Replication in viruses

Virus replication
Virus replication

What is Viral replication?

Viral replication is the formation of biological viruses during the infection process in the target host cells. Viruses must first enter the cell before viral replication can occur. Through the generation of abundant copies of its genome and the packaging of these copies, the virus continues to infect new hosts. The replication between viruses is very varied and depends on the type of genes involved in them. Most DNA viruses assemble in the nucleus, while most RNA viruses replicate only in the cytoplasm.

Steps of replication in viruses

The virus does not have its own metabolic system. The infected host cell has to provide the energy, metabolic machinery and precursor molecules for the synthesis of viral proteins and nucleic acids.

Replication in viruses occurs in six steps which are named below.

  1. Adsorption/ attachment of the virus to host cell
  2. 2- Penetration of Viral components in the host cell
  3. Uncoating
  4. Synthesis of viral components by mRNA production / Transcription
  5. Virion assembly
  6. Release of virus (liberation stage)
Steps of viral replication
Steps of viral replication

1- Adsorption/ attachment of the virus to host cell

It is the first step of viral replication. The virus binds to the cell membrane of the host cell by a specific receptor site on the host cell membrane through binding proteins in the capsid or by 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 imagined by thinking of multiple keys with multiple locks where each key will fit a single specific lock.

 2- Penetration of Viral components in the host cell

Then the virus injects its DNA or RNA into the host to start the infection.

  • Bacteriophage nucleic acid 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. In the plant, the cell membrane of the host cell invaginates the virus particle, enclosing it in a pinocytotic vacuole.
  • Some enveloped viruses enter the cell when the viral envelope fuses directly with the cell membrane of the host cell.

3- Uncoating

Once inside the cell, the viral capsid is broken down by the cellular enzymes (from lysosomes) of the host and the viral nucleic acid is released, which is then available for replication and transcription.

4- Synthesis of viral components by mRNA production / Transcription

The virus uses cellular structures of the host cell to replicate. The replication mechanism depends on the viral genome.

  • DNA viruses generally use proteins and enzymes from the host cell to produce additional DNA that is transcribed into messenger RNA (mRNA), which is then used to direct protein synthesis.
  • RNA viruses generally use their RNA core as a template for the synthesis of viral genomic RNA (to be incorporated in the structure of new virus)  and mRNA. Viral mRNA directs the host cell to synthesize two types of proteins.

a) Structural: The proteins that make up the viral particle are manufactured and assembled.
b) Non-structural: it is not found in viral particles. It is composed of enzymes for the replication of the virus genome.

If a host cell does not provide the enzymes necessary for viral replication, the viral genes provide the information to direct the synthesis of the missing proteins.

Retroviruses, like HIV, have an RNA genome that must be reverse transcribed into DNA, which is then incorporated into the host cell genome.

To convert RNA to DNA, retroviruses must contain genes that encode the enzyme reverse transcriptase for the virus-specific enzyme, which transcribes an RNA template into DNA.

The fact that HIV produces some of its own enzymes not found in the host has allowed researchers to develop drugs that inhibit these enzymes. These drugs, including the reverse transcriptase inhibitor AZT, inhibit HIV replication by reducing enzyme activity without affecting host metabolism. This approach has led to the development of a variety of drugs used to treat HIV and has been effective in reducing the amount of infectious virions (copies of viral RNA) in the blood to undetectable levels in many people infected with HIV.

5- Virion assembly

A virion is simply an intact or active virus particle. At this stage, the newly synthesized genome (nucleic acid) and proteins assemble to form new virus particles.

This can take place in the cell nucleus, the cytoplasm, or in the plasma membrane of most developed viruses.

6- Release of virus (liberation stage)

It is the last stage of viral replication in which the viruses, which are now mature, are released in the host organism. They can then infect adjacent cells and repeat the replication cycle. Viruses are released by sudden cell disruption or by gradual extrusion (budding) of viruses enveloped through the cell membrane.

New viruses can invade or attack other cells, or remain dormant in the cell. In the case of bacterial viruses, the virions are released from the progeny by lysis of the infected bacteria. However, in the case of animal viruses, release generally occurs without cell lysis.

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