, no cell walls in
animal cells; adsorption is largely responsible for tissue specificity of animal viruses -
only cells with a complementary receptor are attacked by a particular virus.
2. Penetration
can occur in 3 ways:
a. viral
envelope fuses with cell membrane, emptying the rest of the virion inside the cell.
b. other
enveloped viruses enter by being phagocytized by a host cell
c. most
naked animal viruses enter as most bacteriophages do - the capsid adsorbs to cell surface
& only the viral nucleic acid enters cell.
3. Uncoating
- Envelopes/capsids are often removed in the penetration process; viruses that enter the
cell partially or completely intact are uncoated inside the cell by the host cell's own
hydrolytic enzymes, sometimes those in its lysosomes.
4. Viral
Synthesis
- The specifics of this process depend on which of the 5 types of nucleic acids is present
in the virus.
5. Maturation
- Assembly not really understood
6. Release
- Enzymes cause lysis of the host cell or viruses "bud." Viruses that kill the host cell by causing lysis
are called cytocidal. Viruses that damage the host cell but do not kill
it are called cytopathic. Persistent viral infections can last for years,
producing new virus particles by budding without killing the infected cell.
C.
Latency
(similar to temperance or lysogeny) - Sometimes the viral nucleic acid is integrated in
the host cell's DNA (called a provirus),
allowing the infected animal cells to function normally for years (just as a lysogenic
bacteriophage or prophage does).
Ex. Typical of DNA viruses belonging to Herpesvirus family - herpes simplex 1 (causes fever
blisters) causes a symptomless latent infection of nerve cells of mouth & lips - infection can be reactivated by a fever, a cold, too much sun, or stress.
Ex.
Varicella Zoster (another Herpsevirus) causes chickenpox as the primary infection &
shingles
as the reactivation.
Ex.
HIV (Human Immunodefiency Virus) – belongs to the Retrovirus family; causes AIDS
(Acquired
Immune Deficiency Syndrome).
D. Some
Animal RNA Viruses
Retroviruses (Retroviridae)
¨ large
group of RNA viruses; includes HIV (Human Immunodeficiency Virus) which causes AIDS
(acquired immune deficiency syndrome); infects T cells (type of white blood cell).
¨ capsid
contains 2 copies of the same (+) sense RNA
molecule (called a diploid virus); capsid also
contains the enzyme reverse transcriptase.
¨ Retro
means "backward." This virus uses
the enzyme reverse transcriptase to make DNA
from its RNA. This DNA can be integrated into
the host cell's chromosome. The proviral DNA
can now be transcribed into mRNA and translated into viral proteins to assemble new
viruses for release; As with prophages, the provirus can stay in a latent stage in which
it is replicated along with host cell DNA, causing the host cell no damage.
¨ AZT
(azidothymidine), which is used against HIV, helps stop reverse transcription by targeting
the enzyme reverse transcriptase.
Flaviviridae
¨ enveloped;
polyhedral capsid; (+) sense RNA
¨ includes
Yellow Fever (hemorrhagic fever)
¨ enveloped;
polyhedral capsid; (+) sense RNA
¨ includes
Rubella virus (Rubella or German measles)
Picornaviridae
¨ naked;
polyhedral capsid
¨ includes
Enterovirus (causes polio); Rhinovirus (common cold); Hepatovirus (Hepatitis A)
¨ Flu
viruses; 3 types (A, B, C); A is the most common, infecting many species of animals,
including humans; A is responsible for many pandemics (worldwide epidemics); B & C
only infect humans & do not cause pandemics; Outbreaks of B occur every 2-3 years; C
causes mild cold-like illnesses.
¨ enveloped RNA viruses; protein spikes in envelope; its (-) sense RNA is divided into 8 separate pieces, each one packaged in a helical capsid
¨ This
virus exhibits antigenic shift- sudden changes in properties that id. the virus as
a foreign invader to the defenses of the human immune system; occurs from genetic changes
that can occur when 2 different flu viruses infect the same cell; when this happens it is
likely that the RNA molecules of the 2 infecting virions recombine in various ways among
the new virions, producing a virus that is significantly different from either of the
original infecting strains. This is why you
can get the flu over and over again!
Rhabdoviridae
¨ enveloped;
helical capsid; (-) sense RNA
¨ includes
Rabies virus
Paramyxoviridae
¨ enveloped;
helical capsid; (-) sense RNA
¨ includes
viruses that cause Mumps, Measles, Viral pneumonia, Bronchitis
Bunyaviridae
¨ enveloped;
segmented RNA; (-) sense RNA
¨ includes
Hantavirus (“4 corners
disease”)
¨ enveloped;
filamentous capsid; (-) sense RNA
¨ includes
Ebola virus
Reoviridae
¨ naked;
polyhedral capsid; ds RNA
¨ includes
Rotavirus (most common cause of diarrhea in infants and young children under the age of 2)
E.
Some
Animal DNA Viruses
Adenoviridae
¨ naked;
polyhedral capsid; ds DNA
¨ mainly
responsible for human respiratory diseases; also causes diarrhea in babies and young
children
Herpesviridae
- enveloped;
polyhedral capsid; dsDNA
¨ Simplex
virus – Herpes
simplex 1 (oral) and 2 (genital & neonatal)
¨ Varicellovirus
–
Varicella zoster – chicken pox and shingles
¨ Roseolovirus
– Roseola
infantum – roseola in infants (rash and fever)
¨ Lymphocryptovirus
– Epstein Barr virus – causes infectious mononucleosis and Burkitt’s
lymphoma; also linked to Hodgkin’s disease.
Poxviridae
¨ enveloped;
brick shaped capsid; ds DNA; largest of all viruses
¨ includes
Orthopoxvirus – small pox & cow pox
Papovaviridae
¨ naked;
polyhedral capsid; ds DNA; replicate in nuclei of host’s cells.
¨ Includes
Papillomavirus – warts (some associated
with cervical cancer)
Hepadnaviridae
¨ enveloped;
mostly ds DNA; hepa = liver
¨ Hepatitis
B virus
Parvoviridae
¨ naked;
ssDNA; uses a helper virus to supply necessary component to produce more viruses.
¨ Includes
Canine parvovirus – causes severe and sometimes fatal gastroenteritis in dogs.
¨ Also
includes Erythrovirus (B19) – causes 5th disease (erythema infectiosum)
– deep red rash on children’s cheeks and ears and both a rash and arthritis in
adults; can cross placenta and damage fetus.
1. Tumors
-
uncontrolled growth of tissue (cells are dividing out of control); most are benign (non-life threatening); some are malignant (they spread or metastasize to surrounding tissues).
2. Cancer
– malignant tumors that metastasize to surrounding tissues.
3. Cause
– Most
human cancers arise form genetic mutations or cellular damage caused by environmental
factors (chemicals - nicotine, pesticides; radiation - UV, X-rays, etc.; diet). About 15% are attributed to viral infections.
4. Examples:
a. Human T-cell leukemia (blood cancer),
b. Epstein-Barr
virus causes Burkitt's lymphoma
c. Hepatitis
B virus causes hepatocellular carcinoma (liver cancer)
d. human
papillomavirus causes skin & cervical cancers.
e. Kaposi’s
sarcoma – thought to be associated with Herpesvirus
5. How? Some
tumor viruses are retroviruses; they convert a normal cell to a tumor cell by introducing
an oncogene into it (oncogenic provirus). Some researchers believe even normal retroviruses
might cause tumors - merely inserting a normal provirus into the host chromosome near a
normal gene might alter its expression and convert it to an oncogene.
G. Viruses and Teratogenesis
Teratogenesis
– the
induction of defects during embryonic development. A
teratogen is
a drug or other agent that induces such defects. Viruses
are teratogens that can be transmitted across the placenta and infect the fetus. Cytomegalovirus (CMV), Herpes Simplex virus (HSV),
and Rubella account for a large number of teratogenic effects. TORCH series is a series of blood tests used to
detect antibodies to these viruses.
¨ Cause
several economically important plant diseases; none known to infect animals
¨ 1/10
the size of the smallest plant virus
¨ How
it causes disease is a mystery; one theory is that it interacts in some way with the host
genome, changing the expression of the host genes to cause disease.
¨ Defined: infectious agent composed only of protein
¨ Affect
the central nervous system.
¨ Ex.
scrapie of sheep, Creutzfeldt-Jakob disease (CJD) of humans, mad cow disease.
¨ Not
known exactly how it causes disease.