Viruses are infectious agents that have the particularity that they are not considered as forms of life itself.
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 themselves.
Next we’ll look at the cycle of viral replication , which will allow us to understand why viruses are so particular and what makes them so extremely strange.
How does a virus reproduce?
The cycle of virus replication 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, whether they are prokaryotes or eukaryotes and whether they have only one of them or, as in the case of animals, millions. Pathogens such as bacteria, however small, 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 proper. It performs several functions: nutrition, development and reproduction.
Viruses, because they do not contain this type of structure and because they are not a cell, are not considered to be living beings, in addition to the fact that they are not capable of performing the three basic functions of any cell on their own . They require a cell to be able to carry out these functions. That is why their reproductive cycle is so surprising, since, as they cannot carry out it by themselves, they require a form of life to be able to multiply. They are agents that cannot continue to exist without the action of an organism.
Viral replication and its stages
The cycle of viral replication consists of the following phases: fixation or absorption, penetration, stripping, 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 pathogen on 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 “squat” for the virus . Depending on the degree of virus-receptor specificity, the virus will be more or less successful in carrying out the infection.
Once bound to the cell surface receptor, viruses induce changes in their capsid proteins, which leads to the fusion of the viral and cell membranes . Some viruses contain DNA (viral DNA), which can enter the cell through endocytosis.
In order to enter the cell, this viral DNA requires the membrane to be ruptured and an anchorage point for the virus to be established there. This is made possible by 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 content into the cytoplasm , that is, the aqueous medium inside the cell. If a cell contains capsides on its cell surface, this indicates that the cell has been infected.
It should be noted that there are also viruses that do not perform this process in the same way. Some are introduced directly into the cell with their capsid and everything. This is where we can talk about two types of penetration.
- Direct: after it is fixed, the virus breaks through 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 removed and degraded, either by viral enzymes or those of the host organism, and the viral DNA is released.
It is called nudity because the virus, if introduced into the body, loses its capsid and exposes its internal material, as if it were naked . Depending on the duration of the phase of synthesis, two modes of the cycle of viral infection 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, which is necessary for its multiplication, and it is here that the reproduction itself would start. This is the most common modality.
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 bacteria continues to carry out its vital functions, as if nothing were wrong. When the bacterial DNA duplicates itself, the viral DNA coupled to it will also duplicate , 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 multiply with each bacterial replication.
This viral DNA will be released from the DNA of the bacterium when the right conditions exist , continuing with its other 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, such as the wart papillomavirus and some retroviruses that are involved in cancer diseases.
Although we have already introduced it in the naked phase, the multiplication phase of the virus is the one in which the actual replication of the virus takes place.
In essence, the aim is to replicate the genetic material of the virus so that its genetic message is transcribed into an RNA molecule and translated into the form of viral proteins , both those that form the capsid and the enzymatic proteins inside it. Different types of viruses must be taken into account at this stage, as DNA is not always found in their capsids.
Viruses with DNA, which conform to the process explained in the previous phase, carry out the replication of their genetic material in a similar way as 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 resorting to cellular DNA . Each strand of RNA works on its own as a template for the synthesis of its complementarities, the cell being a simple environment in which the process takes place.
However new strands of DNA and RNA are formed, then the assembly of the parts to build the new virions takes place. This assembly can take place by the action of enzymes or mechanically.
5. Release of new viruses
After the multiplication of the virus takes place, the new individuals leave, and like their ‘parent’ they will be able to infect other host cells.
On the one hand there is the gemination release . This occurs when 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 moaning is the influenza A virus. The moment the virus is released, it acquires the lipid cover of the host cell.
On the other hand we have the release by lysis , in which the death of the cell that has been infected does occur. Viruses that reproduce with this modality are called cytolytics, since they kill the cell when infected. 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.
the residual viral proteins that remain 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, recognized by T cells.
- 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.