What is a Virus?

The virus is acellular and considered non-living outside the host cell. The replication of the virus occurs only inside the living cells. Only a few genes are present in the smallest virus, whereas many genes are present in the largest virus. These genes encode specific viral proteins.

What is Viral Replication?

Viral replication is defined as the process in which there is the production of viruses.

• The virus cannot replicate on its own; it needs the host's cell machinery.
• The progeny formed after virus replication is ready to infect a new cell.
• Before replication starts, the virus gets inside the cell of the host cell. Then, viruses continue to infect a new host cell by generating multiple copies of their genome and subsequently packaging the genome inside viral proteins. Between viruses, the process of replication varies and depends mainly on the types of genes involved. In the nucleus, the assembly of DNA (deoxyribonucleic acid) virus occurs.
• In the host cell, various biochemical and structural changes occur that result in cell damage. These changes occurring in the cell are cytopathic, which can cause cell damage and alter the cell's function.
• The virus multiplies and forms its replicates in the living cell only. Thus, the synthetic machinery and energy for viral replication are provided by host cells.

Steps Involved in Viral Replication

1. Attachment

Attachment is the first stage of replication of the virus. The host cell surface consists of receptors that bind to the attachment proteins present on the virus's capsid. Next, the virus injects its DNA or RNA (ribonucleic acid)inside the host cell for initiating infection in the host cell.

2. Entry

In the case of animals, the virus enters the cell through endocytosis, which is facilitated by the fusion of the viral envelope and animal cell membrane. In the case of plants, the process of pinocytosis facilitates the entry of the virus into the cells. Thus, the virus particle enters into the host by invagination or formation of the pinocytic vacuole. The genome of the virus enters the cell naked, and the capsid remains outside the cell.

3. Uncoating

In this phase, the virus protein coat is stripped off with the help of an enzyme. Subsequently, the virus's nucleic acid is released inside the host cell and is ready for transcription and translation.

4. mRNA (messenger RNA) production

The mechanism of viral replication depends on the genome of the virus. In the case of DNA viruses, the host cell enzymes and proteins are used to replicate the viral genome. In the case of RNA viruses, the RNA core is used as a template for synthesizing the viral genome. Furthermore, the viral enzymes are synthesized by the viral RNA. Finally, the viral components are synthesized in the host cell by using the mRNA.

5. Synthesis of viral component

Using the host's existing cell organelles, the virus manufactures its proteins. The viral mRNA is translated into two types of proteins on the cellular ribosomes: structural and non-structural. The structural protein synthesizes the virus particles. The viral polymerase mediates the formation of the new viral genome.

6. Virion assembly

Virion can be described as an intact virus particle or active virus particle. For the formation of the new virus particle, proteins and nucleic acid are assembled. The assembly occurs in the cell's cytoplasm, nucleus, and plasma membrane. The newly synthesized viral components are assembled into immature virus particles. In the nucleus, the non-enveloped DNA viruses assemble their nucleocapsid as the nucleus is the site for genome replication. The viral proteins travel through nuclear pores to reach the site of assembly. The assembly of viruses occurs near the plasma membrane as these viruses derive their envelope from the host plasma membrane.

7. Release 

The release is the final step in the process. The mature viruses produced inside the host cell are now ready to infect other cells. These viruses rupture the host cell and invade healthy cells. After release, the virion can continue the new cycle of infection.

The genome of a virus is either DNA or RNA. The packaging of the genome occurs in a protein capsid. Thus, the organization and composition of the genome of the virus vary.

DNA Viruses

The process of transcription and translation is not coupled in the animal DNA viruses. In these viruses, transcription occurs inside the host nucleus, and translation occurs within the cytoplasm (except for poxvirus).

RNA Viruses

• In these viruses, the nucleic acid is RNA. The RNA viruses may have a single-stranded or double-stranded genome. Many diseases are caused by these viruses, such as the common cold, polio, and mumps.
• This virus encodes the RNA-directed RNA polymerase.
• In a few cases, the RNA virus is further classified based on the sense of polarity of its strands. The negative sense and positive sense are the two types of RNA viruses based on polarity.
• A double-stranded RNA virus is a diverse group of RNA viruses. The reovirus is an example of a double-stranded RNA virus.
• Examples of single-stranded RNA viruses include the poliovirus, rhabdovirus, and influenza virus.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelor of Science in Biology
• Bachelor of Science in Zoology
• Master of Science in Zoology
• Master of Science in Microbiology