Polarity (virology)

The polarity of a nucleic acid describes the relationship of a single-stranded viral genome to the reading direction of the later messenger RNA (mRNA), which is derived from this genome.

In general, a nucleic acid that has the correct sequence of base triplets for the later protein in the 5'→3' direction (the reading direction of the ribosomes during translation) is called positive-stranded or sense (reasonable, useful).

For a single-stranded (ss, single-stranded) RNA and a single-stranded DNA with positive polarity, the base sequence in the 5'→3'-direction corresponds to the base sequence of the later mRNA.

In the case of a single-stranded RNA (ssRNA) and a double-stranded DNA (ssDNA) with negative polarity, the genome has a complementary structure to the mRnNA read off.

The difference in polarity of nucleic acids is due to the fact that in a double-stranded nucleic acid (dsRNA or dsDNA, ds=double-stranded) only one strand is used for the transcription of the mRNA.

In viruses, a distinction is made between 3 types of genome polarity:

• the (+) -polarity (sense) - see figure
• the (-) polarity (antisense) and
• the (+) - and (-) -polarity on the same strand (ambisense).

This distinction is of great importance for the taxonomic classification of viruses and reflects different replication strategies of viruses.

For example, the "antisense RNA (aRNA)", also known as natural antisense transcript (NAT). The single-stranded RNA is complementary to a protein-coding messenger RNA (mRNA). Antisense RNAs play an important role in regulatory processes in cells. In research, these RNAs are increasingly used as "tools" for gene knockdown. Antisense RNA can act in various ways. In most cases, it is effective by preventing the translation of a gene. But it can also have an epigenetic or activating effect.