Viruses can only replicate inside living cells. They hijack the host cell's machinery to produce new viral components and assemble them into new virus particles. There are seven basic stages of viral replication: 1) adsorption, 2) entry, 3) uncoating, 4) transcription, 5) synthesis of viral components, 6) assembly, and 7) release. Bacteriophages follow a similar process in bacterial cells, using either a lytic cycle that kills the host or a lysogenic cycle that allows long-term infection. Plant viruses enter through wounds or vectors and replicate using virus-specific RNA polymerases. Animal viruses recognize receptors to enter cells and then use the host to produce new virions.

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REPLICATION OF VIRUS IN PLANTS, ANIMALS & BACTERIA
Introduction:
A virus can be simply defined as an obligate intracellular parasite. Each viral particle, or
virion consists of a single nucleic acid, RNA or DNA, encoding the viral genome surrounded by
a protein coat, and is capable of replication only within the living cells of bacteria, animals or
plants. The name is from a Latin word meaning “slimyliquid” or “poison.”
The earliest indications of the biological nature of viruses came from studies in 1892 by the
Russian scientist Dmitry I. Ivanovsky and in 1898 by the Dutch scientistMartinus W.
Beijerinck.
General characteristics of replication:
Viruses multiply only in living cells. The host cell must provide the energy and synthetic
machinery and the low molecular-weight precursors for the synthesis of viral proteins and
nucleic acids. The virus replication occurs in seven stages, namely;
1. Adsorption.
It is the first step of viral replication. The virus attaches to the cell membrane of the host
cell. It then injects its DNA or RNA into the host to initiate infection. In animal cells these
viruses get into the cell through the process of endocytosis.
2. Entry.
The cell membrane of the host cell invaginates the virus particle, enclosing it in a
pinocytoticvacuole. This protects the cell from antibodies like in the case of the HIV virus.
3. Uncoating.
Cell enzymes (from lysosomes) strip off the virus protein coat. This releases or renders
accessible the virus nucleic acid or genome.
4. Transcription.
The mRNA is used to instruct the host cell to make virus components. The virus takes
advantage of the existing cell structures to replicate itself.
5. Synthesis of virus components.
The following components are manufactured by the virus through the host's existing
organelles:

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 Viral protein synthesis: virus mRNA is translated on cell ribosomes into two types of
virus protein.
 Structural: the proteins which make up the virus particle are manufactured and
assembled.
 Non – structural: not found in particle, mainly enzymes for virus genome replication.
 Viral nucleic acid synthesis (genome replication) new virus genome is synthesized,
templates are either the parental genome or with single stranded nucleic acid
genomes, newly formed complementary strands. By a virus called polymerate or
replicate in some DNA viruses by a cell enzyme. This is done in rapidly dividing
cells.
6. Virion assembly.
A virion is simply an active or intact virus particle. In this stage, newly synthesized
genome (nucleic acid), and proteins are assembled to form new virus particles. This may take
place in the cell's nucleus, cytoplasm, or at plasma membrane for most developed viruses.
7. Release.
The viruses, now being mature are released by either sudden rupture of the cell, or
gradual extrusion (budding) of enveloped viruses through the cell membrane.
REPLICATION OF VIRUS IN BACTERIA
Introduction
Even bacteria can get a virus! The viruses that infect bacteria are called bacteriophages. A
bacteriophage is a virus that infects bacteria. A bacteriophage, or phage for short, is a virus that
infects bacteria. Like other types of viruses, bacteriophages vary a lot in their shape and genetic
material. Two different cycles that bacteriophages may use to infect their bacterial hosts:
 The lytic cycle: The phage infects a bacterium, hijacks the bacterium to make lots of
phages, and then kills the cell by making it explode (lyse).
 The lysogenic cycle: The phage infects a bacterium and inserts its DNA into the bacterial
chromosome, allowing the phage DNA (now called a prophage) to be copied and passed
on along with the cell's own DNA.
Lytic cycle.
In the lytic cycle, a phage acts like a typical virus: it hijacks its host cell and uses the cell's
resources to make lots of new phages, causing the cell to lyse (burst) and die in the process.

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The stages of the lytic cycle are:
1. Attachment: Proteins in the "tail" of the phage bind to a specific receptor (in this case, a
sugar transporter) on the surface of the bacterial cell.
2. Entry: The phage injects its double-stranded DNA genome into the cytoplasm of the
bacterium.
3. DNA copying and protein synthesis: Phage DNA is copied, and phage genes are expressed
to make proteins, such as capsid proteins.
4. Assembly of new phage: Capsids assemble from the capsid proteins and are stuffed with
DNA to make lots of new phage particles.
5. Lysis: Late in the lytic cycle, the phage expresses genes for proteins that poke holes in the
plasma membrane and cell wall. The holes let water flow in, making the cell expand and
burst like an overfilled water balloon.
Fig: Lytic cycle

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Lysogenic cycle
The lysogenic cycle is a method by which a virus can replicate its DNA using a host cell.
Typically, viruses can undergo two types of DNA replication: the lysogenic cycle or the lytic
cycle. In the lysogenic cycle, the DNA is only replicated, not translated into proteins. In the lytic
cycle, the DNA is multiplied many times and proteins are formed using processes stolen from
the bacteria. While the lysogenic cycle can sometimes happen in eukaryotes, prokaryotes or
bacteria are much better understood examples.
Lysogenic Cycle Steps:
Step 1:
A bacteriophage virus infects a bacteria by injecting its DNA into the bacterial cytoplasm
or liquid space inside of the cell wall.
Step 2:
The viral DNA is read and replicated by the same bacterial proteins that replicate
bacterial DNA.
Step 3:
The viral DNA can continue using the bacterial machinery to replicate, or it can switch to
the lytic cycle. If the viral DNA stays in the lysogenic cycle, one copy, or few copies, of the
DNA exist in many bacteria. In the lysogenic cycle, the DNA only gets replicated when the
bacteria are replicating their own DNA.
Step 4:
Eventually, the viral DNA will switch to the lytic cycle, in which the bacterial
mechanisms are used to produce lots of DNA and lots of capsids, or protein covers, for the DNA.
Step 5:
These capsids get released into the environment, infect a new bacteria, and the lysogenic
cycle may start again. If the bacteria is weak or dying, the virus may enter straight into the lytic
cycle, in order to avoid dying with the bacteria.

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Fig: Lysogenic cycle
VIRAL REPLICATION IN PLANT
Plant viruses, like other viruses, contain a core of either DNA or RNA.
1. 75% of plant viruses have genomes that consist of single stranded RNA (ssRNA).
 65% of plant viruses have +ssRNA, meaning that they are in the same sense orientation
as messenger RNA
 10% have -ssRNA, meaning they must be converted to +ssRNA before they can be
translated.
 5% are double stranded RNA and so can be immediately translated as +ssRNA viruses.
2. 17% of plant viruses are ssDNA and very few are dsDNA, in contrast a quarter of animal
viruses are dsDNA and three-quarters of bacteriophage are dsDNA
Transmission of virus
As plant viruses have a cell wall to protect their cells, their viruses do not use receptor-
mediated endocytosis to enter host cells as is seen with animal viruses and must enter the cellular
cytoplasm through mechanically induced wounds or assisted by a biological vector. Plant viruses
can be transmitted by a variety of vectors:

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 through contact with an infected plant’s sap,
 by living organisms such as insects and nematodes, and through pollen.
When plant viruses are transferred between different plants, this is known as horizontal
transmission; when they are inherited from a parent, this is called vertical transmission.
Replication of virus in TMV.
 The viral-RNA after entry first induces the formation of specific enzymes called ‘RNA
polymerases’ the single-stranded viral-RNA synthesizes an additional RNA strand called
replicative RNA.
 This RNA strand is complementary to the viral genome and serves as ‘template’ for
producing new RNA single strands which is the copies of the parental viral-RNA.
 The new viral-RNAs are released from the nucleus into die cytoplasm and serve as
messenger-RNAs (mRNAs). Each mRNA, in cooperation with ribosomes and t-RNA of
the host cell directs the synthesis of protein subunits.
 After the desired protein sub-units (capsomeres) have been produced, the new viral
nucleic acid is considered to organize the protein subunit around it resulting in the
formation of complete virus particle, the virion.
figure showing TMV

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REPLICATION OF VIRUS IN ANIMALS
Introduction:
Like other viruses, animal viruses are tiny packages of protein and nucleic acid. They
have a protein shell, or capsid, and genetic material made of DNA or RNA that's tucked inside
the caspid. They may also feature an envelope, a sphere of membrane made of lipid.Animal
viruses, like other viruses, depend on host cells to complete their life cycle. In order to
reproduce, a virus must infect a host cell and reprogram it to make more virus particles.
1. The first key step in infection is recognition an animal virus has special surface molecules
that let it bind to receptors on the host cell membrane.
2. Once attached to a host cell, animal viruses may enter in a variety of ways:
 by endocytosis, where the membrane folds in;
 by making channels in the host membrane (through which DNA or RNA can be
injected);
 or, for enveloped viruses,
 by fusing with the membrane and releasing the capsid inside of the cell
Following are the steps of replication of virus in animals:
1. Adsorption
Adsorption to the host cell surface is the first step in reproduction cycle of animal viruses.
Adsorption of virion to the host cell surface takes place through a random collision of virion with
a plasma membrane receptor site; the receptor is a protein, and frequently a glycoprotein. A virus
attaches to a specific receptor site on the host cell membrane through attachment proteins in the
capsid or via glycoproteins embedded in the viral envelope. The specificity of this interaction
determines the host (and the cells within the host) that can be infected by a particular virus. This
can be illustrated by thinking of several keys and several locks where each key will fit only one
specific lock.
2. Entry
The nucleic acid of bacteriophages enters the host cell naked, leaving the capsid outside
the cell. Plant and animal viruses can enter through endocytosis, in which the cell membrane
surrounds and engulfs the entire virus. Some enveloped viruses enter the cell when the viral
envelope fuses directly with the cell membrane. Once inside the cell, the viral capsid is degraded

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and the viral nucleic acid is released, which then becomes available for replication and
transcription.
3. Replication and Assembly
The replication mechanism depends on the viral genome.
DNA ANIMAL VIRUSES.
Generally, DNA animal viruses replicate their DNA in the host cell nucleus with the aid
of viral enzymes and synthesize their capsid and other proteins in the cytoplasm by using host
cell enzymes. The new viral proteins move to the nucleus, where they combine with the new
viral DNA to form virions. This pattern is typical of adenoviruses, hepadnaviruses,
herpesviruses, and papovaviruses. Poxviruses are the only exception; their parts are synthesized
in the host cell’s cytoplasm.
figure showing replication of dsDNA virus
RNA animal viruses
Replication of RNA animal virus takes place in a greater variety of ways than is found in
DNA animal viruses.

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Positive sense RNA
In the poliovirus the viral (+) sense RNA serves as mRNA—it is translated immediately to
produce proteins needed for reproduction of the virus. A (-) sense RNA copy is then made,
which serves as a template for the production of more viral (+) sense RNA molecules. Mature
polioviruses lyse the cell during release.
In HIV each (+)sense RNA, copied with the help of reverse transcriptase, forms an ssDNA,
which serves as template for the synthesis of the complementary strand. The dsDNA is then
inserted into the host chromosome, where it can remain for some time. When virus replication
occurs, one strand of the DNA becomes the template for the synthesis of viral ( ) sense RNA
molecules. Mature HIV particles usually do not lyse the cell but rather bud off the cell
surrounded by an envelope.

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Negative sense RNA
In (-) sense RNA animal viruses, such as the viruses causing measles and influenza A, a
packaged transcriptase uses the (-) sense RNA to make (+) sense RNA molecules (mRNA). Prior
to assembly, new (-) sense RNA is made from (+) sense RNA templates. The process is
essentially the same regardless of whether the viral RNA is in one segment (measles) or in many
segments (influenza A).
4. Egress/ release
The last stage of viral replication is the release of the new virions produced in the host
organism. They are then able to infect adjacent cells and repeat the replication cycle. Some
viruses are released when the host cell dies, while other viruses can leave infected cells by
budding through the membrane without directly killing the cell.