Table of Contents
The virus is an obligate intracellular parasite which can reproduce only by attacking and taking over host cells as they lack the cellular machinery for reproduction. A viroid is sub viral, smallest agent of infectious disease.
What is Virus?
A virus is a small non cellular particle made up of genetic material and protein that can invade living cells. Viruses are non cellular, nonliving structures having a protein coat called capsid and nucleic acid (DNA, RNA) core. Viruses can replicate only inside a host cell. Some viruses are enclosed in a protective envelop. Some viruses have spikes which help in the attachment of host cells. Viral subunits are made of individual protein subunits which are called capsomeres. The study of viruses is called virology. Beijerinck coined the Latin name virus which means poison in 1897. He studied filtered plant juices and found they caused healthy plants to become infectious. Wendell Stanley crystallized sap from diseased tobacco plants in1935. He discovered that viruses are composed of nucleic acids and protein. Edward Jenner developed a small pox vaccine using milder cowpox viruses in 1796. Deadly viruses are said to be virulent. Small pox has eradicated now a days in the world. Viruses are smaller than small cells and measured in nanometer. Viruses could not be seen until electron microscope was discovered in the 20th century. Outside of host cells, viruses are inactive, lack ribosomes and enzymes which are needed for metabolism. Viruses come in a variety of shapes. Some have a helical shape for example Ebola virus. Some have polyhedral shapes like influenza virus, and some have very complex shapes like bacteriophages. Viruses which attack bacteria are called bacteriophages. T-phages are a particular class of bacteriophages with icosahedral heads, double stranded DNA, and tails. The most commonly studied T-phages are T4 and T7. They infect E-coli which is an intestinal bacterium. Six small spikes at the base of tail are used to attach to the host cells. Adsorption, penetration, uncoating, synthesis, assembly, and release are modes of virus replication. Common cold, chickenpox, influence, and cold sores are examples of common human diseases caused by viruses. Many chronic diseases such as AIDS, SARS, avian influenza and Ebola virus disease are also produced by viruses. The relative ability of the virus to cause disease is designated in term of virulence. Other diseases are under examination to discover if they have a virus as the causative agent, such as the likely connection between human herpesvirus 6 and neurological diseases such as multiple sclerosis and chronic fatigue syndrome. Viruses are important to study cell and molecular biology as they provide simple systems that can be used to investigate and manipulate the functions of cells.
What is Viroid?
Viroids are sub viral, smallest infectious pathogens. They are made up of a short strand of circular, single strand RNA without a protein coat. All viroids are present on higher plants and cause diseases in them. Some are present in infected host cell nucleolus while others are present in chloroplasts. Viroids cause diseases by RNA silencing. Some diseases are slight, but some cause a lot of economic loss. There are 20 viroids which vary in nucleotide sequence. Viroids resemble prions which are cut from eukaryotic cells. Theodor Otto Diener who was a well-known plant pathologist, discovered viroid first time in U.S Department of Agriculture’s research center in Beltsville, Maryland in 1971. He also characterized viroid on a molecular basis and named it. That first viroid was a pathogenic agent of the potato spindle tuber disease. This viroid is called potato spindle tuber viroid (PSTVd) now a days. Viroids are composed of nucleic acid but do not code for any protein. Replication mechanism of viroids uses RNA polymerase II which is a host cell enzyme. The human causative agent, hepatitis D virus, is a defective RNA virus similar to a viroid. Viroid infections are transferred by cross contamination following damage to the plants as a result of horticultural or agricultural practices. Aphids transmit some, and they can also be shifted from the plant by leaf contact.
Virus vs. Viroid
- Virus is a particle of nucleoprotein.
- A viroid is an infectious RNA particle.
- The nucleic acid of the virus can be DNA or RNA.
- The nucleic acid of viroid is the only
- The virus has a protein covering around genetic material which is called capsid.
- Viroid has no protein covering around RNA.
- The virus has a larger
- Viroid has a smaller
- The virus infects all types of cells.
- Viroid infects only plant cells.