Viruses multiply by taking over the machinery of living cells. The lytic cycle is a common method viruses use to replicate and spread. This process involves the rapid production of new viral particles within a host cell, ultimately leading to the destruction of that cell.
Understanding the Lytic Cycle
In the lytic cycle, the virus commandeers the host cell’s internal resources and molecular machinery. The cell then becomes a factory for producing new viral components. The term “lytic” refers to lysis, the bursting or breaking open of the cell. This destruction releases viruses, allowing them to infect other cells and continue the cycle.
The Stages of Viral Replication
Attachment
The lytic cycle begins with attachment, where the virus binds to the surface of a host cell. Viruses possess specialized proteins, known as attachment proteins or spikes, interacting with specific receptor molecules on the host cell’s membrane. This interaction is highly selective, ensuring that a virus typically infects only certain types of cells or organisms. For instance, bacteriophages, viruses that infect bacteria, attach to components like specific proteins or lipopolysaccharides on the bacterial cell wall.
Entry
Following attachment, the viral genetic material enters the host cell. The method of entry varies depending on the type of virus. Some viruses, particularly bacteriophages, inject their genetic material (DNA or RNA) directly into the host cell while their protein coat remains outside. In contrast, many animal viruses enter the cell through processes like endocytosis, where the host cell engulfs the entire virus, or through membrane fusion, where the viral envelope merges with the host cell membrane, releasing the viral core into the cytoplasm.
Replication
Once inside, the virus takes control of the host cell’s machinery for replication and protein synthesis. The viral genome contains instructions that reprogram the host cell to prioritize the production of viral components over its own cellular processes. This involves using the host cell’s ribosomes, enzymes, and energy to transcribe viral genes into messenger RNA and then translate that mRNA into viral proteins, including enzymes for genome replication and structural proteins for new particles.
Assembly
After the individual viral components are synthesized, they spontaneously or are guided by viral proteins to assemble into new viral particles. This process, often called maturation, involves packaging the replicated viral genetic material into protein capsids. For some viruses, additional components like enzymes or envelope proteins are also incorporated into the new virions.
Lysis and Release
The final stage of the lytic cycle is lysis and release, where viral particles exit the host cell. In many cases, particularly with bacteriophages, the virus produces enzymes like lysozyme that rupture the bacterial cell wall, causing the cell to burst. For animal viruses, release can occur through lysis, similar to bacteria, or through budding, where new virions acquire an envelope from the host cell membrane as they exit.
Lytic Versus Lysogenic Cycles
Viruses can employ different strategies to replicate within a host, with the lytic and lysogenic cycles representing two distinct pathways. The lysogenic cycle involves a period of dormancy where the viral genetic material integrates into the host cell’s genome. In this integrated state, known as a prophage in bacteria or a provirus in eukaryotes, the viral DNA is replicated along with the host cell’s DNA during normal cell division. The virus can remain latent for many generations without producing new viral particles or harming the host cell. However, under certain environmental stresses, the integrated viral DNA can excise itself from the host genome and enter the lytic cycle.
Why the Lytic Cycle Matters
The lytic cycle underlies many common viral infections in organisms. When viruses like those causing the common cold or influenza initiate a lytic cycle within host cells, the destruction of those cells contributes directly to the symptoms of illness. For example, the destruction of respiratory tract cells by cold viruses leads to inflammation and congestion.
The lytic cycle is also important in the context of bacteriophages. These viruses specifically target and destroy bacteria. The ability of bacteriophages to lyse bacteria has led to interest in their use in phage therapy as an alternative to antibiotics. This approach leverages the lytic cycle to combat bacterial infections, offering a promising avenue for treating antibiotic-resistant strains.