The Study of Viruses
Dmitri Ivanovsky was a Russian microbiologist born in 1864 whose work was centered around the Tobacco mosaic virus (TMV). TMV enters plant cells through a minor wound. The virus causes mottling, necrosis, stunting, leaf curling, and yellowing of plant tissue. TMV can also affect other types of crops in addition to tobacco plants. In fruit-bearing plants, TMV causes the plant to have poor fruit yield, delays in ripening, and the nonuniform color of the fruit.
Between 1887 and 1890, Ivanovsky researched the disease-causing destruction in plants. He made a solution of the infectious agent and passed it through a special filter. The pores in the filter were so small that bacteria was able to be filtered out. Ivanovsky found this filtered substance he collected had the ability to infect more tobacco plants. This meant the agent infecting plants had to be smaller than bacterial cells. Ivanovsky did not understand what was transmitting the infection. He thought it was a toxin produced by the bacteria that was infecting plants.
About six years later, a Dutch biologist named Martinus Beijerinck performed the same experiment independently of Ivanovksy's work. His findings were the same. The filtered substance was able to infect other plants. Beijerinck called the infecting organisms a virus. TMV was the first virus to be discovered.
Since the 19th century, scientists have known organisms smaller than bacteria exist. Unfortunately, there were no microscopes powerful enough to detect these organisms. In 1931, Ernest Ruska and his mentor Max Knoll built the first electron microscope. It was almost eight years later TMV was visualized.
Since the discovery of the electron microscope much has been learned about viruses. Virology is the scientific study of viruses.
Below are some key advancements made in virology.
- 1948- DIfference in the viruses causing smallpox and chickenpox were observed
- 1950- Discovery of Colorado tick fever virus
- 1952- First image of poliovirus was taken
- 1953- Discovery of human adenovirus
- 1954- Development of inactivated polio vaccine
- 1969- Discovery relating to replication of viruses
- 1980- Development of recombinant DNA technology
- 1982- Discovery of the human immunodeficiency virus (HIV)
- 1984- First molecular recombinant virus vaccine
- 1985- Invention of polymerase chain reaction (PCR)
- 1990- Modern molecular detection of new viruses
- 2000- Developments in modern viral evolution concepts
The continual drive to understand more about viruses and immunity has continued into the 21st century. New vaccines and actions are continually being developed to combat diseases. A deeper understanding of how the immune system processes and recognizes foreign substances, as well as new technologies to try to mimic and improve on these responses, are at the forefront of virology.
Are Viruses Alive?
Much debate over whether viruses should be considered living or nonliving still remains in the scientific community. Biologists generally agree the following five key properties are found in all living organisms:
- Ability to reproduce
- Ability to maintain internal homeostasis (maintain a stable internal state)
- Ability to respond to stimuli/environment
- Ability to make energy for itself
- Ability to evolve over time
Although viruses do perform some of these processes, they are not able to reproduce on their own. Viruses replicate as they take over host cells in organisms. Once in the host cell, a virus takes over the cell making more viruses. Viruses also evolve. Viruses also do not contain metabolic machinery and are metabolically inert.
In 2003, the discovery of a very large virus Acanthamoeba polyphaga mimivirus collected from a cooling tower of a hospital in England was made. About five years later, a similar virus was isolated from the water cooling tower in Paris. Since this time, several dozen giant viruses from different areas have been isolated. These giant viruses are large and contain genes that make proteins, which smaller viruses are not able to do. They are also able to be parasitized by other viruses, another characteristic smaller viruses are also not able to do. Since then several new giant viruses have been isolated and now make up the family Mimiviridae.
What's a Virus?
A virus is a small collection of genetic coding with deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) that is surrounded by a protein coat. Virology has laid the foundation in molecular biology as well as more in-depth looks at evolutionary studies.
Viruses range in size from 17 nanometers to 1.5 micrometers. The relatively newly discovered giant viruses can be about 700 to 1000 nanometers in length. Most viruses, except the giant viruses, are too small to be seen with a light microscope.
Viruses are not able to reproduce and continue to infect without the help of host cells. In order for infection to continue, a virus must replicate and create new viral structures that can infect additional host cells. When a virus enters the host cell, it must gain entry into the membrane of the target cell. When inside the target cell, the virus releases its genetic information and utilizes the host cell's ribosomes to manufacture proteins the virus needs for replication. The replicated proteins are assembled and released from the host cell where they continue the process of infection.
Virus Structure and Function
A virion is located outside of the host and is defined as an entire virus particle. All virions consist of nucleic acids, capsomeres, capsids, and protomers.
- Nucleic acids: all viruses contain either deoxynucleic acids (DNA) or ribonucleic acid (RNA).
- Capsomeres: individual protein subunits that comprise the capsid.
- Capsid: outer protective shell made of protein subunits (capsomeres) that houses the genetic material (RNA orDNA).
- Protomers: subunits that compose capsomeres
- Viral envelope: present in some viruses. Made of lipids and proteins which aid in protection, increase the ability of the virus to infect its host and determine viral entry and exit methods of the host cell.
- Viral proteins: help the virus attach to host cells. Can also disguise a virus so it can enter the host without detection.
- Capsid proteins: viruses that do not contain an envelope utilize capsid proteins to attach to host cells and determine viral entry and exit methods of the host cell.
Viruses are found in both prokaryotes and eukaryotes.
Prokaryotes are cells that have no membrane-bound organelles. The DNA in a prokaryotic cell is usually found in a region called the nucleoid. The nucleoid is a central region in a prokaryotic cell that contains genetic information The nucleoid but does not have a surrounding membrane.
In a eukaryotic cell DNA is found in a membrane organelle called the nucleus. The nucleus is the part of a eukaryotic cell containing the cell's chromosomes.
Viruses and Bacteria
Below are some of the similarities and differences between bacteria and viruses.
- Invisible to the naked eye
- May cause illness; however many bacteria are helpful and can aid in digestion, absorption, and decomposition of substances.
- Able to be treated with antibiotics
- Free, living cells prokaryotic cells that can live inside/outside a host
- Able to communicate with other bacterial cells through chemical signaling
- Infection spreads through contact with contaminated organisms or surfaces and contaminated food.
- Not generally specific in areas of the organism the bacteria attacks.
- Infection occurs as bacteria grow and reproduce on their own.
- Smaller than bacteria
- May cause illness; however some viruses are helpful and can kill undesirable bacteria and more dangerous viruses.
- Antibiotics are not effective against viruses. Treatment for viruses can be more difficult because of their size and reproduction inside cells.
- Nonliving collection of molecules that need a host for reproduction
- Usually more specific about the cells they attack (liver, respiratory system, blood)
- Infection occurs as virions attach to host cells and utilize the host cell's machinery to manufacture more virions.
It is estimated viruses outnumber bacteria 10 to 1. A bacteriophage is a virus that infects and replicates within bacteria. They are sometimes referred to as phages.
Virology is the scientific study of viruses. A virus is a small collection of genetic coding with DNA or RNA surrounded by a protein coat. Viruses range in size from 17 nanometers to 1.5 micrometers. The relatively newly discovered giant viruses can be about 700 to 1000 nanometers in length. Most viruses, except the giant viruses, are too small to be seen with a light microscope. A virion is located outside of the host and is defined as an entire virus particle. All virions consist of nucleic acids, capsomeres, capsids and protomers. Most scientists consider viruses nonliving because they are not able to reproduce on their own and are metabolically inert. Viruses depend on host cells to reproduce and continue infection of other host cells.
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What is the structure and shape of viruses?
A virus is a small collection of genetic coding with deoxyribonucleic acid (DNA) or ribonucleic acid (RNA) that is surrounded by a protein coat. They are not the same as bacteria.
Which structure is unique to viruses?
A virion is located outside of the host and is defined as an entire virus particle. All virions consist of nucleic acids, capsomeres, capsids and protomers. Once the virus enters the host, the genetic material (DNA or RNA) is injected into the host cell.
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