Viruses: Obligate Intracellular Parasites

Virus particles, called virions, contain a core of genomic information that is either DNA or RNA. The core is surrounded by a protein shell called a capsid, and together the capsid and core are called the nucleocapsid. Some viruses are enclosed by a lipid envelope that is obtained from the host cell, and these viruses are called enveloped viruses. Viruses that lack a lipid envelope are called nonenveloped viruses.
In enveloped viruses, the virion inserts its own proteins and lipids into the host-cell derived membrane. These proteins can include a matrix protein (M protein), which is in contact with the nucleocapsid and lines the inner side of the envelope. Glycoproteins can protrude from the envelope, and can be used in adhesion to the host cell. A well known glycoprotein is hemagluttinin, which is found on the surface of the influenza virus.

Genetic information is carried in viruses by either RNA or DNA. The genome of a virus can be either single stranded RNA (ssRNA), double stranded RNA (dsRNA), single stranded DNA (ssDNA), double stranded DNA ds(DNA), or a mix of ssDNA and dsDNA. In order to undergo translation, the genome must be converted to (+)RNA, or mRNA.

Many ssRNA viruses have genomes that are already (+)RNA, and they can undergo transcription immediately. In replication, a virus-associated RNA polymerase replicates the (+)ssRNA genome to (-)ssRNA, and this is used as a template to make more (+)ssRNA. A (-)ssRNA virus must have its genome replicated to (+)ssRNA, which can now serve as a template for the transcription of proteins. In replication, the (+)ssRNA is used as a template to make more (-)ssRNA. A dsRNA virus must have its genome replicated to (+)ssRNA, which can be used in transcription. RNA polymerase can directly replicate dsRNA.

Single stranded DNA viruses employ the same transcription and replication mechanisms. From the ssDNA, (+)ssRNA is made by host cell RNA polymerase, and is used in transcription. Host cell DNA polymerase is used in replication.
Reverse transcriptase is an enzyme that replicates DNA from RNA. (+)ssRNA viruses such as HIV use reverse transcriptase to replicate their genomes to dsDNA, which is then integrated into the host cell’s chromosome, where it can express viral proteins.

The entry of the virion into the host cell varies by virus, but most employ similar mechanisms. The virion must be able to bind to the host cell and enter it, either by using its own mechanisms or by inducing the host cell to phagocytize it. Once in the host cell, the virus utilizes the host cell’s energy and enzymes to create progeny virions. Cell death is usually caused by the lysis of the cell as a result of a burst of progeny virions. The progeny virions then travel to neighboring cells, infecting them.




© 2003, J.Graf, for comments please contact