Viruses and Bacteria

Viruses

Viruses are parasites, typically ranging from 0.02 to 0.3 μm. They are completely dependent on cells (bacteria, plants or animals) to reproduce. Viruses have an outer covering of protein and sometimes lipids, a core of RNA or DNA, and some have the enzymes needed for the first steps of viral replication.

They are mainly classified according to the nature and structure of their genomes and their methods of replication. Thus, there are DNA viruses and RNA viruses. Each type can contain single or double stranded genetic material.

DNA viruses normally replicate in the nucleus of the host cell and RNA viruses in the cytoplasm.

For infection to occur, the virus first attacks the host cell at one of several receptor molecules on the cell surface. The viral DNA or RNA then separates from the outer layer (uncapsidation) and replicates inside the host cell, in a process that requires specific enzymes. The newly synthesized viral components then form a complete viral particle.

The host cell usually dies, releasing new viruses that infect other host cells. Each step of viral replication recruits different enzymes and substrates and provides an opportunity to interfere with the infection process.

The SARS-CoV-2 virus that causes COVID-19 is transmitted primarily through the droplets that are generated when an infected person coughs, sneezes, or exhales. These droplets are heavy and therefore do not remain suspended in the air for long periods of time, but deposit quickly on floors or surfaces.

Anyone can be infected by breathing in the virus if they are close to someone with COVID-19 or touch a contaminated surface and then touch their eyes, nose, or mouth.

SARS-CoV-2 is a ribonucleic acid (RNA) virus, whose genetic material is represented by a single positive RNA molecule (RNA+). It is classified as RNA+ because of its 5’3′ direction, which means that it can be read directly by the cellular structures of the host cell. It is considered a type of messenger RNA that induces the production of viral proteins. Another characteristic of this type of RNA is the presence of the enzyme RNA polymerase, which either accompanies the virus or is produced by the infected cell. This enzyme allows the production of a negative RNA molecule (RNA-) from the RNA+ molecule. This negative RNA molecule (RNA-) is transient, and from it, numerous positive RNA molecules (RNA+) are produced that are identical to the original positive RNA (RNA+). Therefore, the negative RNA molecule (RNA-) serves as a template for the production of positive RNA molecules (RNA+) that are descendants of the virus that infected the cell, and these descendants will parasitize the cell.

With the viral proteins already produced, assembly of the virions occurs by the inclusion of the positive RNA molecules (RNA+) in protein capsids. As they approach the cell plasma membrane, the virions are surrounded by a lipid bilayer, which opens at the end of the process, releasing a large amount of virus outside the cell.