Viruses are primarily classified by their genetics, the type of nucleic acid that they possess. And nucleic acids show a great deal of variability among viruses. Not only can a viral genome be made of either DNA or RNA, it can be single-stranded DNA, double-stranded DNA, single-stranded RNA or double-stranded RNA (ssDNA, dsDNA, ssRNA or dsRNA respectively). If made of single stranded RNA, then even the type of RNA, called the positive or negative sense strand, is used in viral classification.
Before discussing the difference between positive and negative RNA sense strands, it is helpful to review the relationship between genetic material and the manufacturing of cellular and viral proteins.
Nucleic Acid Transcription & Translation
Transcription is the process by which the DNA sequence is copied to produce a complementary strand of RNA, another type of nucleic acid involved in carrying out the genetic instructions. In other words, it is the transfer of genetic information from DNA into RNA. Transcription leads to the process of translating the genetic code into a peptide or protein (a process actually called translation). Messenger RNA (or mRNA) is one of three types of ribonucleic acid. It is the form that is read by the cell and then translated into proteins.
Making Sense of Sense Strands
Knowing a little about transcription and translation is helpful in making sense of viral sense strands. If a virus’ genome happens to be ssRNA, there are two different possible forms of the RNA strand. This is where the terms positive or negative “sense” or “strand” come into the picture. Generally, “sense” and “strand” mean the same thing, so the terms positive-strand and positive-sense are interchangeable. Whether the viral nucleic acid is positive-sense or negative-sense is an important part of the genetic means of classifying viruses.
Positive-sense ssRNA Viruses
If the genome of a single-stranded RNA virus is positive-sense (alternatively called a “plus-strand” or “plus-sense”), the viral RNA can be directly translated into the desired viral proteins. This means that, in a positive-sense RNA viruses, the viral RNA genome can be directly translated by the host cell to make viral proteins. There are no additional steps required. Essentially positive-sense viral RNA is the same thing as mRNA.
Negative-sense ssRNA Viruses
If the viral genome of an RNA virus is negative-sense (also known as a “minus-strand” or “minus-sense”), this means that the ribonucleic acid strand is complementary to the viral mRNA, and therefore must first be converted to positive-sense RNA before the protein-building instructions can be carried out. So, essentially there are more steps to making viral proteins for a cell infected by a negative-sense virus, since this type of ssRNA can’t be directly translated into protein.