REPRODUCTION OF VIRUSES: PHASES AND CHARACTERISTICS - BIOLOGY - 2024

The reproduction of viruses or viral replication is the event by which a virus particle multiplies by several orders of magnitude, by hijacking the host cell's enzymatic machinery. Since viruses are not made up of cells, they cannot reproduce independently, strictly requiring a cellular host to do so.

There are two general alternatives by which a virus can reproduce: the lytic cycle or the lysogenic cycle. Both processes have been widely studied in viruses that infect bacteria or bacteriophages.

Reproductive cycle of viruses.

1-Fixation

2-Penetration

3-Unfolding

4-Synthesis (4a-Transcription, 4b-Translation, 4c-Genome Replication)

5-Assembly

6-Liberation

Source: Franciscosp2

The lytic cycle culminates in the rupture of the host cell, while in the lysogenic cycle, the cell continues to live with the genetic material of the virus inside.

In the case of the lytic pathway, the virus finds the potential cell that it will infect and attaches itself to it through receptors that it recognizes on the cell surface. It then injects its DNA into the cytoplasm, where the production of structural components will begin. These pieces of nucleic acids and proteins are assembled and released, after which they can infect new hosts.

The lysogenic cycle begins in a similar way, with the exception that the virus DNA will undergo a recombination process and will be integrated into the chromosome of its host. The virus remains latent inside the cell, until the action of some chemical or UV light triggers the lytic cycle.

What is a virus?

Before explaining what the reproduction of viruses consists of, we must be clear about several aspects related to the biology of these entities. Viruses are not cells, but rather simple structures made up of nucleic acids and some proteins.

The complete and developed set of the infectious virus particle is known as a virion.

Unlike organic beings composed of cells, viruses do not have metabolism or exchange substances in a controlled manner with the external environment. But what they can do is reproduce inside biological systems that do present these functions: that is, in living cells.

For this reason, viruses are considered obligate cellular parasites, as they cannot complete their reproduction without the living cell. Its hosts can be vertebrates, invertebrates, plants, protists, bacteria, etc., depending on the virus studied.

For reproduction, viruses must hijack their host's enzymatic machinery. This aspect has consequences when developing drugs to stop the viral infection, since affecting the reproduction of the virus can interfere with the reproduction of the host's cells. We will explore how this process occurs below.

How do viruses multiply?

As we mentioned, viruses are biological particles of a very simple nature. Therefore, the nucleic acid (either DNA or RNA) that they possess contains the information for the production of a few proteins and enzymes to build the virion.

In a single host cell, a virion can produce thousands of viral particles similar to the initial one, using the metabolic machinery of its host.

Despite the fact that viruses and their hosts are highly variable, the reproduction cycle is similar in all. Below we will generalize the process and describe step by step the reproduction of bacteriophages, viruses that infect bacteria. Then we will mention some of the peculiarities in viruses that infect animals.

Multiplication of bacteria viruses (bacteriophages)

Bacteriophages can reproduce in two alternative ways: the lytic cycle or the lysogenic cycle. As its name implies, the last step of the lytic involves the lysis (and thus death) of the host cell. In contrast, the lysogenic cycle involves viral reproduction with the living cell.

-Lithic cycle

The lytic process in bacteriophages T (T2, T4 and T6) in the famous E. coli bacteria is known in great detail. The processes that we will describe below are based on these study models.

It occurs in five distinct stages: fixation, penetration, biosynthesis, maturation, and release.

Fixation

This step is also known as virus adsorption. The first thing that must happen for a virus to multiply is the meeting between the virus particle and the host cell. This collision happens haphazardly.

The virus binds to some complementary receptor that it recognizes on the cell surface; in this case, in the bacterial cell wall. This binding is a chemical interaction where weak bonds occur between the virus and the receptor.

Penetration

Once the virus recognizes the recipient, it proceeds to inject its genetic material. The bacteriophage releases an enzyme that damages a portion of the cell wall. In this context, the viral particle functions as a hypodermic syringe in charge of injecting DNA.

Biosynthesis

When the DNA has reached the host's cell cytoplasm, the biosynthesis of the genetic material and proteins of the organism in question begins. Host protein synthesis is stopped by a series of steps orchestrated by the virus.

The invader manages to sequester both the free nucleotides of the host, ribosomes and amino acids, as well as the enzymes necessary to copy the DNA of the virus.

Maturation

As all the building blocks of viruses are synthesized, the assembly or maturation process begins. The assembly of the components of the viral particles occurs spontaneously, eliminating the need for other genes to assist the process.

Release

At the end of the assembly process, the viruses have to be released into the extracellular environment. As we are explaining the lytic cycle, this final step involves the lysis of the cell that assisted the entire process.

Lysis involves the rupture of the plasma membrane and the cell wall. The degradation of this last component occurs by the action of the enzyme lysozyme, which is synthesized within the cell during the process described.

In this way, the newly synthesized virus particles are released. These can infect neighboring cells and repeat the cycle again.

-Lysogenic cycle

Not all viruses penetrate host cells and destroy them at the cost of their own reproduction. An alternative mode of multiplication is called the lysogenic cycle. Viruses capable of reproducing in this way are known as temperates.

Although some viruses can reproduce through the lytic pathway described in the previous section, they can also reproduce without destroying the cell and remain latent or inactive within it.

To describe it to you, we will use as a model organism the bacteriophage lambda (λ), a lysogenic bacteriophage that has been thoroughly studied.

The stages by which the lysogenic cycle occurs are: penetration into the host, formation of a circular DNA from the linear DNA molecule, and recombination with the host DNA.

Integration of viral DNA into host DNA

The initial stages occur in a very similar way to the previous cycle, with the exception that the DNA of the virus is integrated into the DNA of the host cell, by a process of recombination.

In this state, the virus is latent in the cell, and the viral DNA replicates along with the host's DNA.

Alternation between lytic and lysogenic cycle

On the other hand, a variety of stochastic events can lead to the change from lysogenic to lytic cycle. Among these events are exposure to UV radiation or certain chemicals that lead to the excision of phage DNA and the initiation of lysis.

Consequences of lysogeny

There are important consequences of lysogeny, namely: (i) lysogenic cells are immune to subsequent infections from the same bacteriophage, but not to a different virus; (ii) the cells can acquire new characteristics by integrating the phage's genetic material, such as the production of some toxins and (iii) the specialized transduction process is allowed.

Multiplication of animal viruses

Broadly speaking, animal viruses follow a pattern of multiplication quite similar to that described in viruses that infect bacteria. However, there are some striking differences in both processes.

The most obvious is the cell entry mechanism, due to the differences that exist at the structural level between eukaryotic and prokaryotic cells. In animal cells, the receptors are made up of proteins and glycoproteins anchored in the plasma membrane.

An example of this is the HIV virus. In order to enter the cell, the virus recognizes a receptor called CCR5. Certain individuals have a deletion (that is, portions of DNA are missing) of 32 base pairs in the gene that codes for the cellular receptor that destroys the protein and confers resistance to the dreaded virus.

Many invaders take advantage of the receptors that mediate the endocytosis process to gain entry to the cell, through the formation of vesicles. Viruses that are covered by a membrane can enter the cell by fusing the lipid membranes.

Once the virus has penetrated, the synthesis of the virus particles is somewhat variable. Animal cells have different enzymatic machinery than we find in bacteria.

References

1. Forbes, BA, Sahm, DF, & Weissfeld, AS (2007). Diagnostic microbiology. Mosby.
2. Freeman, S. (2017). Biological science. Pearson Education.
3. Murray, PR, Rosenthal, KS, & Pfaller, MA (2015). Medical microbiology. Elsevier Health Sciences.
4. Reece, JB, Urry, LA, Cain, ML, Wasserman, SA, Minorsky, PV, & Jackson, RB (2014). Campbell biology. Pearson education.
5. Tortora, GJ, Funke, BR, & Case, CL (2016). Microbiology. An introduction. Pearson.