Viruses Replicate inside Host Cells
Cells can increase their number by dividing. Viruses need a suitable host cell to produce new virions, because they carry no machinery for building proteins or copying genetic material on their own.
Viral Genomes Are Copied and Viral Proteins Are Made in Host Cells
Bacteria reproduce by dividing one cell into two. A virus first delivers its genome into a host cell. Enzymes from the virus or host cell then copy the viral genome, while host ribosomes make viral proteins. Copies of the viral genome and the viral proteins are assembled into new virions before release.
For many viruses, infection can proceed only after a viral surface protein binds to a compatible receptor on the host cell.
Entry and genome-copying mechanisms vary among viruses. A bacteriophage may inject only its genome, while many animal viruses enter as a complete particle. The enzymes used to copy the genome may come from the host cell, the virus, or both. Every mechanism still requires a suitable host cell.
Lytic and Lysogenic Cycles in Bacteriophages
Virulent phages reproduce through the lytic cycle. Temperate phages can enter the lysogenic cycle, where their genome is integrated into the host chromosome and copied with the bacterium's DNA. The viral genome can later become active and return to the lytic cycle.
The phage is attached to the host-cell surface. The red path shows the viral genome entering. Viral genomes are then copied, viral proteins are made, and the components are assembled into new virions.
- Event to compare
- The viral genome enters the cell first while the phage body stays outside.
- Biological result
- In the lytic cycle, viral genomes are copied and host ribosomes make viral proteins. These components are then assembled into new virions.
The Lytic Cycle Starts with Attachment
The lytic cycle follows an ordered sequence from cell recognition to the release of newly assembled virions. Reordering the steps breaks the cycle, because each stage prepares the next one.
Receptor Matching Allows the Virus to Attach
The lytic cycle begins with adsorption, attachment of the virus to the host cell. Attachment depends on a match between viral proteins and cell receptors. Without that molecular match, replication is unlikely to begin.
After attachment comes penetration, when the virus delivers its genome or another part needed for replication into the cell. The next stage is synthesis. During this stage, viral genomes are copied and host ribosomes make capsid proteins. Once the genome copies and capsid proteins are available, the components are assembled into new virions. In many lytic examples, the cell then undergoes lysis, the breaking of the cell so new virions can exit.
Component Synthesis and Virion Assembly
During synthesis, viral genomes are copied and viral proteins are made as separate components. During assembly, the genome copies and proteins are arranged into new virions that then leave the cell.
| Phage lytic stage | What happens | Easy confusion to avoid |
|---|---|---|
| Adsorption | The virus attaches to a matching host receptor | Attachment is not yet reproduction |
| Penetration | Viral genome or material needed for replication enters | The whole outside shell may not enter in every virus |
| Synthesis | Viral genomes are copied and viral proteins are made | The components have been produced but are not yet assembled into virions |
| Assembly and release | New virions form and leave | The parts join to form new viruses, which then leave the cell |
During viral replication, viral genomes are copied and the host cell makes viral proteins. The genome copies and proteins are then assembled into new virions.
A Prophage Can Remain in the Host Genome
In the lysogenic cycle, many new virions do not form immediately. The viral genome can integrate into host DNA and be copied when the host cell divides.
Certain conditions can reactivate the viral genome and shift the virus into the lytic cycle.
An integrated phage genome is called a prophage, and its return to the lytic cycle is called induction. Animal viruses can also form long-lasting or inactive infections. The terms latency and persistent infection refer to processes that differ from a prophage in a bacterial chromosome.
Distinguishing Synthesis from Assembly
During synthesis, viral genomes are copied and viral proteins are made, but these parts are not yet complete virions. Assembly joins copies of the viral genome and the viral proteins into new virions. Repeating these steps within one host cell can produce many virions.