Most viruses are significantly smaller than cells, so they can pass through bacterial filters. Their diameters typically ranging from around 20 to 200 nm. Vaccinia virus with a diameter of 200 nm were once considered to be the largest virus. However, Some larger viruses over 500 nm have been discovered one after another. Pandoravirus, which can encode thousands of proteins, even exceeds 1000 nm. Nevertheless, viruses are still much smaller than most cells whose size ranges from a few micrometers to millimeters. Porcine circovirus is one of the smallest viruses with a diameter of 14-17 nm and a genome encoding only a few proteins is approximately 1.76 kb. Due to their small size, most viruses cannot be observed by optical microscopes, and electron microscopes are required to observe their morphology and accurately measure their size.
Structure of Viruses
Their structure is highly simplified. Nucleic acids and proteins are their basic elements. Some viruses also contain carbohydrates and lipids. The central part of a virus is its nucleic acids that is surrounded by a protein coat known as a capsid. In some more complex viruses, the capsid may be further covered by a protein-containing lipid bilayer referred to as an envelope. Some envelopes also have protruding spike proteins. Viruses only carry the genes necessary for replication and lack certain metabolic enzymes and ribosomes, thus they are intracellular parasites.
Genetic Material: Nucleic Acids
The genetic material located at the core of a virus is called the genome, which encodes the viral structural proteins and the enzymes required for metabolism. Viral genomes exhibit diversity: (1) DNA or RNA; (2) double-stranded, single-stranded, or partially single-stranded and partially double-stranded; (3) circular or linear; (4) most viral genomes consist of a single molecule, while some RNA viruses have their genomes divided into multiple segments. Viruses are categorized as DNA viruses or RNA viruses based on the type of their nucleic acid.
Generally, most animal viruses are double-stranded DNA viruses or single-stranded RNA viruses; plant viruses are mainly single-stranded RNA viruses; bacteriophages mainly consist of double-stranded DNA viruses. These genetic materials are typically enveloped within a protective protein shell known as a capsid.
Capsid
It is a protein shell that surrounds and protects the viral genetic material. The group of the capsid and nucleic acid is called the nucleocapsid. The capsid of non-enveloped viruses recognizes host cell glycoproteins for attachment. The capsid also induces humoral and cellular immune responses in the host organism.
The capsid is composed of repeating protein subunits called capsomeres, which assemble together to form a symmetric structure. Virus particles exhibit two types of symmetric structures: helix (rod-shaped) and icosahedron (twenty faces). Some complex viruses are actually a combination of these symmetries. For example, bacteriophages have a head composed of an icosahedral capsid enclosing the genetic material, while the tail is made up of helically arranged capsomeres used for attachment and invasion into bacteria.
Envelope
Some viruses have an envelope surrounding the nucleocapsid. The lipids in the envelope originate from the host cell membrane, endoplasmic reticulum, or nuclear membrane, containing viral proteins and some host membrane proteins. Similar to the capsid, the envelope plays a vital role in protecting the virus, facilitating attachment, invasion, and triggering immune responses.
Spike Proteins
Many viruses, such as HIV, influenza, and coronaviruses, possess spikes or glycoproteins protruding from their envelopes. These structures play a crucial role in virus attachment, entry into host cells, and triggering the immune response by interacting with specific receptors on the host cell surface.