VIRUS REPLICATION

· Virus replication is the process by which viruses reproduce and multiply within a host organism.

· Virus do not undergo binary fission (seen in bacteria), but undergo a complex way of cell division.

· Viruses are microscopic infectious agents that lack the ability to replicate on their own and therefore rely on host cells to carry out the necessary steps for their replication.

· Understanding the process of virus replication is crucial for studying and combating viral infections.

· Viruses have a simple structure consisting of genetic material enclosed in a protein coat called a capsid.

· The genetic material of viruses can be either DNA or RNA.

· Replication of viruses passes through six sequential steps.

VIRUS REPLICATION

· Steps involved in virus replication are as follows:

Attachment → Penetration → Uncoating → biosynthesis →Maturation → Release

1) Attachment: The virus attaches to specific receptors on the surface of the host cell. This attachment is typically mediated by interactions between viral surface proteins and host cell surface receptors. The receptors can be proteins, carbohydrates, or other molecules on the cell surface.

2) Penetration: The virus entry into the host cell. There are two main mechanisms for viral entry:

· Direct Fusion: Some viruses, particularly enveloped viruses, can fuse their viral envelope with the host cell membrane, allowing the viral genetic material to enter the cell.

· Endocytosis: Other viruses are taken up by the host cell through endocytosis, where the cell membrane engulfs the virus, forming a vesicle called an endosome. The virus then uses various mechanisms to escape from the endosome and release its genetic material into the cytoplasm.

3) Uncoating: Once inside the host cell, the virus undergoes uncoating, where the viral capsid (protein coat) is disassembled, exposing the viral genetic material. Uncoating can occur through various mechanisms, including enzymatic digestion or conformational changes triggered by the cellular environment.

4) Replication of Viral Genome/ Biosynthesis :

· In this step the following viral component are synthesized:

a. Nucleic acid

b. Capsid protein

c. Enzyme required for various stages of viral replication

· DNA Viruses: In DNA viruses, the DNA replication occurs in the nucleus except in poxvirus, which synthesized DNA in the cytoplasm. The viral DNA is replicated using host enzymes, and viral proteins are synthesized using the host cell's protein synthesis machinery. DNA viruses that replicate in the nucleus typically integrate their genetic material into the host cell's DNA for long-term persistence.

· RNA Viruses: The process of biosynthesis varies among RNA virus depending on whether the genomic RNA is positive/negative sense or single-stranded RNA (ssRNA) or double-stranded RNA (dsRNA) genome.

· Positive sense ssRNA Viruses: Positive-sense ssRNA viruses can directly serve as mRNA and be translated into viral proteins by the host cell's ribosomes.

· Negative-sense ssRNA viruses : Negative sense ssRNA polarity is opposite to that of mRNA, hence they cannot directly translate into protein.

· dsRNA Viruses: dsRNA viruses carry their own RNA-dependent RNA polymerase, which synthesizes viral RNA strands from the dsRNA template. These viral RNA strands then serve as templates for translation and replication.

5) Viral Protein Synthesis: The viral genes are transcribed and translated to produce viral proteins. These proteins are crucial for various functions, including viral replication, assembly, and evasion of host defenses.

6) Maturation: The newly synthesized viral components, such as viral proteins and genetic material, come together to assemble new viral particles, called virions. Maturation of daughter virions take place either in the host cell nucleus or cytoplasm or membrane.

7) Release:

· Lysis: Some viruses cause the host cell to undergo lysis, resulting in cell death and the release of newly formed virions into the surrounding environment. This lysis can occur due to the accumulation of viral components or the release of enzymes that disrupt the host cell's membrane

· Budding: During budding, the viral particle acquires an envelope from the host cell's membrane, and the newly formed virions are released by budding off from the cell surface. This process allows the virus to exit the host cell without immediate cell death.