General Characteristics of Viruses

Structure

Because most viruses are extremely well adapted to their host organism, virus structure varies greatly. However, there are some general structural characteristics that all viruses share.

Figure 1.1: General virus structure

All viruses have a capsid or head region that contains its genetic material.

The capsid is made of proteins and glycoproteins.

Capsid construction varies greatly among viruses, with most being specialized for a particular virus's host organism.

Some viruses, mostly of the type infecting animals, have a membranous envelope surrounding their capsid. This allows viruses to penetrate host cells through membrane fusion.

The virus's genetical material rests inside the capsid; that material can be either DNA, RNA.

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In addition to the head region, some viruses, mostly those that infect bacteria, have a tail region. The tail is an often elaborate protein structure. It aids in binding to the surface of the host cell and in the introduction of virus genetic material to the host cell.

Though the details of virus infection and replication vary greatly with host type, all viruses share 6 basic steps in their replication cycles. These are:

1) attachment - The virus must first attach itself to the host cell

2) penetration - Either of the whole virus or just the contents of the capsid.

3) uncoating - If the entire capsid enters, the genetic material must be uncoated to make

it available to the cell's replication machinery

4) replication - Replication of genetic material takes place, as well as the production of

capsid and tail proteins

5) assembly - Once all of the necessary parts have been replicated, individual virus

particles are assembled

6)release- Release often takes place in a destructive manner, bursting and killing the

host cell

Genetic Material

Viruses may carry DNA or RNA as their genetic material. DNA may be single- or double-stranded (ssDNA and dsDNA), and it may be circular or linear.

Virus "Life" Cycles

Some viruses have a slightly more complicated replication cycle involving lyticand lysogenicphases. The lytic phase is similar to that described above, with virus particles infecting and being replicated. In the lysogenic phase, however, viral genetic material that has entered the host cell becomes incorporated in the cell and lies dormant. It is passed on to the progeny of the infected cells. Eventually, the lytic phase will start again, and cells that were never infected themselves, but carry the viral genetic material will begin to produce new virus particles.

Lytic cycle

• In viruses, the cycle of viral infection, replication, and cell destruction is called the lytic cycle.
• After the viral genes have entered the cell, they use the host cell to replicate viral genes and to make viral proteins, such as capsids.
• The proteins are then assembled with the replicated viral genes to form complete viruses. The host cell is broken open and releases newly made viruses.

Lysogenic Cycle

• During an infection, some viruses stay inside the cells but instead of producing virus particles, the viral gene is inserted into the host chromosome and is called a provirus.
• Whenever the cell divides, the provirus also divides, resulting in two infected host cells.
• In this cycle, called the lysogenic cycle, the viral genome replicates without destroying the host cell.

Figure1.2: Generalized Replication of Viruses

Host Cell Specificity

• Viruses are often restricted to certain kinds of cells.
• Viruses may have originated when fragments of host genes escaped or were expelled from cells.
• The hypothesis that viruses originated from a variety of host cells may explain why there are so many different kinds of viruses. Biologists think there are at least as many kinds of viruses as there are kinds of organisms.

Structure of HIV—an Enveloped Virus

• The human immunodeficiency virus (HIV) causes acquired immune deficiency syndrome (AIDS).
• Within HIV’s envelope lies the capsid, which in turn encloses the virus’s genetic material.
• In the case of HIV, the genetic material is composed of two molecules of single-stranded RNA.

Attachment

• Studding the surface of each HIV are spikes composed of a glycoprotein.
• Once inside a cell, the HIV particle sheds its capsid. The particle then releases an enzyme called reverse transcriptase.
• Reverse transcriptase copies the naked viral RNA into a complementary DNA version.
• Translation of the viral DNA by the host cell’s machinery directs the production of many copies of the virus.
• For years after the initial infection, HIV continues to replicate (and mutate). Eventually and by chance, HIV’s surface glycoproteins change to the point that they now recognize a new cell surface receptor. This receptor is found on the subset of lymphocytes called T cells.
• Unlike its activity in macrophages, HIV reproduces in T cells and then destroys them.
• It is this destruction of the body’s T cells that blocks the body’s immune response and signals the onset of AIDS.

• Perhaps the most lethal virus in human history is the influenza virus.
• Certain viruses can also cause some types of cancer.
• Viruses associated with human cancers include hepatitis B (liver cancer), Epstein-Barr virus (Burkitt’s lymphoma), and human papilloma virus (cervical cancer).
• Viruses that evolve in geographically isolated areas and are pathogenic to humans are called emerging viruses.
• These new pathogens are dangerous to public health. People become infected when they have contact with the normal hosts of these viruses.
• Examples of emerging viruses include West Nile virus and hantavirus.

• Prions are composed of proteins but have no nucleic acid. A disease-causing prion is folded into a shape that does not allow the prion to function..
• A viroid is a single strand of RNA that has no capsid. Viroids are important infectious disease agents in plants.