Virus is what kind of cell

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You cannot download interactives. People get sick when another organism, big or small, invades their body and infects them. These infectious agents come in all shapes and sizes, and all of them pose different threats to the human body. Some are microscopic, such as bacteria or viruses, which attack human bodies on the cellular level.

Others are larger, like fungi, which are unicellular or multicellular organisms that grow on and feed off organic material, including humans. Finally, parasites such as tapeworms can find their way inside the human body and feed on blood and nutrients without killing their host.

Learn more about infectious agents and their impact on human health with this curated resource collection. Even the most basic parts of a cell can enable complex cellular processes, and multifunctional organelles expand these capabilities to make advanced activities possible for higher life-forms. Organelles are specialized structures that perform various tasks inside cells.

Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Image virus Viruses are microscopic biological agents that invade living hosts and infect their bodies by reproducing within their cell tissue. Photograph by Maryna Olyak.

Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. 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. Most viruses maintain all their genetic information with the single-stranded RNA.

There are two types of RNA-based viruses. In most, the genomic RNA is termed a plus strand because it acts as messenger RNA for direct synthesis translation of viral protein. A few, however, have negative strands of RNA. In these cases, the virion has an enzyme, called RNA-dependent RNA polymerase transcriptase , which must first catalyze the production of complementary messenger RNA from the virion genomic RNA before viral protein synthesis can occur.

The Influenza Flu Virus - Next to the common cold, influenza or "the flu" is perhaps the most familiar respiratory infection in the world. In the United States alone, approximately 25 to 50 million people contract influenza each year.

The symptoms of the flu are similar to those of the common cold, but tend to be more severe. Fever, headache, fatigue, muscle weakness and pain, sore throat, dry cough, and a runny or stuffy nose are common and may develop rapidly. Gastrointestinal symptoms associated with influenza are sometimes experienced by children, but for most adults, illnesses that manifest in diarrhea, nausea, and vomiting are not caused by the influenza virus though they are often inaccurately referred to as the "stomach flu.

Since that time, a tremendous amount of research focusing upon the causative agent of AIDS has been carried out and much has been learned about the structure of the virus and its typical course of action. HIV is one of a group of atypical viruses called retroviruses that maintain their genetic information in the form of ribonucleic acid RNA. The activity of the enzyme enables the genetic information of HIV to become integrated permanently into the genome chromosomes of a host cell.

License Info. Image Use. Rybicki has characterized viruses as a form "at the edge of life. Since viruses are capable of self-replication, they are clearly some type of lifeform, and likely involved with the early evolutionary development of such other simple lifeforms as bacteria and protists. Viruses differ, however, from the simpler autonomous replication of chemical crystals. This is due to the fact that a virus can inherit a genetic mutation and is also subject to similar natural selection processes of cellular organisms.

A virus cannot be labelled simply, therefore, as inanimate or lifeless. Here, we consider it a lifeform, but we adhere to current taxonomy and do not credit it with a parallel domain to other recognized cellular lifeforms. Although there is no detailed catalogue of the evolutionary relationships of viruses and hosts, certain general characterizations can be made.

In such viral groups as poxviruses, papillomaviruses , and tobamoviruses , molecular taxonomy aligns generally with the genetic relationships of their hosts. There are clear examples of an otherwise genetically close group like the tobamoviruses including a genetically outlying host; in particular, the tobamoviruses generally utilize plants of the Solanaceae family, but an orchid and a cactus virus can also be found in the group.

Recombination of genome parts of viruses poses a more vexing puzzle, since the events are virtually random pieces of an evolutionary chain. Retroviruses and luteoviruses are examples of viral groups in which large numbers of recombinations have occurred to produce new organisms.

Sometimes these produced genome splices occur naturally, using fragments that are either viral or cellular in nature. In some cases the product is more of a rearrangement of genomic parts—referred to as pseudo-recombination. The Western Equine Encephalovirus is a known example of this last category.

It is likely that viruses began host relationships with archaea and bacteria about two billion years ago; it has been suggested, however, that the proliferation of terrestrial vascular plants was the watershed event in evolution that enabled the explosion of numbers of viral organisms and pathways.

Within that hierarchy reside 82 families, genera, species. David Baltimore devised an earlier system based on the method of viral messenger RNA synthesis. Although viruses must replicate mRNAs from their genomes to produce proteins and reproduce, distinctly different mechanisms are employed within each viral family. These nuances divide viruses into seven Baltimore groups.

This Baltimore classification scheme is centered around the concept of messenger RNA replication, since viruses generate messenger RNA from their genomic coding to produce proteins and from that point replicate themselves.

The majority of viruses characteristically measure between 10 and nanometers nm , although certain filoviruses extend to a length of up to nm, with a diameter of approximately 80 nm.

A complete virus, known as a virion, consists of nucleic acid encased within an exterior protective coating of proteins termed a capsid—constructed from identical protein subunits called capsomers. Very few viruses cannot be observed with a basic light microscope, but scanning and transmission electron microscopes can be employed to observe the virion. To increase the contrast between viruses and the background, electron-dense staining is invoked; this technique involves solutions of heavy metal salts e.

When virions are coated with positive stain, fine detail is obscured, and negative stains of the background only are applied to complement the positive staining observations. There are four major distinct structural forms:. Helix: This group is characterized by single type of capsomer stacked around a core axis to form a helical structure, which may have a central cavity within the helix. This geometry results in rod-shaped or filamentous structures, which may be quite long, flexible, and filamentous or abbreviated and rigid.

Most often the core genetic macromolecule is single-stranded RNA bound inside the protein helix by polar interactions between the negatively charged nucleic acid and effective positive charge at the protein surface. Tobacco mosaic virus is a prominent example of a helical virus. Envelope: In some cases a cell membrane of the host is utilized for encasement of the virus; this may be either the external cell membrane or the nuclear membrane.

These membranes become the outer lipid bilayer known as a viral envelope. The membrane is studded with proteins coded by the viral genome and host genome; the lipid membrane and any carbohydrates present derive exclusively from the host. Influenza and HIV viruses use this strategy. Icosahedral: These are the main shapes occurring in viruses infecting animal hosts.

They have icosahedral or near-spherical geometries with icosahedral symmetry. A regular icosahedron is nature's optimum method of producing a closed shell from identical subunits. Twelve is the minimum number of identical capsomers required for this formation, each capsomer being comprised of five identical subunits. A number of viruses e. Capsomers at the apices are surrounded by five other capsomers and are called pentons. Capsomers on the triangular faces are surrounded by six other capsomers, and are termed hexons.

Complex structures: More complex viral structures may have a capsid that is neither purely helical, nor purely icosahedral, and that may possess such ancillary structures as protein tails or a complex outer wall. Some bacteriophages, including Enterobacteria phage T4, possess a complex structure of an icosahedral head bound to a helical tail that may have a hexagon-shaped base plate with protruding protein tail fibers. Such a tail performs as a molecular syringe, first attaching to the bacterial host, and then injecting the viral RNA or DNA into the host cell.

Replication The virus, totally dependent upon its host for reproduction, manifests six essential stages in its life cycle:.