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Morphology, Classification, Reproduction/Replication and Cultivation of Viruses

Size, Structure, Shape and Symmetry, Adsorption, Penetration, Uncoating, Biosynthesis, Maturation, Release, Animal inoculation, Embryonated eggs

Morphology of Viruses

Size

Virus particles that are extracellular are called virion. They are small in comparison to bacteria. A light microscope cannot see them. Under a light microscope, poxviruses and other large viruses can be observed when they are stained appropriately. A virus can be 20 nm in diameter or 300 nm in diameter. The poxvirus is among the largest viruses, while the parvovirus is among the smallest. A collodion membrane filter with a graded porosity was the first method of measuring virus particle size. In order to estimate its size, the diameter of the pore in the finest filter that allowed passage of the virion was measured. A second method was developed with the advent of ultracentrifuge. Stoke's law can be used to calculate the particle size based on the sedimentation rate of the virus in the ultracentrifuge. Thirdly, electron microscopy is the most direct way to determine virus size. Viruses can be studied both by their shape and size using this method.

Structure, Shape and Symmetry

There is a nucleic acid coat surrounding a protein coat inside the virion, which is called the capsid. Capsids containing nucleic acids are known as nucleocapsids. Nucleic acid in the capsid is protected from harmful substances in the environment. Each morphological unit of it is composed of several capsomers. In a symmetrical arrangement, polypeptide molecules make up the chemical units of the capsid. The nucleic acid is surrounded by this shell. Both icosahedral (cubic) and helical symmetry are displayed on the capsid. Icosahedrons are polygons that have 12 vertices and 20 sides. It has equilateral triangle-shaped faces.

The icosahedral capsid is composed of two types of capsomers. At the vertices (pentons) are the pentagonal capsomers, and at the facets (hexons) are the hexagonal capsomers. Each penton always has 12 hexons, but the number varies according to the virus group. Adenoviruses and Herpes Simplex Virus have icosahedral symmetry in their capsids. A nucleocapsid with helical symmetry has a helical or spiral tube formed by the capsomers and nucleic acid, such as tobacco mosaic virus. It is not true that all viruses exhibit icosahedral or helical symmetry. There are some viruses with complex symmetry, such as poxviruses.

Virion envelopes and unenveloped viruses are both possible. Viral envelopes are derived from the membranes of host cells. When the virus budding occurs, the virus leaves the host cell. Protein subunits can be seen on the envelope surface as projecting spikes. These are known as peplomers. Two kinds of peplomers are found in the influenza virus: haemagglutinin and neuraminidase. Neuraminidase is shaped like a mushroom. Haemagglutinin is shaped like a triangular spike. Lipid solvents affect envelope function. The envelope is responsible for granting viruses chemical, antigenic, and biological properties. Viral particles vary in size, shape, and composition. Most animals' viruses have a round shape. Rabies has a bullet-like shape. Pneumoviruses resemble bricks.

Classification of Viruses

Viruses had little understanding of their basic properties until about 1950. According to the disease they caused or the place where they were isolated, they were named haphazardly. Tropisms refer to groupings of organisms based on affinity with different systems or organs of the body. As a result, human viruses were classified into four categories: dermotropic, meaning those that produce skin disease (smallpox, chickenpox, measles), neurotropic, meaning those that affect the nervous system (poliomyelitis, rabies), pneumototropic, meaning those that affect the respiratory tract (influenza, common cold) and viscerotropic, meaning those that affect the visceral organs. Bacwden (1941) was among the first to suggest a virological nomenclature and classification based on characteristics of viruses rather than host responses. Viral groups were first classified in the 1950s based on their physiochemical and structural characteristics. The International Committee on Taxonomy of Viruses (ICTV) is the official body for nomenclature and classification of viruses.

RNA viruses are most important in the medical field. Among the most important are Picornaviridae, Orthomyxoviridae, and Paramyxoviridae, Flaviviridae, and Rhabdoviridae. A Picornaviridae virus has a single-stranded RNA genome and is small (20-30nm), non-enveloped, and icosahedral. Polioviruses and coxsackieviruses are examples. Orthomyxoviridae are enveloped viruses that contain haemagglutinin and neuronaminidase peptides. There are eight pieces of singlestranded RNA that make up the genome. Therefore, their genomes are segmented. The influenza virus is a good example. RNA viruses of the Flaviviridae family are enveloped. There are several types of viruses, including yellow fever virus, Japanese encephalitis virus, and dengue virus. Retroviridae are RNA viruses with an envelope that contain reverse transcriptase, an enzyme. Reverse transcriptionase is an enzyme that relies on RNA for DNA replication. DNA is synthesized from RNA by using it. AIDS (acquired immune deficiency syndrome) is caused by the Retroviridae family of viruses. Baltimore (1970) classified viruses into seven groups based on their replication mechanisms. These are called the Baltimore classifications.

Reproduction

Viral replication refers to the proliferation of viruses. Viruses possess genetic information but lack enzymes necessary for replication. For replication they rely on the machinery of host cells. There are six phases in viral replication - adsorption, penetration, uncloaking, biosynthesis, maturation, and release.

Adsorption - In the adsorption phase, the virus attaches itself to the host cell. A specific receptor must be present on the host cell. Those receptors detect surface components from the viral particles. Viruses attach to host cells via cell-virus interactions.

Penetration - At this point, the virus has entered the host cell. The cell wall of bacteria is rigid. Viruses that attack bacteria are unable to penetrate their walls. Virus nucleic acids are the only parts that enter the bacterial cell. Both animals and humans lack a cell wall. Whole viruses enter the cell as a result. Viropexis is a mechanism by which virus particles are engulfed. When encapsulated viruses infect cells, the viral envelope may fuse with their membranes. Following the nucleocapsid release, the nucleoplasm becomes exposed.

Uncoating - It involves the removal of the outer layer and capsid of the virus. The host cell's lysosomes produce most of these enzymes. An uncoating enzyme produced by the virus can also do the job. The virus is then released and enters the cell.

Biosynthesis - The nucleic acid and capsid of the virus are synthesized in this phase. Assemblage, synthesis, and release of viral products are all contributed to by the synthesis of enzymes. Certain proteins called regulators are also synthesized. These proteins inhibit normal cellular activity. Viral components are produced as a result. The nucleus of the host cell synthesizes the majority of DNA virus nucleic acid. The exception is the poxvirus. DNA viruses are synthesized entirely in the cytoplasm of their host cells. All RNA viruses synthesize all of their components within the cell. Exceptions include a few orthomyxoviruses and a few paramyxoviruses. These viruses synthesize certain compounds in host cells. The primary steps in biosynthesis are as follows:
  • mRNA transcripts are produced from viral nucleic acids
  • A process by which mRNA becomes "early proteins" or "nonstructural proteins". Viral components are synthesized by these enzymes.
  • Nucleic acid replication by viruses
  • Creating or synthesizing "late proteins" and "structural proteins". Capsids of daughter virion particles are composed of these proteins.
Maturation - Virions are formed from synthesis of nucleic acids and proteins from viral nucleic acids. Nuclei or cytoplasm can serve as sites of infection. The nucleus is home to herpesviruses and adenoviruses. The nucleus is where picornaviruses and poxviruses assemble.

Release - A bacterium infected with a virus (bacteriophage) is lysed and releases the virus. Viruses released by animals usually do not require cell lysis. Myxoviruses are released when their membranes budding. There is no cellular damage. Infection of other cells is possible with daughter virions that invade the surrounding medium. Viral transmission takes place directly between cells in some cases (e.g. varicella). Free virus does not exist in these cases. Cell lysis may release the poliovirus, which causes damage to cells.

The presence of the virus within the host cell cannot be detected once it has penetrated until it has produced mature daughter virions. There is no evidence of the virus inside the host cell. A new phase is called the eclipse. For bacteriophages, the replication process takes approximately 15-30 minutes. For animal viruses, the time takes up to 30 hours. The progeny virus can be released by a single infected cell in large numbers.

Cultivation

Viruses are obligate parasites that live inside cells. The culture media used to grow bacteria cannot support their growth. A virus is cultivated by:

Animal inoculation - It was Landsteiner and Popper who isolated the poliovirus in 1909 using monkeys. The isolation of coxsackieviruses and arboviruses (dengue, chikungunya) is performed using infant mice. A mouse can be inoculated several ways - intracerebrally, subcutaneously, intraperitoneally, or subcutaneously. Certain situations use other animals such as rabbits, guinea pigs, and ferrets.

Embryonated eggs - A hen's egg embryonated in 1931 was used for the first time to cultivate viruses by Goodpasture. The technique was then refined by Burnet. It is possible to cultivate different viruses using different parts of the egg. A chorioallantoic membrane inoculated with the herpes simplex virus causes visible lesions known as pocks. Influenza viruses are isolated by injecting them into amniotic sacs. Rabies virus is isolated from yolk sacs through inoculation.

Cell culture - Steinhardt and colleagues probably applied tissue culture for the first time in virology in 1913. The virus was maintained in rabbit cornea fragments. Enders, Weller, and Robbins demonstrated poliovirus cultivation in 1949, and this was the turning point. In tissue cultures of non-neural origin, they demonstrated that poliovirus, which was thought to be strictly neurotropic, could also grow.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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