Viral replication: what is it and what are its phases?

The process in which viruses reproduce is called viral replication. Let's see what its stages are.

Viruses are infectious agents that have the particularity that they are not considered as life forms themselves.

The main reason why they are not considered living beings is that, in addition to not having the basic unit of any organism, the cell, they require the existence of an organism in order to reproduce. They are not capable of replicating on their own.

Next we will see the cycle of viral replicationThis will allow us to understand why viruses are so particular and what makes them so strange.

How does a virus reproduce?

The replication cycle of viruses is the term with which reference is made to the reproductive capacity of these infectious agents.. Viruses are acellular forms, that is, they lack cells, something that all organisms do have, whether they are prokaryotic or eukaryotic and whether they have only one cell or, as in the case of animals, millions of cells. Pathogenic agents such as bacteria, however small they may be, contain at least one cell and are therefore living beings.

The cell is the morphological and functional unit of every living being and is considered the smallest element that can be considered a living being itself. It performs several functions: nutrition, development and reproduction.

Viruses, not containing this type of structure and not being a cell, are not considered living beings, in addition to the fact that they are not capable of performing the three functions of nutrition, development and reproduction. are not capable of performing the three basic functions of any cell on their own.. They require a cell to carry out these functions. This is why their reproductive cycle is so surprising, since they cannot carry it out on their own, they require a life form in order to multiply. They are agents that cannot continue to exist without the action of an organism.

Viral replication and its stages

The viral replication cycle consists of the following phases: attachment or absorption, penetration, denudation, multiplication and release of new viruses.

1. Attachment or absorption

The first step in viral infection, culminating in multiplication, is the attachment of the pathogen to the cell membrane where the entire process will take place. The attachment is carried out by means of viral ligands, which are proteins found in the geometric capsid of the virus, called capsid.

These proteins interact with specific receptors on the cell surface that will act as a "squat house" for the virus.. Depending on the degree of virus-receptor specificity, the virus will be more or less successful in carrying out the infection.

2. Penetration

Once bound to the cell surface receptor, viruses induce changes in their capsid proteins, viruses induce changes in their capsid proteins, leading to fusion of the viral and cell membranes.. Some viruses contain DNA (viral DNA), which can enter the interior of the cell by endocytosis.

In order to enter the interior of the cell, this viral DNA requires that the membrane has been ruptured and an anchor point for the virus is established there. This is made possible by hydrolytic enzymes found in the capsid.

Through the rupture, the virus inserts a central tube with which it will inject its viral DNA. inject its viral DNA, emptying its capsid and introducing its contents into the cytoplasm, i.e. the aqueous medium.i.e. the aqueous medium inside the cell. If a cell contains capsids on its cell surface, this indicates that the cell has been infected.

It should be noted that there are also viruses that do not carry out this process in the same way. Some are introduced directly inside the cell, capsid and all. It is here that we can speak of two types of penetration.

• Direct: after attaching itself, the virus opens a gap and enters the cell.
• Endocytosis: the cell creates a vesicle for the virus to get inside.

There are viruses that have a lipid envelope, which is of the same nature as the cell membrane.. This makes the cell prone to fuse its membrane with that of the virus and endocytosis occurs.

Once inside the cell, the capsid, if it has remained intact, is eliminated and degraded, either by viral enzymes or those of the host organism, and the viral DNA is released.

3. Stripping

It is called undressing because the virus, if introduced into the organism, loses its capsid and exposes its internal material, as if it were undressing, The virus loses its capsid and exposes its internal material, as if it were undressing itself.. Depending on the duration of the synthesis phase, two modalities of the viral infection cycle can be distinguished.

On the one hand, we have the ordinary cycle. The viral DNA proceeds immediately to the transcription of its genetic message into viral RNA, necessary for its multiplication, and it is here that the reproduction itself would begin. This is the most common mode.

On the other hand, there is the lysogenic cycle.. The viral DNA closes at its ends, forming a circular DNA, which is similar to that of prokaryotic organisms. This DNA is inserted into the bacterial DNA, in a region where they have a similar nucleotide chain.

The bacterium continues to carry out its vital functions, as if nothing happened. When the bacterial DNA is duplicated, the viral DNA coupled to it will also duplicate, becoming part of the DNA of the two daughter bacteria.The viral DNA will also duplicate, becoming part of the DNA of the two daughter bacteria.

In turn, the daughter bacteria will have their offspring and so on, so that with each bacterial replication the viral DNA will also multiply.

This viral DNA will be detached from the bacterial DNA when the appropriate conditions are met.The lysogenic cycle can also be used to produce new viruses in the bacterial replication cycle, continuing with its remaining infectious phases and producing new viruses while contributing to the death of the bacterium.

The lysogenic cycle can also occur in viruses that affect animal cells, as in the case of the wart papillomavirus and some retroviruses that are involved in oncological diseases.

4. Multiplication

Although we have already introduced it in the denudation phase, the multiplication phase of the virus is the one in which the replication of the virus itself takes place.

In essence, it is a matter of replicating the genetic material of the virus, its genetic message is transcribed into an RNA molecule and this is translated into a form that produces viral proteins, both those that form the capsid and the enzymatic proteins inside it.The viral proteins, both those that form the capsid and the enzymatic proteins inside it, are produced. At this stage, different types of viruses must be taken into account, since DNA is not always found in their capsid.

Viruses with DNA, which conform to the process explained in the previous phase, replicate their genetic material in a similar way to cells, using the cell's DNA as a scaffold for the multiplication of this material.

Other viruses, which contain RNA, replicate their genetic material without the need for cellular DNA.. Each RNA strand works on its own as a mold for the synthesis of its complements, the cell being a simple environment where the process takes place.

However new strands of DNA and RNA are formed, then the assembly of the pieces to build new virions takes place. This assembly can take place either by the action of enzymes or mechanically.

5. Release of the new viruses

After the multiplication of the viruses has taken place, the release of the new individuals takes place, which, like their 'progenitor', will have the capacity to infect other host cells.

On the one hand there is the gemination release. This occurs when the new viruses do not wait for the cell to die before leaving it, but leave it at the same time as they reproduce, so that the cell continues to live while it 'gives birth' to new viruses.

An example of a virus that is released by budding is the influenza A virus. At the moment the virus is released, it acquires the lipid envelope of the host cell.

On the other hand we have the release by lysisin which the cell that has been infected dies. Viruses that reproduce in this way are called cytolytic, since they kill the cell upon infection. An example of these is the smallpox virus.

Once the newly generated virus leaves the cell, some of its proteins remain in the host cell membrane. These will serve as potential targets for nearby antibodies.

residual viral proteins remaining in the cytoplasm can be processed by the cell itself, if it is still alive, and presented on the cell surface together with MHC (major histocompatibility complex) molecules, which are recognized by T cells.

Bibliographic references:

• Collier, L.; Balows, A.; Sussman, M. (1998) Topley and Wilson's Microbiology and Microbial Infections ninth edition, Volume 1, Virology, volume editors: Mahy, Brian and Collier, Leslie. Arnold. ISBN 0-340-66316-2.
• Dimmock, N.J; Easton, Andrew J; Leppard, Keith (2007) Introduction to Modern Virology sixth edition, Blackwell Publishing, ISBN 1-4051-3645-6.