LITHIC CYCLE: PHASES AND REAL EXAMPLE - BIOLOGY - 2025

The lytic cycle is one of the two alternative life cycles of a virus within a host cell, through which the virus that enters the cell takes over the cell's replication mechanism. Once inside, DNA and viral proteins are made and then lyse (break) the cell. Thus, newly produced new viruses can leave the now disintegrated host cell, and infect other cells.

This method of replication is contrasted with the lysogenic cycle, during which the virus that has infected a cell inserts itself into the host's DNA and, acting as an inert segment of DNA, replicates only when the cell divides.

Lambda phage: lytic cycle and lysogenic cycle

The lysogenic cycle does not cause any damage to the host cell, but is a latent state, while the lytic cycle results in the destruction of the infected cell.

The lytic cycle is generally considered the main method of viral replication, as it is more common. Additionally, the lysogenic cycle can lead to the lytic cycle when there is an induction event, such as exposure to ultraviolet light, that causes this latent stage to enter the lytic cycle.

Through a better understanding of the lytic cycle, scientists can better understand how the immune system responds to repel these viruses, and how new technologies can be developed to overcome viral diseases.

In order to learn how to interrupt viral replication and thus address the diseases caused by viruses that affect humans, animals and agricultural crops, many studies are being carried out.

Scientists hope one day to be able to understand how to stop the triggers that start the destructive lytic cycle in viruses of health concern.

Generalities of the lithic cycle

Viral reproduction is best understood by studying viruses that infect bacteria, known as bacteriophages (or phages). The lytic cycle and the lysogenic cycle are the two fundamental reproductive processes that have been identified in viruses.

Based on studies with bacteriophages, these cycles have been described. The lytic cycle involves the virus entering a host cell and taking over the molecules that replicate the cell's DNA to produce viral DNA and viral proteins. These are the two classes of molecules that structurally make up phages.

When the host cell has many newly produced viral particles inside it, these particles promote the breakdown of the cell wall from within.

Through molecular mechanisms of the phage, certain enzymes are produced that have the ability to break the bonds that maintain the cell wall, which facilitates the release of new viruses.

For example, the bacteriophage lambda, after infecting an Escherichia coli host cell, normally inserts its genetic information onto the bacterial chromosome and remains in a dormant state.

However, under certain stress conditions, the virus can begin to multiply and take the lytic pathway. In this case, several hundred phages are produced, at which point the bacterial cell is lysed and the progeny are released.

Phages of a lytic cycle: Example phage T4

Viruses that multiply through the lytic cycle are called virulent viruses because they kill the cell. Phage T4 is the most studied real example to explain the lytic cycle, which consists of five stages.

Fixation / Adhesion to the cell

The T4 phage first attaches itself to an Escherichia coli host cell. This binding is carried out by the fibers of the tail of the virus that have proteins with high affinity for the host cell wall.

The places where the virus attaches itself are called receptor sites, although it can also be attached by simple mechanical forces.

Penetration / Entry of the virus

To infect a cell, the virus must first enter the cell through the plasma membrane and the cell wall (if present). It then releases its genetic material (RNA or DNA) into the cell.

In the case of phage T4, after binding to the host cell, an enzyme is released that weakens a site on the host cell wall.

The virus then injects its genetic material similar to a hypodermic needle, pressing against the cell through the weak spot in the cell wall.

Replication / Synthesis of viral molecules

The nucleic acid of the virus uses the machinery of the host cell to produce large amounts of viral components, both the genetic material and the viral proteins that comprise the structural parts of the virus.

In the case of DNA viruses, DNA transcribes itself into messenger RNA (mRNA) molecules that are then used to direct the cell's ribosomes. One of the first viral polypeptides (proteins) to be produced fulfills the function of destroying the DNA of the infected cell.

In retroviruses (which inject a strand of RNA), a unique enzyme called reverse transcriptase transcribes viral RNA into DNA, which is then transcribed back to mRNA.

In the case of phage T4, the DNA of the E. coli bacteria is inactivated and then the DNA of the viral genome takes over, and the viral DNA makes the RNA of the nucleotides in the host cell using the host cell's enzymes.

Assembly of viral particles

After multiple copies of the viral components (nucleic acids and proteins) have been produced they assemble to form whole viruses.

In the case of T4 phage, the proteins encoded by the phage DNA act as enzymes that cooperate in the formation of the new phage.

All of the host's metabolism is directed toward the production of viral molecules, resulting in a cell filled with new viruses and unable to regain control.

Lysis of the infected cell

After the assembly of the new virus particles, an enzyme is produced that breaks down the wall of the bacterial cell from within and allows the entry of fluids from the extracellular environment.

The cell eventually fills with fluid and bursts (lysis), hence its name. The new viruses released are able to infect other cells and thus start the process again.

References

1. Brooker, R. (2011). Concepts of Genetics (1st ed.). McGraw-Hill Education.
2. Campbell, N. & Reece, J. (2005). Biology (2nd ed.) Pearson Education.
3. Engelkirk, P. & Duben-Engelkirk, J. (2010). Burton's Microbiology for the Health Sciences (9th ed.). Lippincott Williams & Wilkins.
4. Lodish, H., Berk, A., Kaiser, C., Krieger, M., Bretscher, A., Ploegh, H., Amon, A. & Martin, K. (2016). Molecular Cell Biology (8th ed.). WH Freeman and Company.
5. Malacinski, G. (2005). Essentials of Molecular Biology (4th ed.). Jones & Bartlett Learning.
6. Russell, P., Hertz, P. & McMillan, B. (2016). Biology: The Dynamic Science (4th ed.). Cengage Learning.
7. Solomon, E., Berg, L. & Martin, D. (2004). Biology (7th ed.) Cengage Learning.