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12.2.2.1. Lytic cycle

Lytic cycle of viruses

The lytic cycle is so named because the cell infected by a virus dies by rupture (lysis in Greek), when the new viral copies are released. The lytic cycle is the main method of viral reproduction and involves the destruction of infected cells. The lithic cycle consists of the following phases:

  1. Fixation or adsorption.
  2. Penetration.
  3. Eclipse phase (replication and synthesis of viral components).
  4. Assembly of new viruses.
  5. Lysis or release.

Fixation or adsorption phase

The virus binds to the host cell stably. The binding is specific, since the virus recognizes protein, lipoprotein or glycoprotein molecular complexes, present in cell membranes, which act as receptors that allow the adhesion of virions.

In the binding or adsorption phase, the virus binds to the host cell surface. Proteins in the capsid (or envelope or caudal fibers) bind to receptors on the host cell.

The fixation in the bacteriophages is carried out through the tips of the caudal fibers by means of  chemical bonds and then, mechanically, they drive the spines into the bacterial wall.

Other viruses, especially plants, do not attach to specific sites in the cell, but take advantage of areas of rupture or wounds to penetrate, or by the action of transmitting arthropods, mainly insects.

Penetration phase

The penetration of the nucleic acid of the virus into the cell can be in several ways:

  • By injection, as in bacteriophages and certain naked animal viruses (polio viruses). The bacteriophage pierces the cell wall of the bacteria with lysozyme enzymes from its base plate. It then contracts the tail sheath and injects the DNA through the hole, while the capsid , tail, and attachment plate do not enter the bacteria.
  • By endocytosis: viruses that infect cells animale s intact are introduced inside the cell in a vesicle endocytosis . Inside the cell, the virus breaks the membrane of the vesicle with the help of some hydrolytic enzymes and reaches the cytoplasm .
  • By fusion  of the viral envelope (in enveloped viruses ) with the plasma membrane of the cell.
  • Directly  through pores or breakdown areas of the cell surface, as is the case with many plant viruses

If the nucleocapsid also penetrates into the cell, decapsidation occurs, rupturing the capsid and releasing the nucleic acid into the cytoplasm.    

Eclipse phase (replication and synthesis of viral components)

It is called the eclipse phase because in this phase no copies of the virus are observed in the cell, but the RNA synthesis and transcription necessary to generate the capsid protein copies is taking place. The continuous formation of viral nucleic acids and bacterial DNA- destroying enzymes also occurs.

The metabolism cell is canceled and is used to synthesize the elements of the virus. The nucleic acid of the virus uses nucleotides and RNA polymerase from the host cell to synthesize mRNA which, in turn, will synthesize endonuclease enzymes that destroy cellular DNA. Afterwards, the nucleic acid of the virus will be duplicated and the proteins (capsomeres) of the new viruses will be synthesized.

The two main functions of the eclipse stage are:

The retrovirusesviruses having RNA as nucleic acid , such as the HIV , causing the  acquired immunodeficiency syndrome (AIDS), have two identical copies of single stranded RNA, which replicates via an intermediate form of dsDNA. The reverse transcriptase or reverse transcriptase enzyme is responsible for synthesizing DNA from RNA.

Viruses with RNA have much more variability than those with DNA, since RNA polymerase makes more reading errors than DNA polymerase. This implies that RNA viruses have more changes and it is more difficult to find vaccines against them.

Assembly phase

In the assembly phase, newly synthesized viral components unite to form new viruses. The capsomeres come together to form the capsid and the viral nucleic acid is packaged within it.

Lysis or release phase

In the lysis or release phase, the new viruses leave the cell by breaking (lysis) the bacterial wall, by the action of the enzyme endolysin. These new viruses can already infect a new cell. Lysis leads to cell death.

In the release phase, enveloped viruses take their membrane from the newly ruptured cell, after inserting into it proteins encoded by the virus genome.

Fundamental ideas about the lytic cycle of viruses

In the lytic cycle of a virus, the host cell dies by rupture (lysis), when the new copies of the virus are released.

The lithic cycle consists of several phases: