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The coevolution, or "virus-first" hypothesis, conflicts with the definition of viruses, because viruses depend on host cells. [ 20 ] [ 21 ] Also, viruses are recognised as ancient, and to have origins that pre-date the divergence of life into the three domains . [ 22 ]
For the virus to reproduce and thereby establish infection, it must enter cells of the host organism and use those cells' materials. To enter the cells, proteins on the surface of the virus interact with proteins of the cell. Attachment, or adsorption, occurs between the viral particle and the host cell membrane.
The viral genome is mostly silent within the host. At some point, the provirus or prophage may give rise to the active virus, which may lyse the host cells. [6]: 243–259 Enveloped viruses (e.g., HIV) typically are released from the host cell by budding. During this process, the virus acquires its envelope, which is a modified piece of the ...
A permissive cell or host is one that allows a virus to circumvent its defenses and replicate. Usually this occurs when the virus has modulated one or several of the host cellular intrinsic defenses and the host immune system. The permissive state of a host has now been determined to be the primary factor in determining whether a virus will ...
Cell entry by enveloped viruses is more complicated. Enveloped viruses enter the cell by attaching to an attachment factor located on the surface of the host cell. They then enter by endocytosis or a direct membrane fusion event. The fusion event is when the virus membrane and the host cell membrane fuse together allowing a virus to enter.
Once inside host cells, viruses can destroy cells through a variety of mechanisms. Viruses often induce direct cytopathic effects to disrupt cellular functions. [11] [18] This could be through releasing enzymes to degrade host metabolic precursors, or releasing proteins that inhibit the synthesis of important host factors, proteins, DNA and/or ...
Entry, or penetration, is the second step in viral replication. This step is characterized by the virus passing through the plasma membrane of the host cell. The most common way a virus gains entry to the host cell is by receptor-mediated endocytosis, which comes at no energy cost to the virus, only the host cell. Receptor-mediated endocytosis ...
Viruses, however, use a completely different mechanism to cause disease. Upon entry into the host, they can do one of two things. Many times, viral pathogens enter the lytic cycle; this is when the virus inserts its DNA or RNA into the host cell, replicates, and eventually causes the cell to lyse, releasing more viruses into the environment.