Lytic cycle

Before we talk about the lytic cycle, we have to talk a little bit about viruses. These are acellular beings – devoid of cell structure, as in all living things. For this reason, they are completely dependent on the biochemical mechanisms of the host cell for its development and multiplication. Considering this fact, viruses are classified as mandatory intracellular parasites.

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There are two types of cycles through which they reproduce: the lytic cycle and the lysogenic cycle. When it presents reproduction by lytic cycle, which this article is about, the virus is called lytic or virulent. As examples of these, we can mention bacteriophages or phages – viruses that destroy cells.

During this cycle, the virus inserts its genetic material into the host cell material. The normal functions of that cell are then interrupted in the presence of the virus’ nucleic acid. This way, it starts to dominate the cell metabolism and ends up destroying it. This cycle has five phases to be described below.

1st – Adsorption

In this first phase, the virus is recognized and fixed to the cell. Viruses only affect a unique type of cell – that’s why they are specific parasites – and due to the presence of the host cell’s chemicals, they are able to detect it and attach to the membrane.

2nd – Penetration

After the first step, the virus inserts its genetic material into the host cell directly, through endocytosis, or by fusing the viral envelope. In the first situation, only the genetic material is injected, leaving the protein part on the outside. In the second situation, there is the fixation of the virus, promoted by the chemical receptors of the cell membrane, and then the virus is encompassed by its invaginations. In the third case, the viral envelope merges with the cell membrane, the capsid disintegrates and the genome invades the cell. This third case, however, happens only with enveloped viruses.

3rd – Synthesis

During synthesis, the virus begins to determine the cell’s metabolic activities so that enzymes, previously used in protein and nucleic acid synthesis in the cell, begin to be responsible for the production of viral particles.

4th – Assembly

Having been produced, the virus components are organized and start to form new parasites.

5th – Release

In the fifth and final stage, dozens of new parasites produce a viral enzyme known as lysozyme. This disrupts the host cell – performs cell lysis -, a process favored by cell depletion due to its use for the production of viral structures. By destroying the cells, the viruses then release and start new infections in neighboring cells, restarting the entire cycle.

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