Question
Eclipse phase in viral replication.
Answer
The eclipse phase is a distinctive stage in the viral replication cycle during which no infectious virus particles can be detected, either inside the infected cell or extracellularly — occurring after the infecting virus has entered the host cell and been “uncoated” (its genome released from the protective capsid), but before newly assembled, mature, infectious progeny virions have been produced.
Timing and mechanism: begins once the parental virus particle has attached, penetrated the cell, and undergone uncoating — at this point, the viral genome exists in a “disassembled,” non-infectious form, actively directing the synthesis of viral components (nucleic acid replication, viral protein synthesis) using the host cell’s machinery, but no complete, structurally intact, infectious virion yet exists within the cell; the eclipse phase ends when the first fully assembled, mature, infectious progeny virus particles are formed.
Contrast with the “latent period”: the latent period (a related but distinct concept, most classically applied to bacteriophage infection of bacteria in a one-step growth curve experiment) refers to the total time from initial infection until the first release of progeny virus into the extracellular environment — the eclipse phase is essentially the intracellular component of this latent period, ending with intracellular virion assembly, while the latent period extends further, until virion release (e.g., by cell lysis).
Significance: the existence of the eclipse phase is direct experimental evidence that viral replication does not occur by simple binary division of intact virus particles (unlike bacterial/cellular reproduction) — instead, it proceeds by a fundamentally different process of disassembly (uncoating), independent synthesis of components, and reassembly (maturation) into new, complete virions; this was a historically important observation (first demonstrated with bacteriophages by the one-step growth curve experiments of Ellis and Delbrück) that helped establish the basic principles of the modern viral replication cycle.