Here, we will discuss the building blocks of viruses that help them do this job. A simple sketch of a virus: nucleic acid genome, surrounded by a protein coat capsid , additionally surrounded by a membrane envelope. There are all sorts of virus shapes and sizes. However, all virus particles have a protein coat that surrounds and protects a nucleic acid genome. This protein coat is called a capsid , and the instructions for making the protein subunits of the capsid are encoded in the nucleic acid genome of the virus.
The structure resulting from the combination of the capsid and the nucleic acid genome is called the nucleocapsid. A single, fully assembled, infectious virus particle is called a virion. 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.
There are millions of viruses. Why do some cause illness in humans and other animals? There are almost as many different answers to that as there are viruses. But I can give you a general evolutionary explanation. Over long periods of evolution, you have a virus that comes into a host—a species—and spreads.
One of three things can then happen. One is that the virus can just die out for some reason—everybody can get immune, so the virus can no longer find a host, and it dies out. The second is that the host can die out.
Some viruses that we have around with us—like the common cold virus—are like that. There are many viruses like that, such as HIV. This coronavirus probably is, too. One example is cytomegalovirus, CMV. Many of us are infected with CMV, without consequence.
Somebody can have normal vision on Friday and be totally, irreversibly blind on Monday. It varies greatly. You need really close intimate contact, blood transfusion or sexual contact. However, other scientists suggest that because viruses are made up of the same building blocks of life, DNA and RNA, they verge on life.
Viruses come in many different shapes and sizes, but all are made of two essential components: a core of genetic material, either DNA or RNA, which is surrounded by a protective protein coat called a capsid. Packaged together, a single virion comes in four different shapes: helical, polyhedral, spherical, and complex. Viruses like influenza spread so effectively, and as a result can be so deadly, because of their ability to spontaneously self- assemble in large numbers.
Researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences have engineered a new way to observe and track viruses as they assemble. This new method may spot weaknesses in viruses that drug makers can exploit to design drugs that prevent viruses from assembling in the first place. Virus are extremely tiny, even tinier than bacteria, so how do scientists study them? Hannah Gavin, a microbiologist at Harvard, shows us how she does just that in this video.
Though viruses are most often associated with illness, they can be a friend as well as a foe. Scientists have learned to manipulate their unique characteristics as a scientific tool. Bacteriophages are types of viruses that attack bacteria by attaching to bacterial cells and injecting their genetic material into them. Turn recording back on. National Center for Biotechnology Information , U. Show details Baron S, editor. Search term. General Concepts Structure and Function Viruses are small obligate intracellular parasites, which by definition contain either a RNA or DNA genome surrounded by a protective, virus-coded protein coat.
Classification of Viruses Morphology: Viruses are grouped on the basis of size and shape, chemical composition and structure of the genome, and mode of replication. Nomenclature Aside from physical data, genome structure and mode of replication are criteria applied in the classification and nomenclature of viruses, including the chemical composition and configuration of the nucleic acid, whether the genome is monopartite or multipartite.
Structure and Function Viruses are inert outside the host cell. Classification of Viruses Viruses are classified on the basis of morphology, chemical composition, and mode of replication. Morphology Helical Symmetry In the replication of viruses with helical symmetry, identical protein subunits protomers self-assemble into a helical array surrounding the nucleic acid, which follows a similar spiral path.
Figure The helical structure of the rigid tobacco mosaic virus rod. Figure Fragments of flexible helical nucleocapsids NC of Sendai virus, a paramyxovirus, are seen either within the protective envelope E or free, after rupture of the envelope.
Icosahedral Symmetry An icosahedron is a polyhedron having 20 equilateral triangular faces and 12 vertices Fig. Figure Icosahedral models seen, left to right, on fivefold, threefold, and twofold axes of rotational symmetry. Figure Adenovirus after negative stain electron microscopy. Virus Core Structure Except in helical nucleocapsids, little is known about the packaging or organization of the viral genome within the core.
Figure Two-dimensional diagram of HIV-1 correlating immuno- electron microscopic findings with the recent nomenclature for the structural components in a 2-letter code and with the molecular weights of the virus structural glyco- proteins. Figure Schemes of 21 virus families infecting humans showing a number of distinctive criteria: presence of an envelope or double- capsid and internal nucleic acid genome.
Virus Classification On the basis of shared properties viruses are grouped at different hierarchical levels of order, family, subfamily, genus and species. JB Lippincott, Philadelphia, Gajdusek DC. Unconventional viruses and the origin and disappearance of kuru.
Gelderblom HR. Assembly and morphology of HIV: potential effect of structure on viral function. Mattern CFT: Symmetry in virus architecture. Marcel Dekker, New York, Raven Press, New York, Springer-Verlag, New York, Elsevier, Amsterdam, Structure and Classification of Viruses. In: Baron S, editor. Chapter In this Page.
Related information. PubMed Links to PubMed. Similar articles in PubMed. Review Retroviral Virions and Genomes [Retroviruses. Review Molecular mechanisms of virus spread and virion components as tools of virulence. A review. Acta Microbiol Immunol Hung. Review First glimpses at structure-function relationships of the nucleocapsid protein of retroviruses. J Mol Biol. Packaging of a unit-length viral genome: the role of nucleotides and the gpD decoration protein in stable nucleocapsid assembly in bacteriophage lambda.
Epub Sep 3. Review [The great virus comeback].
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