Viral genome

The genomic nucleic acid of a virus exists either as DNA or as RNA. It can consist of a single strand or a double strand. In viruses, three types of "polarity" are distinguished (Note: In virology, the polarity of a nucleic acid describes the ratio of a single-stranded viral genome to the reading direction of the subsequent messenger RNA derived from this genome):

• the (+) polarity(sense)
• the (-)-polarity(antisense)

and the presence of

• (+)/(-) polarity on one and the same RNA strand(ambisense).

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

DNA viruses: The DNA of viruses is usually double-stranded (dsDNA), whereby the DNA can be arranged linearly or circularly. Examples of viruses with a double-stranded DNA genome include Herpeviridae, Papillomaviridae. Examples of ds/ssDNA, i.e. single-stranded DNA genome, are the hepatitis B virus (Hepadnaviridae) and Parvoviridae.

RNA viruses: The RNA of viruses is present in both single-stranded (ssRNA) and double-stranded (ssRNA) forms. The single-stranded RNA can be expressed as:

• ss(+)-RNA(sense) having the polarity of an mRNA. The RNA can be immediately translated into protein by the cell (Picornaviridae, Coronaviridae, Retroviridae)

or as

• ss(-)-RNA (antisense) with the polarity of an anti-mRNA. In this case, a viral RNA polymerase must produce subgenomic (+)-strand copies, which then serve as mRNA for the production of virus-specific proteins (Paramyxoviridae, Pneumoviridae, Arenaviridae)

(-)ssRNA and a (-) ssDNA: In the case of a (-)ssRNA and a (-) ssDNA, each with negative polarity, the genome is complementary to the mRNA. The differentiation of the polarity of nucleic acids results from the fact that in a double-stranded nucleic acid (dsRNA or dsDNA) only one strand is used for the transcription of the mRNA, whereas the second strand is only complementary, so to speak "mirror-inverted upside down" and reproduces the genetic information. All viruses with a (+)ssRNA genome must code for their own RNA-dependent RNA polymerase. In a first step, this is read from the viral RNA strand that has entered the cell.

The viral ssRNA is amplified using 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, synthesis of the mRNA occurs on the codogenic complementary (-)strand. This is only present in the cell during replication. Prior to this, the DNA single strand is completed by cellular DNA polymerases to form a double strand.

Ambisense polarity (+/-) polarity: A special mixed form of a sense and antisense genome is found in some ssRNA and ssDNA viruses. In this case, both polarities are present on a single nucleic acid strand. The different polarity regions are transcribed into mRNA independently of each other according to their reading direction (antisense section). The sense section can function directly as mRNA.

Viral capsid: The viral genome is not unprotected but surrounded by a protein coat, the capsid (see capsid below). Capsids are composed of symmetrical building blocks called caspsomers. Capsomers can consist of a single polypeptide chain or they can be composed of several different polypeptide chains (e.g. poliovirus). The complex of capsid and nucleic acid is called the nucleocapsid of a virus. The complex capsid and nucleic acid is called the nucleocapsid of a virus. Depending on the number of proteins used and the type of nucleic acids, nucleocapsids have two typical shapes:

• the helical symmetry of a nucleic acid arranged in a spiral and packaged in only one protein
• the cubic symmetry in the form of an icosahedron. Here, a polygon with different symmetry axes is formed by using capsomers from several polypeptide chains. The nucleic acids are directly connected to the inner sides of this polygon. A special feature is found in HIV. In this virus, the nucleic acid is not directly connected to the capsid, but it is first loaded with a nucleoprotein. This nucleic acid/protein complex is surrounded by another capsule-like protein structure, which is then called the "core", core or capsid.

Naked viruses: Here the capsids or nucleocapsids (rotavirus, poliovirus) constitute the entire virus structure.

Enveloped viruses: Capsids or nucleocapsids are surrounded by a double lipid envelope (measles virus, mumps virus, rubella virus, etc.) in which viral glycoproteins are embedded. The cellular origin of this lipid envelope means that it can also contain cellular transmembrane proteins.

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