In positive-sense single-stranded RNA (+ssRNA) viruses, the genome has the same base sequence as mRNA. As soon as it is uncoated inside the host cell, it can act directly as mRNA and be translated by host ribosomes.The stages of replication are:1. Attachment and entry: the virus binds to receptors and enters the cell; the RNA is released into the cytoplasm.
2. Translation: host ribosomes translate the viral RNA, often into one long polyprotein that is cut into separate proteins, including RNA replicase (RNA-dependent RNA polymerase).
3. Making a template: RNA replicase uses the + strand as a template to make a complementary negative (−) strand.
4. Copying: the − strand is used as a template to make many new + strands.
5. The new + strands are either translated to make more capsid proteins or packaged as genomes.
6. Assembly and release of new virus particles.+RNA → −RNA → many +RNA Because the genome itself works as mRNA, the virus does not need to carry any enzyme inside the particle. Even purified viral RNA with no capsid can infect a cell and start a full infection: the naked RNA is infectious.Examples of +ssRNA viruses include poliovirus, rhinoviruses (common cold), hepatitis C virus, SARS-CoV-2 and other coronaviruses, and tobacco mosaic virus.Starting translation straight away is an advantage to the virus: protein synthesis can start the moment the RNA enters the cytoplasm.
2. Translation: host ribosomes translate the viral RNA, often into one long polyprotein that is cut into separate proteins, including RNA replicase (RNA-dependent RNA polymerase).
3. Making a template: RNA replicase uses the + strand as a template to make a complementary negative (−) strand.
4. Copying: the − strand is used as a template to make many new + strands.
5. The new + strands are either translated to make more capsid proteins or packaged as genomes.
6. Assembly and release of new virus particles.