Viral replication: what it is and what are its phases

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 unit basic structure of all organisms, the cell, require the existence of an organism in order to be able to breed. They are not capable of replicating on their own.

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

• Related article: "The 5 types of viruses, and how they work"

How does a virus reproduce?

The virus replication cycle is the term used to refer to the reproductive capacity of these infectious agents. Viruses are acellular forms, that is, they lack cells, something that all organisms do have, be they these prokaryotes or eukaryotes and either having only one of them or, as is the case of animals, millions. Pathogens 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 all living beings and is considered the smallest element that can be considered a living being itself. It performs several functions: nutrition, development and reproduction.

Viruses, since they do not contain this type of structure nor are they cells, are not considered living beings, in addition to they are not capable of performing the three basic functions of any cell on their own. They require a cell to carry out these functions. That is why their reproductive cycle is so surprising, given that, since they cannot carry it out on their own, they require a way of life 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: fixation or absorption, penetration, undressing, multiplication and release of the new virus.

1. Fixation or absorption

The first step for the viral infection, which will culminate in its multiplication, is the fixation of the pathogenic agent in the cell membrane where the whole process will take place. The fixation is carried out by means of viral ligands, which are proteins found in the geometric capsule of the virus, called the capsid.

These proteins interact with specific receptors on the cell surface that will act as a "squatter 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, which lead to the 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 for this viral DNA to enter the interior of the cell, the membrane must have been ruptured and an anchor point established there for the virus. This is possible by means of hydrolytic enzymes found in the capsid.

Through the break, the virus introduces a central tube with which will inject its viral DNA, emptying its capsid and introducing its contents into the cytoplasm, that is, 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 said that there are also viruses that do not carry out this process in an identical way. Some go directly inside the cell with its capsid and everything. This is where we can talk about two types of penetration.

• Direct: After fixing itself, the virus opens a gap and gets inside the cell.
• Endocytosis: the cell creates a vesicle for the virus to enter.

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 stripping because the virus, if introduced into the body, loses its capsid and reveals its internal material, as if stripping. Depending on the duration of the synthesis phase, two types of the viral infection cycle can be distinguished.

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

On the other hand is the lysogenic cycle. Viral DNA is closed 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 were wrong. When the bacterial DNA is duplicated, the viral DNA coupled to it will too., becoming part of the DNA of the two daughter bacteria.

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

This viral DNA will detach from the DNA of the bacteria when the right conditions exist for it., continuing with its remaining infectious phases and producing new viruses while contributing to the death of the bacteria.

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

4. Multiplication

Although we have already introduced it in the stripping phase, the virus multiplication phase is the one in which the replication itself occurs.

In essence, it is about replicating the genetic material of the virus, their 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. In this phase, different types of viruses must be taken into account, since DNA is not always found in its capsid.

Viruses with DNA, which conform to the process explained in the previous phase, carry out the replication of their genetic material in in a similar way to how cells do, using the cell's DNA as a scaffold to make the multiplication of that material.

Other viruses, which contain RNA, replicate their genetic material without the need to go to cellular DNA.. Each RNA chain works by itself as a template for the synthesis of its complements, the cell being a simple environment where the process is carried out.

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

• You may be interested in: "Differences between DNA and RNA"

5. release of new viruses

After the multiplication of the virus has occurred, the exit of the new ones takes place. individuals, which, like their 'progenitor', will have the capacity to infect other cells hostesses.

On one side is the budding liberation. This occurs when the new virus does not wait for the cell to die to leave it, but instead leaves it at the same time that they are reproducing, so that the cell continues to live while it 'gives birth' to new ones. virus.

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

On the other we have the release by lysis, in which the death of the cell that has been infected does occur. Viruses that reproduce in this way are called cytolytic, since they kill the cell upon infecting it. An example of these is the smallpox virus.

After 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 that remain in the cytoplasm can be processed by the cell itself, if it is still alive, and presented on its surface together with MHC (major histocompatibility complex) molecules, recognized by T cells.

Bibliographic references:

• Collier, L.; Balows, A.; Susman, 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.