Viral Replication Cycle

Prepared and reviewed by the PrepElephant Academic Review Team

First published 14 Oct 2026 · Spot an error?

Reference basis: Ananthanarayan and Paniker's Textbook of Microbiology; Jawetz, Melnick and Adelberg's Medical Microbiology

On this page

Direct answer
What you must remember
Following one infection from attachment to release
Where the exam tries to catch you
Frequently asked questions
Related topics

Direct answer

The viral replication cycle is the sequence by which a virus attaches to a host cell, releases its genome, uses host machinery to make new genomes and proteins, assembles progeny virions and leaves the cell. The eclipse phase is the early window of the cycle when no infectious progeny can be recovered from the infected cell, even though viral nucleic acid and proteins are already being made.

What you must remember

Attachment: Viral surface proteins bind specific host receptors, such as CD4 with CCR5 or CXCR4 for HIV and CD155 for poliovirus, and receptor distribution largely decides tissue tropism.
Penetration: Enveloped viruses commonly fuse with the plasma membrane, whereas many naked viruses enter by receptor-mediated endocytosis.
Uncoating: The capsid is removed to release the genome, a step that is blocked by amantadine in influenza A virus.
Early and late genes: Early products such as viral polymerases and host-shutoff proteins appear before genome replication, and late products such as structural proteins appear after it.
Eclipse period: No infectious virus is detectable inside the cell until assembly begins, which is why the interval is called the eclipse phase.
One-step growth curve: Plotting infectious titre against time shows the latent period, the rise period and the plateau, and burst size is the number of virions released per infected cell.
Release: Many naked viruses lyse the cell, whereas enveloped viruses such as influenza leave by budding, which usually does not destroy the cell at once.

Following one infection from attachment to release

The cycle is usually memorised as a list, but exams test the timing of each stage on the growth curve. Read the steps as the order in which infectious progeny become possible.

Attachment: The virion binds a specific receptor on the host cell, and this step is usually rapid compared with the rest of the cycle.
Penetration and uncoating: The virion enters by fusion or endocytosis, and the capsid is stripped away so that the genome becomes accessible to the cell's machinery.
Early synthesis: The genome directs production of early proteins, including polymerases and proteins that suppress host gene expression, which takes control of the cell.
Genome replication: The genome is copied, and for many DNA viruses such as herpesviruses and adenoviruses this happens in the nucleus, whereas most picornaviruses replicate in the cytoplasm.
Late synthesis and assembly: Structural proteins are made and packaged with newly copied genomes to form nucleocapsids, which mature into infectious virions.
Release: Virions exit by lysis, budding or exocytosis, and the progeny released at this step are what the growth curve counts.

On a one-step growth curve, the eclipse phase spans the early steps up to assembly, so no infectious progeny appear in the cell. The rise in titre follows release, and the plateau marks the end of productive infection.

Where the exam tries to catch you

Timing questions are where most marks are lost, because the stages overlap in time and are easily mixed up.

Trap: Believing the eclipse phase means the virus is absent from the cell Correct: Viral nucleic acid and proteins are present and being made during eclipse, and only infectious virions are undetectable.
Trap: Treating the latent period and the eclipse phase as the same interval Correct: The latent period runs from infection to the first release, so it is longer, because eclipse ends at assembly.
Trap: Confusing burst size with titre Correct: Burst size counts virions released per infected cell, whereas titre is the number of infectious particles per unit volume of culture.

Frequently asked questions

What is the eclipse phase in a one-step growth curve?

The eclipse phase is the early period after infection when no infectious virus can be recovered from the cell, even though viral nucleic acid and proteins are already present. It ends when assembly of new infectious virions begins.

Why does the latent period last longer than the eclipse phase?

The latent period runs from infection until the first progeny virions are released, so it includes the eclipse phase plus the time needed for maturation and release. Eclipse ends at assembly, which occurs earlier in the cycle than release.

Which receptor does poliovirus use to attach to host cells?

Poliovirus attaches to CD155, a member of the immunoglobulin superfamily, on the surface of susceptible human cells. Receptor expression on intestinal and neural cells helps explain why the virus targets those tissues.

How does influenza virus leave an infected cell?

Influenza virus leaves the host cell by budding, acquiring its envelope from a host membrane as it exits. Budding usually does not lyse the cell at once, unlike the bursting release seen with many naked viruses.

What is the burst size of a virus?

Burst size is the number of infectious virions released from a single infected cell during one replication cycle. It is estimated from the plateau of a one-step growth curve by dividing the total yield by the number of infected cells.