In virology, the polarity of a nucleic acid describes the ratio 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 subsequent protein in the 5 '→ 3' direction (the reading direction of the ribosomes during translation ) is referred to as positive-stranded or sense (reasonable, meaningful). In the case of 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 later mRNA. In the case of an ssRNA and an ssDNA with negative polarity, the genome is complementary to the mRNA. The differentiation of the polarity of nucleic acids results from the fact that with a double-stranded nucleic acid (dsRNA or dsDNA, ds = double-stranded) only one strand is used to transcribe the mRNA, whereas the second strand is only complementary, as if it were mirrored on the head standing, which reproduces genetic information.
There are three types of genome polarity in viruses: (+) polarity ( sense ), (-) polarity ( antisense ) and the occurrence of (+) and (-) polarity on one and the same strand ( ambisense ). This distinction reflects different replication strategies of viruses and is therefore of great importance for the classification of viruses. The members of kinship groups of viruses usually have one and the same polarity. To the extent that viruses of different polarity are increasingly grouped together with the delimitation of kin groups of higher taxonomic ranks (from order upwards), the importance for the taxonomic classification has decreased recently.
(+) - polarity (sense)
In viruses with single-stranded (+) ssRNA as genome, the sequence of the bases corresponds to that of the later mRNA. For viruses with (+) ssRNA corresponding to an mRNA, it will be directly to the ribosomes to protein translated (see figure). All viruses with a (+) ssRNA genome have to code for their own RNA-dependent RNA polymerase , which in a first step has to be read from the RNA strand entering the cell. The replication of the viral ssRNA takes place via a complementary (-) strand as a template for further (+) strands. Well-known examples of the very numerous viruses with (+) ssRNA are the Flaviviridae (e.g. the hepatitis C virus ) and the Picornaviridae .
In the (rare) (+) ssDNA viruses, the synthesis of the mRNA takes place on the codogenic, complementary (-) strand, which is only present in the cell during replication. Before this, the single strand of DNA is supplemented by cellular DNA polymerases to form a double strand. A (+) ssDNA can be found in the bacteriophage families Inoviridae and Microviridae (circular (+) ssDNA), as well as the plant viruses of the Nanoviridae (segmented (+) ssDNA).
(-) - polarity (antisense)
A (-) ssRNA is always completed to form an RNA double strand, the newly formed complementary RNA strand of which corresponds to the mRNA and can be translated into protein. Eukaryotic cells do not have an enzyme that catalyzes the complementation of an ssRNA to form a dsRNA strand. Viruses with a (-) ssRNA genome must therefore always have at least one molecule of a viral RNA polymerase built into their virion , which can then initiate viral protein and RNA synthesis at the beginning of virus replication in the cell. Viruses with negative-stranded ssRNA make up the majority of all virus species. They include viruses from the order Mononegavirales (which owe their name part -nega to the negativity of the non-segmented genome), as well as the family of the Orthomyxoviridae (e.g. the genera influenza virus ).
In the case of (-) ssDNA viruses, the mRNA is synthesized directly on this strand or is previously completed by cellular DNA polymerases to form a double strand. The base sequence of the complementary DNA strand corresponds to the mRNA. A (-) ssDNA genome is found in the Geminiviridae (circular (-) ssDNA, plant viruses) and the genus Gyrovirus of the family Circoviridae (circular (-) ssDNA, vertebrate viruses).
(+/−) - polarity (ambisense)
Some ssRNA and ssDNA viruses have a special mixed form of a sense and an antisense genome. Both polarities are present on a single nucleic acid strand, which is referred to as ambisense polarity or (+/-) polarity. The genes are then distributed over at least two nucleic acid strands, one of which occasionally first has to be generated as a complementary sequence. The two different polarity areas are either transcribed in mRNA independently of one another (in antisense areas, according to their reading direction) or can be read directly as mRNA (sense areas) or correspond to the mRNA. The ambisense polarity must not be confused with the simultaneous or alternative presence of individual (+) and (-) strands (e.g. in Parvoviridae ) in the viral genome. Not even with the presence of open reading frames in both directions of a double-stranded nucleic acid (e.g. in the case of the Polyomaviridae ).
Viruses with ambisense ssRNA are the Arenaviridae (e.g. Lassa fever virus ) and with partial ambisense polarity the Bunyaviridae . An ambisense ssDNA is found in the genus Circovirus of the family Circoviridae .
- SJ Flint, LW Enquist, VR Racaniello, and AM Skalka: Principles of Virology. Molecular Biology, Pathogenesis, and Control of Animal Viruses. 2nd edition, ASM-Press Washington DC 2004, ISBN 1-55581-259-7 , pp. 67ff