A kinetic class, also known as a temporal class, is a grouping of genes in a viral genome that are expressed at the same time during the viral replication cycle. Five of the human DNA viral families have multiple kinetic classes:

Poxviridae ''Poxviridae'' is a family of double-stranded DNA viruses. Vertebrates and arthropods serve as natural hosts. The family contains 22 genera that are assigned to two subfamilies: ''Chordopoxvirinae'' and ''Entomopoxvirinae''. ''Entomopoxvirinae'' ...

Herpesviridae ''Orthoherpesviridae'', previously named and more widely known as ''Herpesviridae'', is a large family of DNA viruses that cause infections and certain diseases in animals, including humans. The members of this family are commonly known as herp ...

Adenoviridae Adenoviruses (members of the family (biology), family ''Adenoviridae'') are medium-sized (90–100 nanometer, nm), nonenveloped (without an outer lipid bilayer) viruses with an icosahedral nucleocapsid containing a double-stranded DNA genome. ...

Papillomaviridae ''Papillomaviridae'' is a family of non- enveloped double-stranded DNA viruses whose members are known as papillomaviruses. Several hundred species of papillomaviruses, traditionally referred to as "types", have been identified infecting all car ...

, and

Polyomaviridae ''Polyomaviridae'' is a family of DNA viruses whose natural hosts are mammals and birds. As of 2024, there are eight recognized genera. Fourteen species are known to infect humans, while others, such as Simian Virus 40, have been identified i ...

. All of the genes in a particular kinetic class are activated by the same mechanism: either by the process of the virus entering the cell and uncoating, or by the products of an earlier kinetic class in what is known as a transcriptional cascade. Generally speaking, earlier kinetic classes code for

enzymes An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as pro ...

that direct the viral replication process, and later kinetic classes code for

structural proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respon ...

to be packaged into virions

Viruses with three kinetic classes: Poxviridae, Herpesviridae, and Adenoviridae

Multiple naming schemes exist in virus families that have three kinetic classes; some refer to the three classes as 1) immediate-early 2) early and 3) late, and some refer to these same classes as 1) early 2) intermediate and 3) late. The former naming scheme is adopted below because it emphasizes the relative similarity in function of the first two classes compared to the third.

Immediate-early class

The first genes expressed, in the immediate-early class, prepare the virus for replication and protect it from host defenses. These genes are activated in Poxviruses by enzymes packaged in the virion itself. They are responsible for dissolving the virion capsid, directing DNA replication (in Poxviruses), protecting the virus from the host's interferon and complement defenses (in Poxviruses), and activating the early class of genes.

Early class

The second class of genes continues to combat host defenses, stimulates DNA replication (in Herpesviruses), and activates the late class. In Herpesviruses, the early class is known as the beta class and replicates the DNA genome starting from three different origins of replication. In Adenoviruses, it consists of two transcription units, IVa2 and IX.

Late class

The late class consists primarily of structural proteins and assembly enzymes, and is dependent in all three families on the prior synthesis of viral DNA copies. In Herpesviruses, for example, the late class consists of gamma-1 and gamma-2 genes. After the late class is expressed, the virions can assemble and exit the cell.Acheson, Nicholas H. Fundamentals of Molecular Virology, 2nd Edition. Wiley, 2011.{{page needed, date=July 2020

Viruses with two kinetic classes: Papillomaviridae and Polyomaviridae

Early class

The early proteins produced in Papillomaviruses and Polyomaviruses regulate the cell cycle and activate DNA replication. In Papillomaviruses, they are called E1 through E7 and also stimulate cellular replication. In Polyomavirus, the early proteins are T antigens.

Late class

The late proteins make up the virus capsid. In Polyomaviruses, they are known as VP1, VP2, and VP3; in Papillomaviruses they are called L1 and L2 (for “late”).

References

Viral genes