Virology: Understanding Virus Life Cycle and Assembly

Questions and Answers

To induce the final conformation in the capsid proteins

In the nucleus

To translated into non-structural proteins

To define and limit the host species and cell type infected Signup and view all the answers

Production of new viral genomes for incorporation into progeny virions Signup and view all the answers

To build the virion structure Signup and view all the answers

DNA viruses replicate mainly in the nucleus, while RNA viruses replicate mainly in the cytoplasm Signup and view all the answers

Formation of the virus particle Signup and view all the answers

Budding from the cell surface Signup and view all the answers

To copy the negative sense genome into mRNA Signup and view all the answers

Endocytosis Signup and view all the answers

Uncoating Signup and view all the answers

The cellular secretory pathway Signup and view all the answers

To accumulate in the plasma membrane during budding Signup and view all the answers

Attachment/Adsorption Signup and view all the answers

To condense adjacent to the cell membrane during budding Signup and view all the answers

Study Notes

Virus life cycle consists of several stages: attachment/adsorption, penetration and uncoating, nucleic acid replication, viral protein synthesis, assembly and release.

Viral attachment protein specifically binds to a receptor on the cell plasma membrane.
Receptors can be proteins (e.g. ICAM-1 for most rhinoviruses, ACE2 for SARS-CoV-2) or carbohydrates (sialic acid for influenza virus).
Receptors are not for the cell's benefit but are involved in normal processes, and viruses have evolved to utilize them to gain entry.

After attachment, the coat of enveloped viruses may fuse with the host cell membrane and release the virus nucleocapsid into the host cytoplasm.
Other viruses may enter the cell by a process of endocytosis, which involves invagination of the cell membrane to form vesicles in the cell cytoplasm.
Uncoating refers to the release of the viral genome from its protective capsid to enable the nucleic acid to be transported within the cell and transcribed to form new progeny virions.

DNA viruses replicate mainly in the nucleus, while RNA viruses replicate mainly in the cytoplasm.
Nucleic acid replication produces new viral genomes for incorporation into progeny virions.
Messenger RNA (mRNA) is transcribed from viral DNA (or formed directly from some RNA viruses) and codes for viral proteins that are translated by the host cell.

"Early" proteins are usually non-structural (e.g. DNA or RNA polymerase) and later proteins are structural, e.g. capsid proteins, building blocks of the virion.
RNA polymerase is a special enzyme used by negative sense viruses, such as influenza virus, to copy its negative sense genome into mRNA.

Non-enveloped animal viruses have an icosahedral structure, which can be formed through spontaneous assembly of the capsid proteins around the nucleic acid genome.
Virions accumulate in the cytoplasm or nucleus and are only released when the cell eventually lyses.
Enveloped viruses release may take place by budding from the cell surface, where patches of viral envelope glycoproteins accumulate in the plasma membrane, and the capsid proteins and nucleic acid condense directly adjacent to the cell membrane.

Some enveloped viruses utilize the cellular secretory pathway to exit the cell, where virus particles bud into Golgi-derived vesicles and are released to the outside of the cell when the transport vesicle fuses with the cell membrane.
Viral membrane fusion is used by some viruses to induce fusion of the viral envelope and the cell membrane.