What We Mean By Positive & Negative RNA Viruses

Positive-sense RNA viruses (or +RNA viruses) are a type of viruses that have a single-stranded RNA genome which can act as messenger RNA (mRNA) and can be directly translated by the host cell's ribosomes into viral proteins. These viruses are called "positive-sense" because their RNA genome has the same sequence as the viral mRNA, which means it can be directly used to synthesize proteins.

Here are some key characteristics and examples of positive-sense RNA viruses:

1. Genome Structure: The genomic RNA of positive-sense RNA viruses is essentially the same as the viral mRNA. This allows the virus to start replicating and producing proteins as soon as it enters the host cell.

2. Replication: Positive-sense RNA viruses encode their own RNA-dependent RNA polymerase (RdRp), an enzyme responsible for replicating the RNA genome within the host cell. The RdRp uses the viral RNA as a template to synthesize new viral RNA genomes.

3. Translation: Because the viral RNA can be directly translated by the host cell ribosomes, these viruses start producing viral proteins almost immediately after infection.

4. Viral Proteins: Positive-sense RNA viruses encode a variety of viral proteins, including those required for genome replication, viral assembly, and interactions with the host cell.

5. Examples:
- Picornaviruses: These include well-known viruses like the common cold-causing rhinoviruses, as well as enteroviruses (e.g., poliovirus and coxsackievirus). Picornaviruses have a small RNA genome and are responsible for a range of diseases.
- Flaviviruses: This family includes viruses like dengue virus, Zika virus, and hepatitis C virus. These viruses are transmitted primarily by arthropods and can cause diseases in humans ranging from mild to severe.
- Coronaviruses: While most positive-sense RNA viruses have small genomes, coronaviruses have larger genomes and are responsible for diseases like the common cold, as well as more severe diseases like SARS (Severe Acute Respiratory Syndrome), MERS (Middle East Respiratory Syndrome), and COVID-19 (caused by SARS-CoV-2).
- Togaviruses: These include viruses like chikungunya virus and rubella virus. Chikungunya virus, for example, can cause symptoms similar to dengue fever and is transmitted by mosquitoes.
- Caliciviruses: Norovirus, a member of this family, is a major cause of gastroenteritis outbreaks worldwide.

Positive-sense RNA viruses can cause a wide range of diseases in humans and other animals. They often adapt rapidly to their hosts and can evolve quickly, leading to the emergence of new strains and potentially causing outbreaks or epidemics.