A virus is a small infectious agent that replicates only inside the living cells of other organisms. A virus is a small parasite that cannot reproduce by itself. Once it infects a susceptible cell, however, a virus can direct the cell to produce many more viruses. Viruses can infect all types of life forms, such as animals and plants to microorganisms, including bacteria and archaea. Genetic materials of viruses: Most viruses have either RNA or DNA as their genetic material. The nucleic acid may be single- or double-stranded.
The entire infectious virus particle, which is called a virion, consists of the nucleic acid and an outer shell of protein. The simplest viruses contain only enough RNA or DNA to encode four proteins. The most complex can encode 100 – 200 proteins. Host of viruses: Viruses depend on the host cells that they infect to reproduce, which make them host specific. When found outside of host cells, viruses exist as a protein coat or capsid, sometimes enclosed within a membrane. The capsid encloses either DNA or RNA which codes for the virus elements. Host specificity is due to specific attachment sites on the host cells called receptor.
Receptor sites for bacteriophage are found in bacterial cell walls or fimbriae or flagella. Animal cell membranes contain receptors for animal virus’s availability of cellular factors required for viral multiplication in the host cells. Size of viruses: In terms of their absolute numbers, viruses appear to be the most abundant biological entities on planet Earth. The best current estimate is that there are a whopping 1031 virus particles in the biosphere. For every human on the planet there are nearly Avogadro’s number worth of viruses. This corresponds to roughly 108 viruses to match every cell in our bodies.
However, because of their extremely small size, the mass tied up in these viruses is only approximately 5% of the prokaryotic biomass. The assertion about the total number of viruses is supported by measurements using both electron and fluorescence microscopy. Capsid morphology: The protective protein shell of each virus is called a ‘capsid. ‘ This capsid is made up of protein subunits called capsomeres, which are in turn made of subunits called protomers. The capsid encloses the genetic material of the virus. Capsids are broadly classified according to their structure and size.
Some viruses, such as bacteriophages, have developed more complicated structures due to constraints of elasticity. Some viruses are enveloped, meaning that the capsid is coated with a lipid membrane known as the viral envelope. The envelope is acquired by the capsid from an intracellular membrane in the virus’ host; examples include the inner nuclear membrane, the Golgi membrane, and the cell’s outer membrane. Once the virus has infected a cell and begins replicating itself, new capsid subunits are synthesized according to the genetic material of the virus, using the protein biosynthesis mechanism of the cell.
During the assembly process, a portal subunit is assembled at one vertex of the capsid. Through this portal, viral DNA or RNA is transported into the capsid. Structural analyses of major capsid protein architectures have been used to categorize viruses into families. Viral shapes: One type of viral shape is a helical virus. This is a virus that has its capsid shaped into the shape of a spring, taking the space of a cylinder or rod-shaped structure. This type of shape has a central cavity that encloses its nucleic acid. Some of these viruses are short, while others are very long.
Many allow for a lot of flexibility or a lot rigidity depending on how the capsomeres are arranged. Another type of viral shape for transporting viral nucleic acids is called icosahedral. An icosahedral virus is a virus consisting of identical subunits that has 20 equilateral triangular faces which is arranged in a symmetrical fashion. A special type of icosahedral shape, called a prolate, is a variant of the icosahedral viral shape and is found in bacteriophages. A lot of viruses are either helical or icosahedral in shape.
Many animal viruses, which include those that infect humans, are icosahedral in shape. The icosahedral shape has been shown to be the most optimal way of forming a viral capsid for numerous reasons, but namely due to the fact that it provides the virus with a very stable shape with a lot of room inside for the storage for the nucleic acid. In addition, because the protein subunits that make up the shape are identical, the virus doesn’t have to waste a lot of its genome on encoding many different kinds of proteins for its capsid.
This leads to conservation of energy and genetic economy. Viral envelope: The viral envelope is an additional layer around the virus that is made up of a lipid bilayer. It’s layer it is not for protection, but more for ease of infection. Some viruses such as influenza and many animal viruses have this viral envelope covering their protective protein capsids. The envelopes typically come from portions of the host cell membranes phospholipids and proteins, but include some viral glycoproteins. They may help viruses avoid the host immune system using the envelope as a boundary.
Classification of viruses: Viruses are not usually classified into conventional taxonomic groups but are usually grouped according to such properties as size or morphology, the type of nucleic acid they contain, the size and structure of the capsid and the number of protein subunits in it, host species, and immunological characteristics. Viral Replication: Viral replication is the formation of biological viruses during the infection process in the target host cells. Viruses must first get into the cell before viral replication can occur.
From the perspective of the virus, the purpose of viral replication is to allow production and survival of its kind. By generating abundant copies of its genome and packaging these copies into viruses, the virus is able to continue infecting new hosts. Replication between viruses is greatly varied and depends on the type of genes involved in them. Most DNA viruses assemble in the nucleus while most RNA viruses develop solely in cytoplasm. Lytic replication of bacteriophages: A temperate bacteriophage has both lytic and lysogenic cycles.
In the lytic cycle, the phage replicates and lyses the host cell. In the lysogenic cycle, phage DNA is incorporated into the host genome, where it is passed on to subsequent generations. The lytic cycle results in the destruction of the infected cell and its membrane. A key difference between the lytic and lysogenic phage cycles is that in the lytic phage, the viral DNA exists as a separate molecule within the bacterial cell, and replicates separately from the host bacterial DNA. Lysogeny: Lysogeny was first explained by the French biologist Andre Lwoff in the early 1950s.
It is a type of life cycle that takes place when a bacteriophage infects certain types of bacteria. The collection of genes in the nucleic acid core of a virus, known as a genome, of the bacteriophage enters into the chromosome of the host bacterium and replicates in concert with it. No progeny viruses are produced. Instead, the infecting virus lies dormant within the bacterium’s chromosome until the bacterium is exposed to certain stimuli, such as ultraviolet light. Next the virus enters a life cycle in which its genome is excised from the host chromosome and begins to multiply, forming new progeny viruses rapidly.
Ultimately the bacterial host is destroyed and the virus particles are released into the environment and infect new bacterial cells. Replication of animal viruses: The basic life cycle stages of animal viruses differ from bacteriophages in some key ways. 1. ) Attachment- requires specific interactions between host cell plasma membrane proteins and viral “spike” proteins (enveloped) or capsid proteins (non-enveloped). 2) Entry- by endocytosis or fusion of the envelope with plasma membrane. Involves uncoating of the virus (release of DNA, RNA). Some viruses may become lysogenic or latent following entry. )
Biosynthesis- *Replication of viral RNA occurs in cytoplasm. *Replication of viral DNA occurs in nucleus. 4) Maturation- RNA viruses typically assemble in cytoplasm. DNA viruses typically assemble in nucleus. 5) Release- via lysis (rupture of plasma membrane) or budding. host cell is not necessarily killed. The role of viruses in Cancer: When the DNA or RNA affects the host cell’s genes, it can push the cell toward becoming cancer. Several viruses are linked with cancer in humans. Both DNA and RNA viruses have been shown to be capable of causing cancer in humans.
Epstein-Barr virus, human papilloma virus, hepatitis B virus, and human herpes are the four DNA viruses that are capable of causing the development of human cancers. Human T lymphotropic virus type 1 and hepatitis C viruses are the two RNA viruses that contribute to human cancers. An estimated fifteen percent of all human cancers worldwide may possibly be attributed to viruses, representing a large portion of the global cancer burden today. Our growing knowledge of the role of viruses as a cause of cancer has led to the development of vaccines to help prevent certain human cancers.
