Module 6 - Viruses
Module 6 - Viruses
Module 6 - Viruses
MCB100-Module 6
• Viruses are not cells and thus are nonliving, they nonetheless possess a genome
encoding the information they need in order to replicate.
• They rely on host cells to provide the energy and materials needed for replicating their
genomes and synthesizing their proteins.
• They can exist in either extracellular or intracellular forms.
• In its extracellular form, a virus is a microscopic particle containing nucleic acid surrounded by a
protein coat and sometimes, depending on the specific virus, other macromolecules.
• A virion is the extracellular form of a virus and contains either an RNA or a DNA
genome inside a protein shell.
• The virus genome may enter a new host cell by infection.
• The virus redirects the host metabolism to support virus replication.
• Viruses are classified by their nucleic acid and type of host.
General Properties of Viruses
• Viruses can be classified on the basis of the hosts they infect as well as by their
genomes (i.e. bacterial viruses, archaeal viruses, animal viruses, plant viruses, and
viruses that infect other kinds of eukaryotic cells.
• Bacterial viruses, sometimes called bacteriophages (or phage for short; from the
Greek phagein, meaning “to eat”), have been intensively studied as model systems
for the molecular biology and genetics of virus replication.
• Species of both Bacteria and Archaea are infected by specific viruses.
General Properties of Viruses
• Virions come in many sizes and shapes. Most viruses are smaller
than prokaryotic cells, ranging in size from 0.02 to 0.3 µm (20–300
nm). A common unit of measure for viruses is the nanometer,
which is one-thousandth of a micrometer.
• Smallpox virus, one of the largest viruses, is about 200 nm in
diameter (about the size of the smallest cells of Bacteria).
Poliovirus, one of the smallest viruses, is only 28 nm in diameter
(about the size of a ribosome).
• The largest known viral genome, that of Mimivirus, consists of
1.18 Mbp of double-stranded DNA. This virus, which infects
protists such as Amoeba, is one of a few viruses currently known
whose genome is larger than some cellular genomes.
Nature of the Virion
• In the virion of a naked virus, only nucleic acid and protein are present, with the nucleic acid on the inside; the whole unit is
called the nucleocapsid. Enveloped viruses have one or more lipoprotein layers surrounding the nucleocapsid.
• The nucleocapsid is arranged in a symmetric fashion, with a precise number and arrangement of structural subunits
surrounding the virus nucleic acid.
• Although virus particles are metabolically inert, one or more key enzymes are present within the virion in some viruses.
Viral Structure
• The nucleic acid of the virion is always located within the
particle, surrounded by a protein shell called the capsid.
• A few viruses have only a single kind of protein in their
capsid, but most viruses have several distinct proteins
that are associated in specific ways to form assemblies
called capsomeres.
• The complete complex of nucleic acid and protein
packaged in the virion is called the virus nucleocapsid.
• Some viruses are naked, whereas others possess lipid-
containing layers around the nucleocapsid called an
envelope.
General Structural Properties
• All virions, even if they possess other constituents, are con- structed
around a nucleocapsid core (indeed, some viruses con- sist only of a
nucleocapsid).
• The nucleocapsid is composed of a nucleic acid, either DNA or RNA, held
within a protein coat called the capsid, which protects viral genetic
material and aids in its transfer between host cells.
• There are four general morphological types of capsids and virion structure.
1. Some capsids are icosahedral in shape. An icosahedron is a regular
polyhedron with 20 equilateral triangular faces and
12 vertices. These capsids appear spherical when viewed at low
power in the electron microscope.
General Structural Properties
• 2. Other capsids are helical and shaped like hollow protein cylinders,
which may be either rigid or flexible.
• 3. Many viruses have an envelope, an outer membranous layer
surrounding the nucleocapsid. Enveloped viruses have a roughly
spherical but somewhat variable shape even though their
nucleocapsid can be either icosahedral or helical.
• 4. Complex viruses have capsid symmetry that is neither purely
icosahedral nor helical. They may possess tails and other structures
(e.g., many bacteriophages) or have complex, multilayered walls
surrounding the nucleic acid (e.g., poxviruses such as vaccinia).
Helical and
Icosahedral Capsids
• Helical capsids are shaped much like hollow tubes with
protein walls. The tobacco mosaic virus provides a well-
studied example of helical capsid structure.
• A single type of protomer associates together in a helical or
spiral arrangement to produce a long, rigid tube, 15 to 18 nm
in diameter by 300 nm long.
• The RNA genetic material is wound in a spiral and positioned
toward the inside of the capsid where it lies within a groove
formed by the protein subunits.
• Not all helical capsids are as rigid as the TMV capsid. Influenza
virus RNAs are enclosed in thin, flexible helical capsids folded
within an envelope
• The icosahedron is one of nature’s
favorite shapes (the helix is probably
most popular).
• Viruses employ the icosahedral shape
Principles
Chorodopoxvirinae contains poxviruses of
vertebrates.
• Within the subfamily are several genera that are
of Virus
distinguished based on immunologic
characteristics and host specificity. The genus
Orthopoxvirus contains several species, among
them variola major (the cause of smallpox),
Taxonomy
vaccinia, and cowpox.
• Viruses are divided into different taxonomic
groups based on characteristics that are related
to the type of host used, virion structure and
composition, mode of reproduction, and the
nature of any diseases caused. Some of the
more important characteristics are:
Principles of Virus Taxonomy
2. Nucleic acid characteristics—
DNA or RNA, single or double
stranded, molecular weight,
1. Nature of the host—animal, 3. Capsid symmetry— 4. Presence of an envelope and
plant, bacterial, insect, fungal segmentation and number of icosahedral, helical, binal ether sensitivity
pieces of nucleic acid (RNA
viruses), the sense of the strand
in ssRNA viruses
Nucleic Acid
RNA is also the mRNA.
• In other viruses, the virus genome is a
template for the formation of viral
and Protein
mRNA, and in certain cases, essential
transcriptional enzymes are contained
in the virion.
The Baltimore • The virologist David Baltimore, who along with Howard Temin
and Renato Dulbecco shared the Nobel Prize for Physiology or
Classification Scheme Medicine in 1975 for the discovery of retroviruses and reverse
transcriptase, developed a classification scheme for viruses.
and DNA Viruses
The Baltimore Classification Scheme and DNA Viruses
The Baltimore classification scheme is based on the relationship of the
viral genome to its mRNA and recognizes seven classes of viruses, such as
the following:
• Class I: Double-stranded (ds) DNA viruses.
• The mechanism of mRNA production and genome replication of class I viruses is
the same as that used by the host cell genome, although different viruses use
different strategies to ensure that viral mRNA is expressed in preference to host
mRNA.
• Class II: are single-stranded (ss) DNA viruses.
• Before mRNA can be produced from such viruses, a complementary DNA strand
must be synthesized because RNA polymerase uses double-stranded DNA as a
template.
• These viruses form a dsDNA intermediate during replication that is also used for
transcription.
The Baltimore Classification Scheme and DNA Viruses
• Class III: Double stranded RNA [dsRNA (+/-)] virus
• Class IV: Single stranded RNA (+) virus
• Class V: Single stranded RNA (-) virus
The Baltimore Classification Scheme and DNA Viruses
• Class VI: Single stranded RNA (+) retrovirus
• The retroviruses are animal viruses that are responsible for causing certain kinds
of cancers and acquired immunodeficiency syndrome, AIDS.
• Retroviruses have ssRNA in their virions but replicate through a dsDNA
intermediate.
• The process of copying the information found in RNA into DNA is called reverse
transcription, and thus these viruses require an enzyme called reverse
transcriptase.
• Class VII: viruses are those that have double-stranded DNA in their
virions but replicate through an RNA intermediate.
• These unusual viruses also use reverse transcriptase. The strategy these viruses
use to produce mRNA is the same as that of class I viruses, although their DNA
replication is very unusual because the genome is only partially double-stranded.
Enzymes in Virions
• Virions do not carry out metabolic processes and thus a virus is metabolically inert
outside a host cell.
• However, some virions do contain enzymes that play important roles in infection. Some of
these enzymes are required for very early events in the infection process.
• Some bacteriophages contain the enzyme lysozyme, which they use to make a
small hole in the bacterial cell wall.
• This allows the virus to inject its nucleic acid into the cytoplasm of the host cell.
• Lysozyme is again produced in large amounts in the later stages of infection, causing lysis of
the bacterial cell and release of the new virions.
• Many viruses contain their own nucleic acid polymerases for replication of the viral
genome and for transcription of virus-specific RNA.
• Some viruses contain enzymes that aid in their release from the host.
• Certain animal viruses contain surface proteins called neuraminidases, enzymes
that cleave glycosidic bonds in glycoproteins and glycolipids of animal cell
connective tissue, thus liberating the virions.
The Virus Host
• Some viruses do not cause recognizable effects in cell cultures yet cause death in
whole animals.
• In such cases, quantification can be done only by titration in infected animals. The
general procedure is to carry out a serial dilution of the virus sample, generally at 10-
fold dilutions, and to inject samples of each dilution into several sensitive animals.
• After a suitable incubation period, the fraction of dead and live animals at each
dilution is tabulated and an end point dilution is calculated.
• This is the dilution at which, for example, half of the injected animals die (the lethal
dose for 50% or LD50).
The virus replication cycle can be divided into five stages: attachment
(adsorption), penetration (injection), protein and nucleic acid synthesis,
assembly and packaging, and virion release.
2.Penetration (entry, injection) of the virion or its nucleic acid into the
host cell.
Penetration
viral genome cannot be read.
• Different viruses have different strategies for
penetration. Uncoating refers to the process in which
the virions lose their outer coat and the viral genome is
exposed.
• Some enveloped animal viruses are uncoated at the
cytoplasmic membrane, releasing the virion contents
into the cytoplasm.
• Some enveloped viruses are uncoated in the cytoplasm.
Others (such as influenza) are uncoated at the nuclear
membrane and the viral genome then enters the
nucleus. In animal cells, wherever uncoating occurs, the
viral genome must eventually enter the nucleus to be
replicated, except in a few rare cases.
Tailed Bacteriophage
and Penetration
• Cells that have cell walls, such as most bacteria,
are infected in a manner different from animal
cells, which lack cell walls.
• The most complex penetration mechanisms
have been found in viruses that infect bacteria.
• The bacteriophage T4, which infects
Escherichia coli, is a good example.
• The virion has a head, within which the viral
linear double- stranded DNA is folded, and a
long, fairly complex tail, at the end of which is a
series of tail fibers and tail pins.
• The T4 virions first attach to E. coli cells
by means of the tail fibers.
Virus Restriction and Modification by the Host
• Animals can often eliminate invading viruses by immune defense mechanisms before the
viral infection becomes widespread or sometimes even before the virus has penetrated
target cells.
• In addition, eukaryotes, including animals and plants, possess an antiviral mechanism
known as RNA interference.
• Although they lack immune systems, both Bacteria and Archaea possess an antiviral
mechanism similar to RNA interference, known as CRISPR (Clustered Regularly
Interspaced Short Palindromic Repeats)
• CRISPR (/ˈkrɪspər/) is a family of DNA sequences found in
the genomes of prokaryotic organisms such as bacteria and archaea.
• These sequences are derived from DNA fragments of bacteriophages that had previously infected
the prokaryote.
• They are used to detect and destroy DNA from similar bacteriophages during subsequent
infections.
• These sequences play a key role in the antiviral (i.e. anti-phage) defense system of prokaryotes
and provide a form of acquired immunity
Virus Restriction and Modification by the Host
• In addition, prokaryotes destroy double-stranded viral DNA after it has
been injected by using restriction endonucleases, enzymes that cleave
foreign DNA at specific sites, thus preventing its replication. This
phenomenon is called restriction and is part of a general host mechanism
to prevent the invasion of foreign nucleic acid.
• Restriction enzymes are specific for double-stranded DNA, and thus single-
stranded DNA viruses and all RNA viruses are unaffected by restriction
systems.
• Although host restriction systems confer significant protection, some DNA
viruses have over- come host restriction by modifying their own DNA so
that they are no longer subject to restriction enzyme attack.
• Two patterns of chemical modification of viral DNA are known:
glucosylation and methylation.
Overview of Bacterial
Viruses
• Bacterial viruses, or
bacteriophages, are very diverse.
• The best- studied bacteriophages
infect bacteria such as Escherichia
coli and are structurally quite
complex, containing heads, tails,
and other components.
• Most known bacteriophages
contain dsDNA genomes, and this
type of bacteriophage is thought to
be the most common in nature.
Overview of Bacterial Viruses
• There are two contrasting viral
life cycles: virulent and
temperate.
• In the virulent (or lytic) mode,
viruses lyse or kill their hosts
after infection, whereas in the
temperate (or lysogenic) mode,
viruses replicate their genomes
in step with the host genome
and without killing their hosts.
• A similar phenomenon is seen
with viruses that infect higher
organisms. When animal viruses
divide in step with host cells,
this is known as a “latent”
infection.
Virulent Bacteriophages and T4
• Virulent viruses kill their hosts after
infection. The first such viruses to be
studied in detail were bacteriophages with
linear, dsDNA genomes that infect
Escherichia coli and a number of related
Bacteria.
• These phages were designated T1, T2, and
so on, up to T7, with the “T” referring to
the tail these phages contain. We have
already briefly mentioned how one of
these viruses, T4, attaches to its host and
how its DNA penetrates the host.
Virulent Bacteriophages and T4
• After a virion of T4 attaches to a host cell and the DNA penetrates into the cytoplasm, the expression of viral genes is regulated so
as to redirect the host synthetic machinery to the production of viral nucleic acid and protein.
• New virions are then assembled and are released by lysis of the cell. T4 has a double- stranded DNA genome that is circularly
permuted and terminally redundant.
The Replication Cycle of a Temperate Phage
• Temperate phages may enter the virulent mode after
infecting a host cell or they may establish lysogeny.
• During lysogeny, the temperate virus does not exist as a
virus particle inside the cell. Instead, the virus genome
is either integrated into the bacterial chromosome (e.g.,
bacteriophage lambda) or exists in the cytoplasm in
plasmid form (e.g., bacteriophage P1).
• f the phage repressor is inactivated or if its synthesis is
pre- vented, the prophage is induced. New virions are
produced, and the host cell is lysed. Altered conditions,
especially damage to the host cell DNA, induce the lytic
pathway in some cases (e.g., in bacteriophage lambda).
If the virus loses the ability to leave the host genome
because of mutation, it becomes a cryptic virus.
Bacteriophage Lambda
• Bacteriophage lambda, which infects
Escherichia coli, has been studied in great
detail. As with other temperate viruses, both
the virulent and the temperate pathways are
possible.
• Lambda virions resemble those of other tailed
bacteriophages, although no tail fibers are
present in the commonly used laboratory
strains.
• Wild-type lambda does have tail fibers. The
lambda genome consists of linear dsDNA.
However, at the 5’ terminus of each strand is a
single-stranded region 12 nucleotides long.
Bacteriophage
Lambda
• When lambda is lysogenic, it integrates into
the E. coli chromosome at a unique site
known as the lambda attachment site, attƛ.
• Integration requires the enzyme lambda
integrase, which recognizes the phage and
bacterial attachment sites.
• When lambda enters the virulent (lytic)
pathway, it synthesizes long, linear
concatemers of DNA by rolling circle
replication.
Lambda: Lysis or
Lysogeny?
• Whether lysis or lysogeny occurs during lambda infection
depends on an exceedingly complex genetic switch.
• The key elements are two repressor proteins, the
lambda repressor, or cI protein , and the repressor
protein Cro.
• To establish lysogeny, two events must happen: (1) The
production of late proteins must be prevented; and (2) a
copy of the lambda genome must be integrated into the
host chromosome.
• If cI is made, it represses the synthesis of all other
lambda-encoded proteins and lysogeny is established.
Conversely, Cro indirectly represses the expression of
the lambda cII and cIII proteins, which are needed to
maintain lysogeny, by inducing synthesis of the cI.
• Lysogeny is a state in which lytic events
are repressed. Viruses capable of entering
the lysogenic state are called temperate
viruses. In lysogeny the virus genome
becomes a prophage, either by integration
into the host chromosome or by
replicating like a plasmid in step with the
host cell.
• However, lytic events can be induced by
certain environmental stimuli.
Classification of
Animal Viruses
• There are animal viruses with all known
modes of viral genome replication.
• Many animal viruses are enveloped,
picking up portions of host membrane as
they leave the cell.
• Not all infections of animal host cells result
in cell lysis or death; latent or persistent
infections are common, and a few animal
viruses can cause cancer.
Consequences of Virus
Infection in Animal Cells
• Viruses can have several different effects on
animal cells. Virulent infection results in the
destruction of the host cell.
• With enveloped viruses, however, release of
virions, which occurs by a kind of budding process,
may be slow, and the host cell may not be lysed.
• The infected cell may therefore remain alive and continue
to produce virus indefinitely. Such infections are called
persistent infections.
Defective
• Some of these so-called defective viruses
merely rely on intact helper viruses of the
same type to provide necessary functions.
• Far more interesting are those defective
Viruses
viruses, referred to as satellite viruses, for
which no intact version of the same virus
exists; these defective viruses rely on
unrelated viruses as helpers.
• Defective viruses are parasites of intact helper
viruses.
• The helper viruses supply proteins that the
defective virus no longer encodes. Some
defective viruses rely on closely related but
intact helper viruses.
• However, satellite viruses rely on unrelated
intact viruses that infect the same host cells to
complete replication events.
Viroids