Virus, Its types, Origin, Viral structure, Replication and Function
Virus a small infectious agent that can replicate only inside the living cell of an organism. Viruses can infect all kinds of organism, from animals and plants to bacteria and archaea. Viruses cause a number of disease in Eukaryotes. In human., smallpox, herpes, polio, rabies, Ebola, Santa fever, and AIDS are the example of viral diseases.
Types of virus:
These pathogens are one of the most widespread of all organisms and are capable of infecting every species of animals from mammals down to the insects, protozoa and even bacteria and plants. In fact, there are more species of viruses than all other creatures put together. Some are harmless while some are extremely dangerous and like HIV causing , AIDS or Ebola and Marburg virus etc.
Origin of viruses:
Researchers have found the resemblance of the genome of viruses with genes of higher animals that can 'jump' from one chromosome to another chromosome. Some believe that virus may have originated from bacterial plasmid while other believes that they have also originated from degenerate of bacteria that have become an obligate parasite. Plasmids are the little packets of genes lying outside the bacteria chromosome and capable of being transferred to another bacterium.
Characteristics of virus:
1: Virus is extremely small ranging in size from 20 to 300nm and simple organism.
2: They are so tiny i.e they are sub-microscopic particles that can only be seen with a special, very powerful microscope called an electron microscope.
3: There are six characteristics of the living viruses they are:
a: Adaptation to the environment.
b: Cellular make up.
c: The Metabolic process that obtains and use energy.
d: Movement response to the environment growth and development.
e: They are obligate intracellular parasites.
f: They can infect animals, plants or microorganism.
Viruses of plants or animals:
a: Plant viruses:
Plant viruses are the viruses affecting plants like all other viruses. Plant viruses are obligate, intracellular parasites that don't have the molecular machinery to replicate without a host. There are over 2000 known viruses that infect plants. They cause an estimated 60 dollar loss on yields worldwide each other. Common symptoms of viral infection include bleached or brown spots on leaves and fruits stunted growth and damaged flowers and roots.
b: Animal viruses;
They are intracellular obligate parasite virus gain entry into host cells via several sites such as skin, gastrointestinal tract, and respiratory tracts. Once an infection has occurred the virus may be replicated into host cells. At the site of infection or they may be spread to the other location. Animal viruses typically spread through the body mainly by the way of blood stream but can also be spread via the nervous system. The virus has several methods to counter host immune system responses. Some virus of animals is HIV, less body immune system cells.
Viral structure, replication, and function:
Viral nucleic acid:
The protein coat is the covering that covers the nucleic acid. This covering protects the nucleic acid. Protein coat has the symmetrical structure and this is built of one or more subunits packets like a chemical crystals.
Lipid membrane ( Envelope ):
This covers the capsid many viruses do not have this envelope and are called naked viruses. Lipid membrane is usually acquired but the virus from the host cell in the process of leaving the cell. This coat enables the virus to survive outside the cell sufficiently long to spread elsewhere via the blood.
The capsid and the entire structure of viruses can be divided into four types:
There is a capsomer coiled around a central axis to form a helical structure. This is a common structure seen in a single standard RNA virus. Tobacco mosaic virus is a helical virus.
These are near spherical shape and this shape is adopted because the coat forms a closed shell. Rotavirus has twelve capsomers and appears spherical.
The virus is covered with a lipid membrane in a modified form of one of the cell membranes. The outer membrane is from the infected host cell and internal membranes from a nuclear membrane or endoplasmic reticulum forming a lipid bilayer known as a viral envelope. This membrane is studded with proteins or receptors.
There is a capsid that is neither purely helical nor purely icosahedral. There may be extra features like protein tails or a complex outer wall. Bacteriophages are examples of this type of viral structure.
The virus in order to infect the cell it must enter into the cell to infect it. However, viruses are not taken up by the cell directly. They must attach to the receptor on the cell surface first in order to gain the entry. If the receptor is not a necessary one, the cell once infected with the virus may go on to remove the receptor altogether.
Each virus has its specific receptor and it is a vital component of the cell surface so that the cell cannot get rid of it to avoid the infection. The selectivity of the viruses determines the cell preference.
For example; Rhinovirus have a preference for cell lining for cells lining the nose airways and the lungs. HIV virus with CD4, CCR5, CXCR4 viruses, Epstein Barr ( EBV ) with rabies virus with CR2, acetylcholine receptor influenza virus with neuraminic acid on the red blood cell.
HIV infects mainly T lymphocytes and macrophages because only they carry a surface molecule known as CD4 receptor and EBV infects B lymphocytes carrying the complement receptors CR2.
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- A Virus is extremely small ranging in size from 20 to 300nm and simple organism.
They are so tiny i.e they are sub-microscopic particles that can only be seen with a special, very powerful microscope called an electron microscope.
The nucleic acid is the core of the virus. It is the either DNA or RNA ( DOXYERIBONUCLEIC ACID AND RIBONUCLEIC ACID ) . The DNA or RNA hold all of the information for the virus that makes it unique and helps multiply.
DNA viruses are the simplest. They use the host cell's RNA polymerase and can make mRNA that translates the host ribosomes to make viral protein.
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