The viruses are composed of:
All viruses contain a nucleic acid that can be DNA (in DNA viruses) or RNA (in RNA viruses or retroviruses), but never DNA and RNA in the same virus. This nucleic acid can be circular or linear, and single-stranded or double-stranded. Nucleic acid can be made up of a single molecule, as in the case of DNA viruses, or segmented, as in some RNA viruses.
- DNA viruses: Most DNA viruses have double-stranded DNA, and will take advantage of the cellular mechanism to replicate their DNA and synthesize the mRNA that produces the proteins necessary for the virus. Examples of viral DNA are adenoviruses, herpesviruses, and bacteriophage T4.
- RNA viruses: There are several types, depending on whether the RNA sequence is directly translated into proteins (equivalent to mRNA, as in poliovirus) or not (complementary to mRNA, as rhabdoviruses). In this case, the RNA polymerase enzyme will form the mRNA, or, as in retroviruses, such as HIV, the reverse transcriptase enzyme, which forms DNA from viral RNA and integrates it into the host cell's DNA.
All viruses that have double-stranded RNA, and some single-stranded RNA, have several independent RNA molecules. They are said to have a fragmented genome. DNA viruses have only one DNA molecule.
- Structural proteins: are responsible for forming the capsid.
- Enzymes proteins: they are involved in the synthesis of new viral nucleic acids.
- Binding proteins: facilitate adherence to the host cell membrane.
The capsid is a protein structure that surrounds and protects nucleic acid. It is formed by the union of capsomeres (globular proteins). Although its main function is to protect the nucleic acid, in viruses that are not enveloped by a membrane, it is also responsible for recognizing the cells it is going to parasitize.
The capsid is usually formed by the repetition of capsomeres with the same type of proteins, so the virus can create a complex capsid, with many capsomeres, with little genetic information. The more proteins you need to synthesize, the more genetic information you need, and the space for nucleic acid in a virus is very limited.
The term nucleocapsid refers to the genetic material wrapped in its capsid.
The capsomeres, to the joining, can form three types of capsids: icosahedral, helical and complex.
- The icosahedral capsid is a polyhedral structure with 12 vertices, 20 triangular faces and 30 edges, in which the capsomeres are arranged to form an icosahedron. This arrangement allows to have a closed structure with few capsomeres.