**3. Structure**

Adenoviruses are non-enveloped viruses, 80–90 nm in diameter. The icosahedral capsid of adenoviruses consists of hexones carrying group-specific and type-specific antigens and pentones containing mainly group-specific antigens at each apex. From each pentone there is a fiber with a head at the end (**Figure 1**).

Structural proteins of adenoviruses are designated by Roman numerals in descending order of molecular weight. The adenovirus capsid consists of seven structural proteins; three major capsid proteins hexon, fiber, and penton; and four minor proteins protein IIIa (pIIIa), VI, VIII, and protein IX [5]. Fibers provide binding to cellular receptors and participate in the discrimination of infected cells, causing inhibition of the synthesis of cellular macromolecules [6]. Soluble proteins of pentone cause a cytopathic effect similar to the action of infectious adenoviruses,

but it manifests itself much faster (after 4–6 h). The pathogenetic significance of hexons is confirmed by the fact that antibodies against their epitopes demonstrated a neutralizing effect. They may be involved in the development of receptor-dependent endocytosis initiated by fibers, the main target for neutralizing antibodies [7].

A feature of adenoviral DNA is the presence of a terminal protein (TP), which is covalently linked to the 5′ end of each of the DNA strands. One of the possible functions of TP is the DNA attachment to the nuclear matrix after viral genome has entered the nucleus. Due to the interaction of TP, DNA is retained in the form of a ring structure, thus increasing the efficiency of transfection of adenoviral DNA isolated from virions [8]. The size of DNA is (20–25) 103 kDa, which corresponds to approximately 36,000 base pairs. This would be enough for about a dozen medium-sized proteins, but the information capacity of DNA is much more: adenoviruses synthesize about 40 proteins. This is achieved by reading information from both DNA strands and alternative splicing, which provides several types of mRNA based on the primary transcript of one gene. The 13 proteins are included in the mature virion, the rest belong to nonstructural components, functioning at the stage of intracellular reproduction of the virus [9].
