Question

What types of genomes do viruses have

Answer 1

Group I: Double-stranded DNA viruses

These types of viruses must enter the host nucleus before they are able to replicate. Furthermore, these viruses require host cell polymerases to replicate the viral genome and, hence, are highly dependent on the cell cycle. The virus may induce the cell to forcefully undergo cell division, which may lead to transformation of the cell and, ultimately, cancer. Examples include Herpesviridae, Adenoviridae, and Papovaviridae.

There is only one well-studied example in which a class 1 virus is not replicating within the nucleus: the Poxvirus family, a highly pathogenic virus that infects vertebrates and includes the smallpox virus.

Group II: Single-stranded DNA viruses

Viruses in this category include the Anelloviridae, Circoviridae, and Parvoviridae (which infect vertebrates), the Geminiviridae and Nanoviridae (which infect plants), and the Microviridae (which infect prokaryotes). Most of them have circular genomes (the parvoviruses are the only known exception). Eukaryote-infecting viruses replicate mostly within the nucleus - usually via a rolling circle mechanism, forming double-stranded DNA intermediate in the process. A prevalent but asymptomatic human Anellovirus, called Transfusion Transmitted Virus (TTV), is included within this classification.

Group III: Double-stranded RNA viruses

As with most RNA viruses, this class replicates in the "Core" capsid that is in cytoplasm, not having to use the host replication polymerases to as much a degree as DNA viruses. Replication is monocistronic and includes individual, segmented genomes, meaning that each of the genes codes for only one protein, unlike other viruses that exhibit more complex translation.

Group IV & V: Single-stranded RNA viruses

The ssRNA viruses belong to Class IV or V of the Baltimore classification. They could be grouped into positive sense or negative sense according to the sense of polarity of RNA. The single stranded RNA is the common feature of these viruses.

Group IV: Single-stranded RNA viruses - Positive-sense

The positive-sense RNA viruses and indeed all RNA defined as positive-sense can be directly accessed by the host ribosomes immediately to form proteins.

Group V: Single-stranded RNA viruses - Negative-sense

The negative-sense RNA viruses and indeed all genes defined as negative-sense cannot be directly accessed by host ribosomes to immediately form proteins. Instead, they must be transcribed by viral polymerases into a "readable" form, which is the positive-sense reciprocal.

Examples in this class include thefamilies Arenaviridae, Orthomyxoviridae, Paramyxoviridae, Filoviridae, and Rhabdoviridae (the latter of which includes the rabies virus).

Group VI: Positive-sense single-stranded RNA viruses that replicate through a DNA intermediate

A well-studied family of this class of viruses include the retroviruses. One defining feature is the use of reverse transcriptase to convert the positive-sense RNA into DNA. RNA for templates of proteins, they use DNA to create the templates. which is spliced into the. Replication can then commence with the help of the host cell's polymerases.

Group VII: Double-stranded DNA viruses that replicate through a single-stranded RNA intermediate

This small group of viruses, exemplified by the Hepatitis B virus (which is in the Hepadnaviridae family), have a double-stranded, gapped genome that is subsequently filled in to form a covalently closed circle (cccDNA) that serves as a template for production of viral mRNAs and a subgenomic RNA. The pregenome RNA serves as template for the viral reverse transcriptase for production of the DNA genome.