Lytic Cycle

The lytic cycle is the replicative strategy of bacteriophages and some eukaryotic viruses in which the infected host cell is destroyed and progeny virions are released.

Stages and Mechanism

During a lytic infection a virion attaches to specific receptors on the host cell surface, injects its genome and uses the host’s machinery to transcribe and translate viral genes. Early phage genes encode proteins that degrade host DNA and redirect transcription and translation. Viral genomes are replicated either by host polymerases or by virus‑encoded enzymes, and late genes produce structural proteins that self‑assemble into capsids and tails. Once enough virions are assembled, lytic enzymes such as holins and endolysins disrupt the cytoplasmic membrane and cell wall, causing osmotic lysis and the release of dozens to hundreds of new phage particles. In temperate bacteriophages, the decision to enter the lytic cycle or establish lysogeny is regulated by repressor proteins and influenced by environmental conditions; DNA damage can induce lysogenic prophages to switch to lytic replication. Eukaryotic viruses with lytic life cycles similarly hijack the host and produce rapid cell death, leading to acute diseases.

Examples and Significance

The Escherichia coli phages T4 and T7 are classic models for understanding lytic replication; their plaques on bacterial lawns form clear zones where cells have been lysed. Lytic phages are the basis of plaque assays used to quantify virus particles, and they have found renewed interest as therapeutics for antibiotic‑resistant bacterial infections because they eliminate bacteria without integrating into genomes. Some human viruses, including rhinoviruses and influenza viruses, replicate via lytic cycles and cause self‑limiting infections. In aquatic ecosystems, lytic phages play a major role in controlling bacterial populations and recycling nutrients.

The lytic cycle exemplifies aggressive viral reproduction, leading to rapid amplification and host cell destruction. Understanding its steps informs virology research, phage therapy development and ecological models of microbial turnover.

Related Terms: Lysogeny, Temperate Phage, Bacteriophage, Virulent Phage, Burst Size