Virus: Structure, Characteristics, and Replication Mechanisms
Microorganisms: Viruses
Microorganisms without cellular organization: Viruses are entities with a simple structure, often described as subcellular. They are not considered living organisms and do not perform metabolic functions on their own. Instead, they rely on the metabolic machinery of a host cell to reproduce. This makes them obligate intracellular parasites, infecting bacteria (bacteriophages or phages), plant cells, and animal cells.
Virus particles, also called virions, consist of a molecule of DNA or RNA (never both in the same virus) enclosed within a protein capsule. Sometimes, they also have a membranous envelope.
Structure and Characteristics of Viruses
As mentioned, all viruses are formed by a protein coat called a capsid, which encloses the nucleic acid. Some complex viruses may also have a membranous envelope composed of proteins and lipids. Viruses are very small and can only be seen with an electron microscope. Their size ranges from 10 nm (poliovirus) to 300 nm (smallpox virus, tobacco mosaic virus). Viruses differ in size, structural characteristics of the capsid, the nature of their nucleic acid, the mode of penetration into the host cell, and the replication mechanism.
Constitution and Morphology
All viruses, without exception, have a protein envelope called a capsid. The capsid is composed of one or more protein subunits called capsomeres, often arranged in concentric layers. The geometry of the capsid is one of the criteria used to classify viruses into four groups: icosahedral, helical, complex, and enveloped.
Icosahedral Viruses
These viruses have a spherical appearance, and their capsid adopts the structure of an icosahedron (a polyhedron with 20 triangular faces, 30 edges, and 12 vertices). Examples include adenoviruses, poliovirus, and picornaviruses.
Helical or Cylindrical Viruses
These viruses, such as the tobacco mosaic virus and the rabies virus, have an elongated appearance. Their capsomeres are arranged in a helical manner, forming a cylinder with a hollow core, similar to the steps of a spiral staircase.
Complex Viruses
Complex viruses, such as bacteriophages (viruses that parasitize bacteria), appear to combine the two previous structures. They have an icosahedral head containing DNA and a helical tail. The tail has fibers and spines that help the virus attach to and infect bacteria.
Enveloped Viruses
Most animal viruses, such as influenza, smallpox, hepatitis, and HIV, have a membranous envelope in addition to the capsid. This envelope is derived from the host cell’s plasma membrane during the virus’s exit process. The lipid bilayer of the envelope contains virus-encoded glycoproteins that protrude from the exterior as spikes. These spikes act as an anchor system for receptors on the host cell membrane, facilitating penetration through endocytosis or membrane fusion. The membranous envelope is very important from an immunological point of view.
Nucleic Acid
The nucleic acid is the essential component of the virus. It can be single-stranded DNA (e.g., in phage φX174) or double-stranded DNA (e.g., in phage T4 and adenoviruses). There are also double-stranded RNA viruses (e.g., rotaviruses) and single-stranded RNA viruses (e.g., retroviruses, including influenza, measles, rabies, and certain oncogenic viruses that cause cancer). Retroviruses contain a special enzyme called reverse transcriptase, which allows them to transcribe RNA into DNA within the infected cell.
Mechanisms of Replication: The Viral Life Cycle
Although the genome of a virus contains a small number of genes, it is sufficient to force the host cell to transcribe and translate its message. The mechanisms of penetration and the cellular compartments used for replication vary among different types of viruses.
Lytic Cycle
- Attachment and Entry: The bacteriophage attaches its tail to specific receptors on the bacterial cell wall. An enzyme located in the tail of the virus weakens the bonds in the cell wall. The phage then contracts its helical sheath, injecting the contents of its head through the tubular tail, and the nucleic acid enters the cell.
- Multiplication: Once inside, the viral DNA uses the host cell’s enzymes to synthesize viral mRNA. This viral mRNA is used to synthesize viral proteins (capsomeres, endonucleases, endolysins). The viral DNA is replicated multiple times using the host cell’s enzymatic complexes. The replicated nucleic acids and synthesized viral components are then assembled into new viruses.
- Lysis and Liberation: In a single bacterium, up to 100 bacteriophages can be formed. The enzyme endolysin weakens the bacterial cell wall, causing it to rupture and release the new viruses, which can then infect new cells.
Lysogenic Cycle
Cell lysis does not always occur immediately. Some phages can integrate their DNA into the bacterial DNA through a process of crossing over. These phages are called prophages and replicate passively with the bacteria. Bacteria that can establish this relationship with attenuated phage DNA are called lysogenic. The prophage can remain latent for several generations until a stimulus induces the start of a typical lytic cycle. While the cell is immune, the prophage DNA prevents the same virus from infecting it. Other bacteriophages can also have lysogenic cycles.