Viruses are generally classified by the organisms they infect, animals, plants, or bacteria. Since viruses cannot penetrate plant cell walls, virtually all plant viruses are transmitted by insects or other organisms that feed on plants. Certain bacterial viruses, such as the T4 bacteriophage, have evolved an elaborate process of infection. The virus has a "tail" which it attaches to the bacterium surface by means of proteinaceous "pins. Viruses are further classified into families and genera based on three structural considerations: 1 the type and size of their nucleic acid, 2 the size and shape of the capsid, and 3 whether they have a lipid envelope surrounding the nucleocapsid the capsid enclosed nucleic acid.
There are predominantly two kinds of shapes found amongst viruses: rods, or filaments, and spheres. The rod shape is due to the linear array of the nucleic acid and the protein subunits making up the capsid. The sphere shape is actually a sided polygon icosahedron. The nature of viruses wasn't understood until the twentieth century, but their effects had been observed for centuries.
British physician Edward Jenner even discovered the principle of inoculation in the late eighteenth century, after he observed that people who contracted the mild cowpox disease were generally immune to the deadlier smallpox disease.
By the late nineteenth century, scientists knew that some agent was causing a disease of tobacco plants, but would not grow on an artificial medium like bacteria and was too small to be seen through a light microscope.
Advances in live cell culture and microscopy in the twentieth century eventually allowed scientists to identify viruses. Advances in genetics dramatically improved the identification process. Capsid - The capsid is the protein shell that encloses the nucleic acid; with its enclosed nucleic acid, it is called the nucleocapsid.
This shell is composed of protein organized in subunits known as capsomers. They are closely associated with the nucleic acid and reflect its configuration, either a rod-shaped helix or a polygon-shaped sphere.
The capsid has three functions: 1 it protects the nucleic acid from digestion by enzymes, 2 contains special sites on its surface that allow the virion to attach to a host cell, and 3 provides proteins that enable the virion to penetrate the host cell membrane and, in some cases, to inject the infectious nucleic acid into the cell's cytoplasm.
Under the right conditions, viral RNA in a liquid suspension of protein molecules will self-assemble a capsid to become a functional and infectious virus. Envelope - Many types of virus have a glycoprotein envelope surrounding the nucleocapsid.
The envelope is composed of two lipid layers interspersed with protein molecules lipoprotein bilayer and may contain material from the membrane of a host cell as well as that of viral origin. The virus obtains the lipid molecules from the cell membrane during the viral budding process. However, the virus replaces the proteins in the cell membrane with its own proteins, creating a hybrid structure of cell-derived lipids and virus-derived proteins.
Many viruses also develop spikes made of glycoprotein on their envelopes that help them to attach to specific cell surfaces. Nucleic Acid - Just as in cells, the nucleic acid of each virus encodes the genetic information for the synthesis of all proteins. While the double-stranded DNA is responsible for this in prokaryotic and eukaryotic cells, only a few groups of viruses use DNA.
Viruses have different shapes and sizes. Scientists categorize viruses according to various factors, including:. Examples of viruses with an envelope include the influenza virus and HIV. Within these categories are different types of viruses. A coronavirus, for example, has a sphere-like shape and a helical capsid containing RNA. It also has an envelope with crown-like spikes on its surface.
Seven coronaviruses can affect humans, but each one can change or mutate, producing many variants. Learn more about coronaviruses here. Just as there are friendly bacteria in the intestines that are essential to gut health , humans may also carry friendly viruses that help protect against dangerous bacteria, including Escherichia coli. Viruses do not leave fossil remains, so they are difficult to trace through time.
Scientists use molecular techniques to compare the DNA and RNA of viruses and find out more about where they come from. Three competing theories try to explain the origin of viruses. In reality, viruses may have evolved in any of these ways.
The regressive, or reduction, hypothesis suggests that viruses started as independent biological entities that became parasites. Over time, they shed genes that did not help them parasitize, and became entirely dependent on the cells they inhabit. In this way, they gained the ability to become independent and move between cells. The virus-first hypothesis suggests that viruses evolved from complex molecules of nucleic acid and proteins either before or at the same time as the first cells on Earth appeared, billions of years ago.
When a viral disease emerges, it is not always clear where it comes from. A virus exists only to reproduce. When it reproduces, particles spread to new cells and new hosts. The features of a virus affect its ability to spread. Some viruses can remain active on an object for some time. If a person with the virus on their hands touches an item, the next person can pick up that virus by touching the same object. The object is known as a fomite. Viruses often change over time.
Some of these changes are very small and do not cause concern, but others can be more significant. Significant changes could make a virus more transmissible, as has been the case with the B. They may also help the virus evade the immune system or existing treatments. For example, doctors use several drugs in combination to treat HIV so that it is harder for the virus to develop resistance to treatment.
Influenza viruses can also do so-called antigenic shift. This can happen if a host cell has become infected with two different types of influenza virus. For instance, pigs can often serve as a mixing vessel for avian and human influenza viruses.
Some viruses, such as HPV, can lead to cancer. The full impact of a virus can take time to appear, and sometimes there may be a secondary effect. For example, the herpes zoster virus can cause chickenpox. The person recovers, but the virus may stay in the body. Years later, it may cause shingles in the same individual.
Coronaviruses are a large family of viruses and include viruses that cause the common cold. However, it has changed many times since scientists first identified it in China. Viruses have been found everywhere on Earth. Researchers estimate that viruses outnumber bacteria by 10 to 1. Viruses are submicroscopic, which means that you cannot see them in the microscope. What's interesting about viruses is that they have two or three components.
Starting from the inside, you will have a nucleic acid, which can be either RNA or DNA, and in both cases the nucleic acid can be either single-stranded or double-stranded. Then surrounding the nucleic acid will be a protein coat that's in the form of capsid, or little small units that are assembled in a certain way.
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