The lytic cycle is a method of viral reproduction characterized by its rapid and destructive nature. A virus infects a host cell, replicates its genetic material, produces new viral particles, and then destroys the host cell to release these new viruses.
Understanding the Viral Invaders
Viruses are microscopic infectious agents that can only replicate inside the living cells of other organisms. Bacteriophages are a common example, specifically targeting bacteria. A bacteriophage typically consists of genetic material, either DNA or RNA, encased within a protein shell called a capsid. Some bacteriophages, like the T4 phage, possess a more complex structure with a polyhedral head, a tail, and tail fibers, which are essential for interacting with their bacterial hosts.
The Stages of Viral Replication
The lytic cycle unfolds through a series of distinct stages, ensuring the efficient production of new viral particles.
Attachment (Adsorption)
The lytic cycle begins with attachment, also known as adsorption, where the bacteriophage recognizes and binds to the surface of a specific host bacterial cell. This binding is highly selective, occurring between proteins on the phage’s tail fibers and specific receptor molecules located on the bacterial cell wall, such as lipopolysaccharides or proteins. This ensures a particular phage can only infect certain types of bacteria. Firm attachment is necessary for subsequent steps.
Entry (Penetration)
Following successful attachment, the bacteriophage injects its genetic material into the host cell. For bacteriophages like T4, the tail sheath contracts, acting like a hypodermic needle to push a hollow tube through the bacterial cell wall and membrane. The viral DNA or RNA then enters the bacterial cytoplasm, while the protein capsid typically remains outside the cell.
Replication (Biosynthesis)
Once inside the host cell, the viral genetic material takes control of the bacterium’s cellular machinery. The phage expresses genes that produce enzymes to degrade the host cell’s DNA, effectively shutting down its normal functions. The host cell’s resources, including ribosomes and nucleotides, are then redirected to synthesize viral components such as new viral DNA or RNA, and viral proteins like capsomeres, tail proteins, and enzymes. This process rapidly produces the building blocks for new phages.
Assembly (Maturation)
After a sufficient number of viral components have been synthesized, assembly commences. During this phase, the newly created viral genetic material and proteins spontaneously come together to form complete, infectious viral particles, known as virions. This involves the viral genome being packaged into newly formed protein capsids, and other structural components like tails and tail fibers attaching to create fully functional bacteriophages. This process ensures that many new phages are constructed within the infected cell.
Release (Lysis)
The final stage of the lytic cycle is release, or lysis, where the newly assembled virions exit the host cell. The bacteriophage produces enzymes, such as lysin (also called endolysin) and holin, that weaken and ultimately break down the bacterial cell wall. Holins create pores in the bacterial cell membrane, allowing endolysin to access and degrade the peptidoglycan layer of the cell wall. This degradation causes the bacterial cell to swell due to osmotic pressure and eventually burst, releasing hundreds of new phage particles into the surrounding environment.
The Host Cell’s Fate
The ultimate consequence of the lytic cycle for the host cell is its destruction. The bursting of the bacterial cell, known as lysis, liberates a large number of progeny phages. The release of these new virions allows them to infect other susceptible bacterial cells, perpetuating the cycle and potentially leading to a rapid spread of the viral infection within a bacterial population.