Virus and type of virus

Together, the nucleic acid and the capsid form the nucleocapsid of the virion Fig. Viral capsid proteins protect the fragile genome, composed of nucleic acid, from the harsh environment. The capsid and nucleic acid together are known as the nucleocapsid. Remember that the genomes of most viruses are very small. Genes encode the instructions to make proteins, so small genomes cannot encode many proteins. It is for this reason that the capsid of the virion is composed of one or only a few proteins that repeat over and over again to form the structure.

The nucleic acid of the virus would be physically too large to fit inside the capsid if it were composed of more than just a few proteins. In the same way that a roll of magnets will spontaneously assemble together, capsid proteins also exhibit self-assembly. The first to show this were H. Fraenkel-Conrat and Robley Williams in They separated the RNA genome from the protein subunits of tobacco mosaic virus, and when they put them back together in a test tube, infectious virions formed automatically.

This indicated that no additional information is necessary to assemble a virus: the physical components will assemble spontaneously, primarily held together by electrostatic and hydrophobic forces. Most viruses also have an envelope surrounding the capsid. Virus capsids are held together by some of the same bonds that are found in living organisms.

Rarely are covalent bonds found in capsids; these are the strongest of bonds that are formed when atoms share electrons with each other. Hydrogen bonds are also weak electrostatic forces that occur between slightly charged atoms, usually between hydrogen slightly positively charged and another atom that is partially negatively charged, such as oxygen.

Van der Waals forces are weak interactions that occur when an atom becomes slightly charged due to random asymmetry of its electrons. The properties of water also contribute to virus assembly and attachment to cells. Water is a polar molecule, meaning that the molecule has two distinct ends, much like a battery or magnet has a positive and a negative end. Molecules that do not have distinct ends are termed nonpolar. Other polar molecules are attracted to water, since water is polar too.

This explains the phenomenon of oil nonpolar not mixing with water polar. These viruses often have proteins, called matrix proteins , that function to connect the envelope to the capsid inside. A virus that lacks an envelope is known as a nonenveloped or naked virus Fig. Each virus also possesses a virus attachment protein embedded in its outer-most layer.

This will be found in the capsid, in the case of a naked virus, or the envelope, in the case of an enveloped virus. The virus attachment protein is the viral protein that facilitates the docking of the virus to the plasma membrane of the host cell, the first step in gaining entry into a cell. The capsid of an enveloped virion is wrapped with a lipid membrane derived from the cell.

Virus attachment proteins located in the capsid or envelope facilitate binding of the virus to its host cell. Each virus possesses a protein capsid to protect its nucleic acid genome from the harsh environment.

Virus capsids predominantly come in two shapes: helical and icosahedral. The helix plural: helices is a spiral shape that curves cylindrically around an axis. It is also a common biological structure: many proteins have sections that have a helical shape, and DNA is a double-helix of nucleotides. In the case of a helical virus, the viral nucleic acid coils into a helical shape and the capsid proteins wind around the inside or outside of the nucleic acid, forming a long tube or rod-like structure Fig.

The nucleic acid and capsid constitute the nucleocapsid. In fact, the protein that winds around the nucleic acid is often called the nucleocapsid protein. Once in the cell, the helical nucleocapsid uncoils and the nucleic acid becomes accessible. A Viral capsid proteins wind around the nucleic acid, forming a helical nucleocapsid. B Helical structure of tobacco mosaic virus. Graph , 12, —44 using a 2xea PDB assembly J.

There are several perceived advantages to forming a helical capsid. First, only one type of capsid protein is required. This protein subunit is repeated over and over again to form the capsid. This structure is simple and requires less free energy to assemble than a capsid composed of multiple proteins. In addition, having only one nucleocapsid protein means that only one gene is required instead of several, thereby reducing the length of nucleic acid required.

Because the helical structure can continue indefinitely, there are also no constraints on how much nucleic acid can be packaged into the virion: the capsid length will be the size of the coiled nucleic acid.

Helical viruses can be enveloped or naked. The first virus described, tobacco mosaic virus, is a naked helical virus. In fact, most plant viruses are helical, and it is very uncommon that a helical plant virus is enveloped. In contrast, all helical animal viruses are enveloped. These include well-known viruses such as influenza virus, measles virus, mumps virus, rabies virus, and Ebola virus Fig. A Vesicular stomatitis virus forms bullet-shaped helical nucleocapsids.

Fred A. B Tobacco mosaic virus forms long helical tubes. C The helical Ebola virus forms long threads that can extend over nm in length.

Of the two major capsid structures, the icosahedron is by far more prevalent than the helical architecture. In comparison to a helical virus where the capsid proteins wind around the nucleic acid, the genomes of icosahedral viruses are packaged completely within an icosahedral capsid that acts as a protein shell.

Initially these viruses were thought to be spherical, but advances in electron microscopy and X-ray crystallography revealed these were actually icosahedral in structure. An icosahedron is a geometric shape with 20 sides or faces , each composed of an equilateral triangle. An icosahedron has what is referred to as 2—3—5 symmetry , which is used to describe the possible ways that an icosahedron can rotate around an axis.

If you hold an icosahedral die in your hand, you will notice there are different ways of rotating it Fig. A helix is mathematically defined by two parameters, the amplitude and the pitch, that are also applied to helical capsid structures.

The amplitude is simply the diameter of the helix and tells us the width of the capsid. The pitch is the height or distance of one complete turn of the helix. In the same way that we can determine the height of a one-story staircase by adding up the height of the stairs, we can figure out the pitch of the helix by determining the rise , or distance gained by each capsid subunit.

A staircase with 20 stairs that are each 6 inches tall results in a staircase of 10 feet in height; a virus with This is the architecture of tobacco mosaic virus. Your pencil would be right in the middle of a triangle facing up and a triangle facing down. If you rotate the icosahedron clockwise, you will find that in degrees you encounter the same arrangement symmetry : a triangle facing up and a triangle facing down.

Continuing to rotate the icosahedron brings you back to where you began. This is known as the twofold axis of symmetry, because as you rotate the shape along this axis your pencil , you encounter your starting structure twice in one revolution: once when you begin, and again when rotated degrees. On the other hand, if you put your pencil axis directly through the center one of the small triangle faces of the icosahedron, you will encounter the initial view two additional times as you rotate the shape, for a total of three times.

This is the threefold axis. Similarly, if your pencil axis goes through a vertex or tip of the icosahedron, you will find symmetry five times in one rotation, forming the fivefold axis. It is for this reason that an icosahedron is known to have 2—3—5 symmetry, because it has twofold, threefold, and fivefold axes of symmetry.

This terminology is useful when dealing with an icosahedral virus because it can be used to indicate specific locations on the virus or where the virion has interactions with the cell surface. For instance, if a virus interacts with a cell surface receptor at the threefold axis, then you know this interaction occurs at one of the faces of the icosahedron.

A protein protruding from the capsid at the fivefold axis will be found at one of the vertices tips of the icosahedron. All of the illustrations of viruses in Fig. How many twofold axes of symmetry are found in one icosahedron? How about the number of threefold or fivefold axes? How many faces, edges, and vertices are found in an icosahedron? A Icosahedron faces fuchsia triangles , edges red rectangles , and vertices violet pentagons are indicated on the white icosahedron.

B The twofold axis of symmetry occurs when the axis is placed through the center of an edge. The threefold axis occurs when the axis is placed in the center of a face C , and the fivefold axis passes through a vertex of the icosahedron D.

Viral proteins form each face small triangle of the icosahedral capsid. Viral proteins are not triangular, however, and so one protein subunit alone is not sufficient to form the entire face. Therefore, a face is formed from at least three viral protein subunits fitted together Fig. These can all be the same protein, or they can be three different proteins. The subunits together form what is called the structural unit.

The structural unit repeats to form the capsid of the virion. A Virus capsids are composed of viral protein subunits that form structural units. Related Articles. Computer Network Fundamentals.

Physical layer. Data Link layer. Network layer. Transport layer. Application layer. Network Security. Computer Network Quizes. Table of Contents. Improve Article. Save Article. Like Article. A virus is a fragment of code embedded in a legitimate program. Viruses are self-replicating and are designed to infect other programs. They can wreak havoc in a system by modifying or destroying files causing system crashes and program malfunctions. On reaching the target machine a virus dropper usually a trojan horse inserts the virus into the system.

For more details, refer to this. Various types of viruses: File Virus: This type of virus infects the system by appending itself to the end of a file.

It changes the start of a program so that the control jumps to its code. After the execution of its code, the control returns back to the main program.

The arrangement of capsomeres into an icosahedral shell compare Fig. This requires the identification of the nearest pair of vertex capsomeres called penton: those through which the fivefold symmetry axes pass and the distribution of capsomeres between them. Adenovirus after negative stain electron microscopy. A The capsid reveals the typical isometric shell made up from 20 equilateral triangular faces. The net axes are formed by lines of the closest-packed neighboring capsomeres.

In adenoviruses, the h and k axes also coincide with the edges of the triangular faces. This symmetry and number of capsomeres is typical of all members of the adenovirus family. Except in helical nucleocapsids, little is known about the packaging or organization of the viral genome within the core. Small virions are simple nucleocapsids containing 1 to 2 protein species.

The larger viruses contain in a core the nucleic acid genome complexed with basic protein s and protected by a single- or double layered capsid consisting of more than one species of protein or by an envelope Fig.

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. SU stands for more Because of the error rate of the enzymes involved in RNA replication, these viruses usually show much higher mutation rates than do the DNA viruses.

Mutation rates of 10 -4 lead to the continuous generation of virus variants which show great adaptability to new hosts. The viral RNA may be single-stranded ss or double-stranded ds , and the genome may occupy a single RNA segment or be distributed on two or more separate segments segmented genomes. In addition, the RNA strand of a single-stranded genome may be either a sense strand plus strand , which can function as messenger RNA mRNA , or an antisense strand minus strand , which is complementary to the sense strand and cannot function as mRNA protein translation see Ch.

Sense viral RNA alone can replicate if injected into cells, since it can function as mRNA and initiate translation of virus-encoded proteins. Antisense RNA, on the other hand, has no translational function and cannot per se produce viral components. 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.

DsRNA viruses, e. Each segment consists of a complementary sense and antisense strand that is hydrogen bonded into a linear ds molecule. The replication of these viruses is complex; only the sense RNA strands are released from the infecting virion to initiate replication. The retrovirus genome comprises two identical, plus-sense ssRNA molecules, each monomer 7—11 kb in size, that are noncovalently linked over a short terminal region.

Retroviruses contain 2 envelope proteins encoded by the env-gene, 4—6 nonglycosylated core proteins and 3 non-structural functional proteins reverse transcriptase, integrase, protease: RT, IN, PR specified by the gag-gene Fig. This DNA, mediated by the viral integrase, becomes covalently bonded into the DNA of the host cell to make possible the subsequent transcription of the sense strands that eventually give rise to retrovirus progeny.

After assembly and budding, retroviruses show structural and functional maturation. In immature virions the structural proteins of the core are present as a large precursor protein shell. After proteolytic processing by the viral protease the proteins of the mature virion are rearranged and form the dense isometric or cone-shaped core typical of the mature virion, and the particle becomes infectious. Most DNA viruses Fig. The papovaviruses, comprising the polyoma- and papillomaviruses, however, have circular DNA genomes, about 5.

Three or 2 structural proteins make up the papovavirus capsid: in addition, nonstructural proteins are encoded that are functional in virus transcription, DNA replication and cell transformation. Single-stranded linear DNA, 4—6 kb in size, is found with the members of the Parvovirus family that comprises the parvo-, the erythro- and the dependoviruses.

The virion contains 2—4 structural protein species which are differently derived from the same gene product see Ch. The adeno-associated virus AAV, a dependovirus is incapable of producing progeny virions except in the presence of helper viruses adenovirus or herpesvirus. It is therefore said to be replication defective.

Circular single-stranded DNA of only 1. The isometric capsid measures 17 nm and is composed of 2 protein species only. On the basis of shared properties viruses are grouped at different hierarchical levels of order, family, subfamily, genus and species. More than 30, different virus isolates are known today and grouped in more than 3, species, in genera and 71 families. Viral morphology provides the basis for grouping viruses into families.

A virus family may consist of members that replicate only in vertebrates, only in invertebrates, only in plants, or only in bacteria. Certain families contain viruses that replicate in more than one of these hosts. This section concerns only the 21 families and genera of medical importance. Besides physical properties, several factors pertaining to the mode of replication play a role in classification: the configuration of the nucleic acid ss or ds, linear or circular , whether the genome consists of one molecule of nucleic acid or is segmented, and whether the strand of ss RNA is sense or antisense.

Also considered in classification is the site of viral capsid assembly and, in enveloped viruses, the site of nucleocapsid envelopment. Table lists the major chemical and morphologic properties of the families of viruses that cause disease in humans. The use of Latinized names ending in -viridae for virus families and ending in -virus for viral genera has gained wide acceptance.

The names of subfamilies end in -virinae. Vernacular names continue to be used to describe the viruses within a genus. In this text, Latinized endings for families and subfamilies usually are not used.

Table shows the current classification of medically significant viruses. In the early days of virology, viruses were named according to common pathogenic properties, e. From the early s until the mids, when many new viruses were being discovered, it was popular to compose virus names by using sigla abbreviations derived from a few or initial letters.

Thus the name Picornaviridae is derived from pico small and RNA; the name Reoviridae is derived from respiratory, enteric, and orphan viruses because the agents were found in both respiratory and enteric specimens and were not related to other classified viruses; Papovaviridae is from papilloma, polyoma, and vacuolating agent simian virus 40 [SV40] ; retrovirus is from reverse transcriptase; Hepadnaviridae is from the replication of the virus in hepatocytes and their DNA genomes, as seen in hepatitis B virus.

Hepatitis A virus is classified now in the family Picornaviridae, genus Hepatovirus. Although the current rules for nomenclature do not prohibit the introduction of new sigla, they require that the siglum be meaningful to workers in the field and be recognized by international study groups.

Several viruses of medical importance still remain unclassified. Some are difficult or impossible to propagate in standard laboratory host systems and thus cannot be obtained in sufficient quantity to permit more precise characterization.

Hepatitis E virus, the Norwalk virus and similar agents see Ch. The fatal transmissible dementias in humans and other animals scrapie in sheep and goat; bovine spongiform encephalopathy in cattle, transmissible mink encephalopathy; Kuru, Creutzfeldt-Jakob disease, and Gerstmann-Straussler-Scheinker syndrome in humans see Ch.