What do all viruses consist of

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. By September , scientists had logged over 12, mutations, and the development continues. Some variants are more transmissible and more likely to cause severe illness than others.

The main concern with new variants is the unpredictability of their impact. The main symptoms of COVID are dry cough , fatigue , and fever, but there are many possible symptoms. Anyone who has symptoms should seek a test. It is also important to self-isolate until 10 days after symptoms appear and when no fever has been present for 24 hours.

If a person has difficulty breathing , they should seek emergency medical attention. However, special T cells, known as cytotoxic T cells, can recognize cells that contain viruses, and release substances that kill those cells.

Some viruses can escape detection by cytotoxic T cells, but other immune cells — natural killer cells — can cause the cell containing the virus to die. In addition, body cells that contain a virus emit proteins called interferons, which warn other cells that a virus is present. This gives healthy cells a chance to defend themselves by changing the molecular makeup of their surface.

Antibodies can also help fight a virus before it enters a cell. They do this by neutralizing or damaging the virus or by changing its features so that it can no longer enter healthy cells. Antibiotics treat bacterial infections , but they cannot treat a viral infection. People will need either a vaccination to prevent infection, or antiviral drugs to treat any symptoms. Sometimes, the only option is symptom relief. In recent decades, scientists have developed antiviral drugs, largely in response to the AIDS pandemic.

These drugs do not destroy the virus, but they slow or prevent its development. With antiviral treatment for HIV, for example, the level of virus in the body can become so low that tests cannot detect it. At this point, it becomes untransmittable, which means that a person cannot pass the virus on to another person. Antivirals are also available to treat infection with HSV, hepatitis B , hepatitis C , influenza, shingles, and chickenpox. Tamiflu is an example of an antiviral drug. Unlike any microorganism, many viruses can, in suitable cells, reproduce themselves from their genome, a single nucleic acid molecule, that is, their nucleic acid alone is infectious.

Outside a susceptible cell, the virus particle like a bacterial spore is metabolically inert; on the other hand, when replicating in a cell, it exhibits all the characteristics of life. The new group of microorganisms are known as the filterable viruses. The filtration studies has shown that virus particles virions range from about the size of the smallest unicellular microorganisms nm down to objects little bigger than the largest protein molecules 20 nm.

In the simpler viruses, the virion consists of a single molecule of nucleic acid surrounded by a protein coat, the capsid; the capsid and its enclosed nucleic acid together constitute the nucleocapsid. The unicellular microorganisms can be arranged in order of decreasing size and complexity: protozoa, fungi, bacteria, mycoplasmas, rickettsiae, and chlamydiae.

These microorganisms, however small and simple, are cells. They always contain DNA as the repository of their genetic information, they contain RNA, and they have their own machinery for producing energy and macromolecules. Microorganisms grow by synthesizing their own macromolecular constituents nucleic acid, protein, carbohydrate, and lipid , and they multiply by binary fission. Viruses, on the other hand, are smaller and simpler in construction than unicellular microorganisms, and they contain only one type of nucleic acid—either DNA or RNA, never both.

Furthermore, since viruses have no ribosomes, mitochondria, or other organelles, they are completely dependent on their cellular hosts for energy production and protein synthesis.

Indeed, unlike any microorganism, many viruses can, in suitable cells, reproduce themselves from their genome, a single nucleic acid molecule; i. Are viruses alive? The question is rhetorical. Outside a susceptible cell, the virus particle, like a bacterial spore, is metabolically inert; on the other hand, when replicating in a cell it exhibits all the characteristics of life.

The key differences between viruses and microorganisms are listed in Table Several important practical consequences flow from these differences. For example, some viruses but no microorganisms may persist in cells by the integration of their DNA or a DNA copy of their RNA into the genome of the host cell, and they are not susceptible to antibiotics that act against specific steps in the metabolic pathways of bacteria.

For many years it has been known that viruses are smaller than unicellular microorganisms. Independently, in —, the Dutch plant pathologist Beijerinck, working on tobacco mosaic disease, and the German veterinarians Loeffler and Frosch, working on foot-and-mouth disease of cattle, showed that these diseases could be transmitted by material which could pass through a filter with pores too small to allow passage of bacteria.

Only with the advent of the electron microscope did it become possible to study their morphology properly. In , our knowledge of viral ultrastructure was transformed when Brenner and Horne applied negative staining to the electron microscopy of viruses.

Potassium phosphotungstate, which is electron-dense, fills the interstices of the viral surface, giving the resulting electron micrograph a degree of detail not previously possible. Electron micrographs of negatively stained preparations of the virions of all families of viruses of vertebrates are shown in the relevant chapters of Part II of this book.

In the simpler viruses the virion consists of a single molecule of nucleic acid surrounded by a protein coat, the capsid; the capsid and its enclosed nucleic acid together constitute the nucleocapsid. In some of the more complex viruses the capsid surrounds a protein core Fig.

The capsid is composed of morphological units called capsomers, which are held together by noncovalent bonds. Individual capsomers, which consist of one or more polypeptide molecules, are usually visible by electron microscopy. In helical nucleocapsids, the viral nucleic acid is folded throughout its length in a specific relationship with the capsomers Fig. Features of virion structure, exemplified by adenovirus A,B , tobacco mosaic virus C , and paramyxovirus D.

Not to scale. A,B Icosahedral structure of adenovirion. C The structure of helical nucleocapsids has been elucidated by studies of a nonenveloped plant virus, tobacco mosaic virus, but the principles apply to animal viruses with helical nucleocapsids, all of which are enveloped. In tobacco mosaic virus a single polypeptide is folded to form a capsomer. A total of capsomers assemble in a helix with a pitch of 2.

The 6-kb RNA genome sits in a groove on the inner part of the capsomer, and is wound to form an RNA helix of the same pitch, 8 nm in diameter, which extends the length of the virion. The virion is nm long and 18 nm in diameter, with a hollow cylindrical core 4 nm in diameter. D All animal viruses with a helical nucleocapsid and some of those with an icosahedral capsid are enveloped.

The envelope consists of a virus-specified matrix protein M; absent in Arenaviridae, Bunyaviridae, and Coronaviridae, as well as in the enveloped viruses with icosahedral capsids , beneath a lipid bilayer in which are inserted numerous glycoprotein peplomers. Fraenkel-Conrat and R. Wagner, eds. Plenum Press, New York, B, by John Mack, from R.

Jurnak and A. McPherson, eds. Wiley, New York, ; C, from C. Nayak, ed. Dekker, New York, ; and D, modified from D. Caspar et al. Within an infected cell, the capsomers of the simpler viruses self-assemble to form the capsid. The manner of this assembly is strictly defined by the nature of the bonds formed between individual capsomers, which imparts symmetry to the capsid.

Only two kinds of symmetry have been recognized: icosahedral and helical Fig. It has axes of two-, three-, and fivefold rotational symmetry, passing through its edges, faces, and vertices, respectively see Fig. The icosahedron is the optimum solution to the problem of constructing, from repeating subunits, a strong structure to enclose a maximum volume.

Features of icosahedral structure. A regular icosahedron viewed along twofold A , threefold B , and fivefold C axes of symmetry. In negatively stained electron micrographs, virions may appear hexagonal in outline upper row or apparently spherical middle row.

Various clusterings of capsid polypeptides give characteristic appearances of the capsomers in electron micrographs lower row. As such, there is some debate as to whether or not viruses should be considered living organisms.

A virus that is outside of a host cell is known as a virion. Not only are viruses microscopic, they are smaller than many other microbes, such as bacteria. Most viruses are only 20— nanometers in diameter, whereas human egg cells, for example, are about micrometers in diameter, and the E. Viruses are so small that they are best viewed using an electron microscope , which is how they were first visualized in the s. Viruses generally come in two forms: rods or spheres.

However, bacteriophages viruses that infect bacteria have a unique shape, with a geometric head and filamentous tail fibers. No matter the shape, all viruses consist of genetic material DNA or RNA and have an outer protein shell, known as a capsid. There are two processes used by viruses to replicate: the lytic cycle and lysogenic cycle. Some viruses reproduce using both methods, while others only use the lytic cycle. In the lytic cycle, the virus attaches to the host cell and injects its DNA.

Then fully formed viruses assemble. These viruses break, or lyse, the cell and spread to other cells to continue the cycle. Like the lytic cycle, in the lysogenic cycle the virus attaches to the host cell and injects its DNA. In humans, viruses can cause many diseases. For example, the flu is caused by the influenza virus. Typically, viruses cause an immune response in the host, and this kills the virus. However, some viruses are not successfully treated by the immune system, such as human immunodeficiency virus, or HIV.

Viruses strive to be as simple as possible while still maintaining their basic function, a concept that scientists call genetic economy. This means that the nucleic acid genome of the virus can be very tiny, providing instructions for only a few types of capsid protein subunits, each of which get produced in large numbers. Amazingly, these protein subunits can self-assemble in a stable and repetitive way, with each subunit forming the maximum number of contacts with the next subunit.

As a result, the capsid of viruses is like a coat of armor for the nucleic acid genome, and the repetitive nature of the identical protein subunits give nearly all viruses geometrical symmetry.

As mentioned above, the capsid of a virion is metastable — with the right kind and amount of perturbation, the capsid can become undone, allowing host cellular machinery to get access to the viral genome. The ability of the virion to disassemble is afforded by the fact that viral capsid subunits are NOT covalently bound, and will release from each other with the appropriate signal. Some viruses, such as the now famous coronavirus, also have a lipid membrane that surrounds the capsid.

These sugar-protein complexes are found on the surface of a virus particle, and are called glycoproteins. While glycoproteins are not specific to viruses there are many examples of glycoproteins throughout all life , they do provide a way for viruses to attach themselves to host cells.

Since viral glycoproteins are one of the key ways viruses can infect cells, many scientists are working on medicines that can impact how the glycoproteins work in order to prevent viral illnesses in people, pets, and plants. In addition to being varied in their shapes and sizes, viruses also demonstrate diversity when it comes to their nucleic acid genomes. The primary function of a viral genome is to store the instructions for building more virus particles.

Regardless of which type of genome a virus has, there are two main routes for packing it: viruses can either assemble their capsid shell around their nuclear genome, or viruses can make a capsid shell, and insert their nuclear genome into it.

Viruses also need to make sure that they are packaging their genomes, and not the genomes of their host cells. Because there are millions of different viruses, there are millions of different viral genomes.

So far, scientists have mapped the genomes of 75, viruses, but that is merely a fraction of what is out there.