What are the steps of replication for an animal virus?

The flu virus is probably the first thing that comes to your mind when you hear the word virus. We’ve all probably had the flu at one point or another in our lives. So just how does the flu invade our bodies?

Watch this video from NPR for a quick introduction to the flu virus:

As we’ve seen, viruses go through as specific cycle as they replicate—and they target and infect cells in order to do so. Let’s look at the process of a flu virus replicating:

Each of the steps in viral replication has a name:

• Step 1: Attachment: The virus attaches itself to the target cell.
• Step 2: Penetration: The virus is brought into the target cell.
• Step 3: Uncoating and Replication: The enveloped virus loses its envelope, and viral RNA is released into the nucleus, where it is replicated.
• Step 4: Assembly: Viral proteins are assembled.
• Step 5: Egress (Release): New viral particles are released.

In the case of the flu, the target cell is not killed; however, other viruses, like the T4 bacteriophage, cause cell death to release the newly created copies of itself.

Remember: The correct order of the different steps is as follows:

Attachment, penetration, uncoating and replication, assembly, and egress (release)

As we mentioned earlier, some viruses kill the cells they infect. The influenza virus is packaged in a viral envelope that fuses with the plasma membrane. This way, the virus can exit the host cell without killing it. What advantage does the virus gain by keeping the host cell alive?

The host cell can continue to make new virus particles.

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Introduction:

As a virus is an intracellular pathogen, it cannot replicate without any metabolism and machinery of a host cell. Hence a virus must first invade a cell before viral replication can occur.

Viral Replication or Reproduction

A virus will multiply only in living cells and organisms. The host cell provides energy, synthetic machinery, and low-weight molecules for protein synthesis and nucleic acid.

Seven basic stages are essential for viral replication. They are:

1. Attachment: This is the first stage of the process. The viruses attach to the cell membrane of the host cell and inject their DNA or RNA into the host, which will initiate the infection process. In animals, the virus invades the cell through a process called endocytosis - fusing the virus and viral envelope with the host cell's cell membrane. In plants, the virus invades the cell through pinocytosis - working on pinching the virus.
2. Penetration or Entry: The cell membranes of the host cells transpose the virus particles enclosing this in pinocytotic vacuoles. This action protects the cells from antibodies, for example, as in the case of HIV viruses.
3. Uncoating: The enzymes of the cell, especially lysozymes, break the virus's protein coat. This releases the viral genomic nucleic acid.
4. Replication: This process is also called Transcription or mRNA Production. In this stage, for some of the RNA viruses, infected RNA will produce messengers in the form of mRNA and translate and change the genome products into protein products. For a virus that carries negative-stranded DNA or RNA, viruses are produced by transcription and later by translation. The main function of mRNA is to instruct the host cell to make components of viruses, and the viruses also take full advantage of the cell structures to replicate themselves
5. Synthesis of components of virus: The components are manufactured using the host cell's existing organelles: a. Viral structural proteins: they are proteins that make up the virus particle b. Viral Nonstructural proteins: they are proteins found mainly in the enzymes that help viral genome replication. c. Viral nucleic acid: New genomes are synthesized, and the templates are either newly formed complementary strands or the parental genome.
6. Assembly of virions: Virions are active and intact virus particles. At this stage, newly synthesized nucleic acid and genomes and proteins are assembled to form new virus particles. This occurs in the cytoplasm, plasma membrane, and cell's nucleus for developed viruses.
7. Virion release: Also called the liberation stage. The virus particles are now matured and are released in 2 ways. Either sudden ruptures of the cell or gradual forcing out of enveloped viruses through the cell's membranes. These new viruses can invade and attack other cells or remain dormant in the cells.

Viruses are released from the host cell by the budding process. After the release of virions, some viral proteins remain inside the host cell's membrane. The residual viral proteins that remain inside the host cell's cytoplasm will be processed and presented at the surface of the cells.

Reproduction and multiplication of a virus in short:

The reproductive cycle of a virus is also called the Lytic Cycle. All viruses follow the same basic steps:

1. A virus particle attaches to the host cell
2. The particle releases its genetic instructions into the host cell and invades it.
3. The injected genetic material uses the host cell enzymes.
4. Enzymes make a part of many more new viral particles.
5. These new particles assemble these parts into new viruses.
6. The newly-formed viruses break free from the host cell, resulting in the death of the host cells.
7. The newly-formed viruses are now capable of infecting the host cells.

Conclusion:

• Virologists and scientists define the formation of viruses during infection processes in the target host cells as the process of viral replication.
• As discussed above, they are carried out in a series of 7 steps that is: attachment, penetration and entry, uncoating, replication, synthesis of virus components, assembly of virions and lastly the release of virions.

FAQs:

1. What are the steps in viral replication?

The steps in viral replication are Attachment, Penetration, Uncoating, Replication, Synthesis of viral components, Assembly of virions, and release of virions.

2. What are the two types of replication in viruses?

The two types of viral replications are the Lytic cycle and the lysogenic cycle. Viruses attach themselves to the host cells and inject their DNA into the lytic cycle. Using the host cell's metabolism, the viral DNA replicates and forms proteins. Then, fully formed viruses assemble, and these viruses break and the cells spread to other cells to continue the cycles. Although the virus attaches itself to the host cell and injects its DNA into the lysogenic cycle, the difference is that the viral DNA will get incorporated into the host DNA and host cells. Hence, each time the viral cell undergoes replication, the viruses' DNA will also undergo replication, thus spreading the genetic information throughout the host cell.

3. What is the viral replication cycle called?

The reproductive cycle of a virus is also called the Lytic Cycle.

4. How long does a virus last?

This depends on the type and function of a particular virus and also the place it acts on. It can last from a few hours to several days. The amount of virus, the temperature, surface of action of the virus, and humidity of the environment determine how long a virus will last.

5. How do RNA viruses replicate?

The RNA viruses replicate their genomes through two unique pathways, and they are: RNA dependant RNA synthesis and RNA dependant DNA synthesis, also called reverse transcription. This is followed by DNA replication and transcription.

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