What do viruses contain

There are many different modes of entry. HIV, a virus with an envelope, fuses with the membrane and is pushed through. Another enveloped virus, the influenza virus, is engulfed by the cell. Some non-enveloped viruses, such as the polio virus, create a porous channel of entry and burrow through the membrane. Once inside, viruses release their genomes and also disrupt or hijack various parts of the cellular machinery. Viral genomes direct host cells to ultimately produce viral proteins many a time halting the synthesis of any RNA and proteins that the host cell can use.

Ultimately, viruses stack the deck in their favor, both inside the host cell and within the host itself by creating conditions that allow for them to spread. For example, when suffering from the common cold, one sneeze emits 20, droplets containing rhinovirus or coronavirus particles, according to "Molecular Biology of the Cell.

Understanding the relationships between viruses began with noting similarities in size and shape, whether viruses contained DNA or RNA, and in which form. With better methods to sequence and compare viral genomes, and with the constant influx of new scientific data, what we know about viruses and their histories is constantly being fine-tuned. Until , the notion that viruses were much smaller than bacteria, with tiny genomes was taken for granted.

That year scientists discovered a bacteria-like structure within some amoebae in a water-cooling tower, according to Wessner. As it turns out, what they discovered was not a bacterial species, but a very large virus, which they dubbed Mimivirus. The virus is about nm in size and may also have the same staining properties as gram-positive bacteria.

This was followed by the discovery of other large viruses such as the Mamavirus and Megavirus. However, their genome still contains many remnants of genes associated with the process of translation. It is possible that Mimiviruses may have once been independent cells. Or they could have simply acquired and accumulated some host genes, Wessner wrote.

Such discoveries bring up new questions and open new avenues of research. In the future these studies may provide answers to fundamental questions about the origins of viruses, how they reached their present parasitic state, and whether viruses should be included in the tree of life.

It may also be linear or circular. While most viruses contain a single nucleic acid, others have genomes that have several, called segments. DNA viruses cause human diseases, such as chickenpox, hepatitis B, and some venereal diseases, like herpes and genital warts. These RNA polymerase enzymes are more likely to make copying errors than DNA polymerases and, therefore, often make mistakes during transcription.

This causes them to change and adapt more rapidly to their host. Human diseases caused by RNA viruses include hepatitis C, measles, and rabies. Viruses display a wide diversity of shapes and sizes, called morphologies. In general, there are five main morphological virus types:. Viral structure : An outline of the structures of some common viral types.

A complete virus particle, known as a virion, consists of nucleic acid surrounded by a protective coat of protein called a capsid. These are formed from identical protein subunits called capsomeres. The capsid is made from proteins encoded by the viral genome and its shape serves as the basis for morphological distinction. Virally coded protein subunits will self-assemble to form a capsid, in general requiring the presence of the virus genome.

Complex viruses code for proteins that assist in the construction of their capsid. Proteins associated with nucleic acid are known as nucleoproteins, and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid. The capsid and entire virus structure can be mechanically physically probed through atomic force microscopy. Most viruses that have been studied have a diameter between 20 and nanometers.

Some filoviruses have a total length of up to nm; their diameters are only about 80 nm. Most viruses, such as virions, cannot be seen with an optical microscope, so scanning and transmission electron microscopes are used to visualize them. These are solutions of salts of heavy metals, such as tungsten, that scatter the electrons from regions covered with the stain. When virions are coated with stain positive staining , fine detail is obscured.

Negative staining overcomes this problem by staining the background only. Complex viruses are often asymetrical or symetrical in combination with other structures such as a tail. Viruses have many structural shapes, often falling into certain categories.

Some bacteriophages, such as Enterobacteria phage T4, have a complex structure consisting of an icosahedral head bound to a helical tail, which may have a hexagonal base plate with protruding protein tail fibers. This tail structure acts like a molecular syringe, attaching to the bacterial host and then injecting the viral genome into the cell. T4 Bacteriophage : T4 is a bacteriophage that infects E.

Although it has an icosahedral head, its tail makes it asymmetrical, or complex in terms of structure. The poxviruses are large, complex viruses that have an unusual morphology. The viral genome is associated with proteins within a central disk structure known as a nucleoid. The nucleoid is surrounded by a membrane and two lateral bodies of unknown function. The virus has an outer envelope with a thick layer of protein studded over its surface.

The whole virion is slightly pleiomorphic, ranging from ovoid to brick shape. Mimivirus is the largest characterized virus, with a capsid diameter of nm. 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.

As next generation sequencing and analysis continues to grow in its sophistication, scientists will continue building knowledge when it comes to viral genomes! Gelderblom, H. Structure and classification of viruses.

Baron Ed. University of Texas Medical Branch. Holmes, E.