Viral Genome and Structure Organization

Exam Tip: When identifying a virus, always check if the genome is segmented. A segmented genome (like Rotavirus) means the virus carries multiple separate pieces of nucleic acid rather than one single continuous strand.

Structural Organization: Capsids and Envelopes

The genome of a virus cannot function in isolation; it must be protected from the environment. This protection is provided by the virus's physical structure, which is primarily composed of proteins and, in some cases, lipids.

The Capsid

The fundamental building block of viral structure is the capsid, a protective protein shell that encloses the viral genome. The specific arrangement of these proteins determines the shape and stability of the virus.

Non-enveloped vs. Enveloped Viruses

Viruses are broadly classified into two groups based on whether they possess an outer lipid layer:

1. Non-enveloped (Naked) Viruses: These viruses consist only of a capsid protecting the genome. They are often more resilient to environmental stressors. Examples include the Tobacco Mosaic Virus (TMV), a naked ssRNA virus, and the Canine Parvovirus, a naked ssDNA virus.
2. Enveloped Viruses: These possess a capsid that is further surrounded by an external membrane (the envelope). This membrane is typically derived from the host cell's own membrane as the virus "buds" out of the cell during the replication process. Examples include the Ebola virus and Herpes simplex.

Specialized Replication: Retroviruses

A highly specialized group of viruses known as retroviruses (such as HIV) employs a unique method of genetic integration that defies the standard "DNA $\rightarrow$ RNA $\rightarrow$ Protein" central dogma of molecular biology.

While all retroviruses are RNA viruses, it is important to remember that not all RNA viruses are retroviruses. The defining characteristic of a retrovirus is the presence of a specific gene that encodes the enzyme reverse transcriptase.

The Process of Reverse Transcription

Retroviruses use reverse transcriptase to perform reverse transcription, a process that converts their ssRNA genome into DNA. Once this viral DNA is synthesized, it is spliced into the host cell's own genome. Once integrated, the host's cellular machinery treats the viral DNA as its own, transcribing it back into RNA to facilitate the production of new viral particles.

Key Distinction: If a question asks about the difference between a standard RNA virus and a retrovirus, the answer lies in the enzyme. Retroviruses use reverse transcriptase to turn RNA into DNA; standard RNA viruses do not integrate into the host genome in this manner.