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Replication of Plant and Animal Virus

In this replication of plant and animal virus post we have briefly explained about virus replication process, animal viral replication cycle (replication of RNA viruses, replication DNA viruses) and plant viral replication cycle.

Viral Replication Cycle

The need to reproduce is one feature viruses have in common with actual biological beings. As we’ve seen, all viruses are obligatory intracellular parasites, which means they must successfully enter a host cell in order to multiply.

Much of the ‘machinery’ required for viral replication cycle is provided by the host cell. All viral growth cycles follow a similar general series of events, with some variances across types governed by viral structure and the host cell’s nature.

Main Stages of Viral Replication Cycle

BacteriophagesViral Replication Cycle

Bacteriophages (phages for short) are viruses that infect bacterial cells. Bacteriophages literally mean “bacteria eaters.”

The T-evenphages, a kind of bacteriophage that infects E. coli, have one of the best-understood viral reproduction cycles of all.

These viruses are huge and complicated, with a distinct head and tail shape. The icosahedral head holds the double-stranded, linear DNA genome, which comprises about 100 genes.

Because the growth cycle terminates in the lysis (=bursting) of the host cell, it is referred to as lytic.

T-evenphages

Lytic Viral Replication Cycle

Adsorption

T4 binds to complementary receptors on the surface of the host cell using specialised tail fibre proteins. One of the most important aspects in defining a virus’s host specificity is the type of these receptors.

Penetration

The phage’s tail sheath contains the enzyme lysozyme, which weakens the cell wall at the point of attachment, causing the phage’s core to be pushed down into the cell, releasing the viral DNA into the bacterium’s interior.

The capsid remains totally outside the cell, as Hershey and Chase’s famous experiment neatly illustrated.

Replication

Phage genes turn off the host’s protein and nucleic acid synthesis, directing all of the host’s metabolic apparatus to the production of phage DNA and proteins.

Phage-encoded enzymes destroy host nucleic acids, providing a supply of nucleotide building blocks.

The phage DNA is replicated by host enzymes, which is subsequently transcribed into mRNA and translated into protein.

The Lytic Cycle of Phage T4

Assembly

Capsid and DNA components assemble spontaneously into viral particles once sufficient numbers have been synthesised.

The head and tail portions are synthesised separately, then the head is filled with the DNA genome and the tail is put together.

Release

The lysozyme encoded by the phage weakens the cell wall, causing cell lysis and the release of viral particles, which can then infect new host cells and restart the cycle.

The host cell contains phage components but no full particles during the early stages of infection. The eclipse period is the name given to this time period.

The latent period (also known as the burst time) is the time between the attachment of a phage particle to the cell surface and the release of freshly synthesised phages; for T4 under ideal conditions, this is roughly 22 minutes.

Lysogenic Viral Replication Cycle

Virulent phages, such as T4, are phages that cause cell lysis. In addition to the lytic cycle described above, temperate phages can also go through a different type of growth cycle.

The phage DNA is incorporated into the host’s genome as a prophage at this point. The host cell is unharmed in this state of lysogeny.

This is due to the fact that the phage’s repressor proteins prohibit most of the other phage genes from being transcribed.

These genes, on the other hand, are replicated together with the bacterial chromosome, so the integrated prophage is present in all bacterial offspring.

When the host cell’s existence is threatened, usually by an external element such as UV light or a chemical mutagen, the lysogenic stage is terminated.

The phage DNA is excised and takes on a circular shape in the cytoplasm once the repressor protein is inactivated.

It commences a lytic cycle in this form, culminating in the demise of the host cell. Bacteriophage (Lambda), which infects some strains of E. coli, is an example of a temperate phage. Lysogens are bacterial strains that can assimilate phage DNA in this way.

Replication cycle of a temperate phage

Replication of Plant and Animal Virus

Animal Viral Replication Cycle

Viruses that infect multicellular organisms like animals may be specific not only to the organism, but also to the cell or tissue type they infect.

This is referred to as the virus’s tissue tropism, and it happens because the virus attaches to the host cell surface via specialised receptors.

Animal viruses have the same basic phases as bacteriophages, although some of the details may differ significantly. The majority of these changes are due to structural differences between bacterial and animal host cells.

Adsorption and penetration

Animal viruses do not have the same head and tail structure as phages; hence their attachment process is different. The specific engagement with a host receptor is accomplished through a component of the capsid or, in the case of enveloped viruses, unique features such as spikes (peplomers).

Host antibody molecules can usually block viral attachment sites; however, certain viruses (such as rhinoviruses) have solved this by placing their attachment sites in deep depressions that are inaccessible to antibodies.

Unlike bacteriophages, which inject their nucleic acid component from the outside, animal viruses have a more complex process, which is reflected in the length of time it takes to complete.

Animal viruses don’t have to deal with strong cell walls, and in many situations, the entire virion is internalised. This demands an additional step of uncoating, which is performed by host enzymes.

Many animal viruses have an envelope, which allows them to enter the cell via fusion with the cell membrane or endocytosis.

While some non-enveloped types release only their nucleic acid component into the cytoplasm, others require additionally that virus-encoded enzymes be introduced to ensure successful replication.

Animal viral replication cycle

Replication DNA viruses

Unlike bacteria, animal cells’ DNA is compartmentalised within a nucleus, and it is here that viral DNA replication and transcription take place.

The messenger RNA is then translated by ribosomes in the cytoplasm. A double-stranded intermediate is generated in the case of viruses with ssDNA genomes, which serves as a template for mRNA production.

Assembly

Translation products are finally returned to the nucleus for assembly into new virus particles.

Release

Naked (non-enveloped) viruses are usually discharged when the host cell is lysed. Release is more gradual in the case of wrapped forms.

The host’s plasma membrane is altered by the insertion of virus-encoded proteins before the virus particle is engulfed and released by a budding process. ] Internalisation through fusion might be thought of as the inverse of this process.

Replication of RNA viruses

So far, all of the phage and animal viral development cycles we’ve documented have used double-stranded DNA genomes.

Many viruses, as you may recall, employ RNA rather than DNA as their genetic material, and we now need to look at how these viruses complete their reproduction cycles.

RNA viruses replicate in the host’s cytoplasm; the details vary depending on whether the RNA is single-stranded or double-stranded, and whether it is in (+) or () sense.

A (+) sense single-stranded RNA virus’ genome operates directly as an mRNA molecule, resulting in a massive polyprotein that is then cleaved into the virus’s different structural and functional proteins.

To duplicate (+) sense RNA, a complementary () sense strand is created, which serves as a template for the synthesis of new (+) sense RNA.

Replication of RNA viruses

A virally encoded RNA polymerase must first use the RNA of a (-) sense RNA virus as a template for the synthesis of its complementary sequence.

The (+) sense RNA thus produced has two purposes: I as mRNA for translation into the virus’s different proteins, and (ii) as a template for the synthesis of more genomic (-) sense RNA.

Double-stranded RNA viruses are segmented in every way. By transcription of the (-) strand of their genome, they produce distinct mRNAs for each of their proteins.

These are all translated and then combined with appropriate proteins to produce a subviral particle, which serves as a template for the manufacture of a double-stranded RNA genome, ready to be incorporated into a new viral particle.

The enzyme reverse transcriptase, initially identified in 1970, is involved in two final, quite intricate variations on viral replication cycle.

Replication of RNA viruses

Retroviruses

These viruses, which include some of the most dangerous human diseases, contain a genome that alternates between RNA and DNA at different stages of replication.

Retroviruses have a (+) sense ss-RNA genome that is diploid, making them unusual among viruses. The enzyme reverse transcriptase uses the two copies of the genome as templates to create a complementary strand of DNA.

The RNA component of this hybrid is then destroyed, allowing a second strand of DNA to be synthesised. This proviral DNA enters the nucleus and is incorporated into the genome of the host.

A host RNA polymerase produces mRNA, which is translated into viral proteins and also acts as genomic material for new retrovirus particles.

A good example of a retrovirus is the human immunodeficiency virus (HIV), which causes acquired immune deficiency syndrome.

Retroviruses Replication 

Hepadnaviruses

DNA and RNA phases alternate in two virus families (hepadnaviruses and caulimoviruses), but their appearance order is inverted, resulting in a dsDNA genome.

This is made feasible by reverse transcriptase, which occurs later in the virus particle’s development.

Plant Viral Replication Cycle

Plant viral infections can spread through one of two main routes. Horizontal transmission is the introduction of a virus from the outside, and it is most commonly accomplished by insect vectors, who use their mouthparts to break through the cell wall and deliver the virus.

Inanimate objects, such as garden tools, can also be used to transmit this type of infection. The virus is conveyed vertically from a plant to its offspring, either by asexual propagation or through infected seeds.

Although DNA variants of plant viruses, such as the caulimo viruses, have been identified, the bulk of plant viruses discovered so far have an RNA genome.

Depending on the nature of the viral genome, replication is comparable to that of animal viruses.

When an infection spreads throughout the plant, it becomes significant (a systemic infection).

This is accomplished by viral particles travelling through plasmodesmata, which are naturally occurring cytoplasmic strands that connect adjacent plant cells.

Further Readings

Reference

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