Virology 1 PDF - General Properties of Viruses

lecture one virology Dr.Maryam General Properties of Viruses Introduction: Viruses are the smallest infectious agents (ranging from about 20 to 300 nm in diameter) and contain only one kind of nucleic acid (RNA or DNA) as th...

lecture one virology Dr.Maryam General Properties of Viruses Introduction: Viruses are the smallest infectious agents (ranging from about 20 to 300 nm in diameter) and contain only one kind of nucleic acid (RNA or DNA) as their genome. The nucleic acid is - encased in a protein shell, which may be surrounded by a lipid-containing membrane. The entire infectious unit is termed a virion. The other infectious agents namely, bacteria, fungi, protozoa, and worms, are either single cells or composed of many cells. Cells are capable of independent replication, can synthesize their own energy and proteins, and can be seen in the light microscope. In contrast, viruses are not cells; they are not capable of independent replication, can synthesize neither their own energy nor their own proteins, and are too small to be seen in the light microscope. Viruses are characterized by the following features: & Viruses are particles composed of an internal core containing either DNA or RNA (but not & both) covered by a protective protein coat. Some viruses have an outer lipoprotein & membrane, called an envelope, external to the coat. Viruses do not have a nucleus, cytoplasm, mitochondria, or ribosomes. The cells, both prokaryotic and eukaryotic cells, have both DNA and RNA. Eukaryotic cells, such as fungal, protozoal, and human cells, have a nucleus, cytoplasm, mitochondria, and ribosomes. Prokaryotic cells, such as bacteria, are not divided into nucleus and cytoplasm and do not have mitochondria but do have ribosomes; therefore, they can synthesize their own proteins. 1. Viruses must reproduce (replicate) within cells, because they cannot generate energy or synthesize proteins. Because they can reproduce only within cells, viruses are obligate intracellular parasites. (The only bacteria that are obligate intracellular parasites are chlamydiae and rickettsiae. They cannot synthesize sufficient energy to replicate independently.) 2. Viruses replicate in a manner different from that of cells, i.e., viruses do not undergo binary fission or mitosis. One virus can replicate to produce hundreds of progeny viruses, whereas one cell divides to produce only two daughter cells. 3. Table –1 compares some of the attributes of viruses and cells. lecture one virology Dr.Maryam Table –1 Comparison of Viruses and Cells Property Viruses Cells Type of nucleic acid DNA or RNA but not both DNA and RNA Proteins Few Many Lipoprotein membrane Envelope present in some viruses Cell membrane present in all cells Ribosomes Absent1 Present Mitochondria Absent Present in eukaryotic cells Enzymes None or few Many Multiplication by binary fission or mitosis No Yes 1 Arenaviruses have a few nonfunctional ribosomes. Shape of virus Viruses range from 20 to 300 nm in diameter; this corresponds roughly to a range of sizes > from that of the largest protein - to that of the smallest cell. Their shapes are frequently - - referred to in colloquial terms, e.g., spheres, rods, bullets, or bricks, but in reality they are complex structures of precise geometric symmetry. The shape of virus particles is determined by the arrangement of the repeating subunits that form the protein coat (capsid) of the virus. Virus Size & Structure Viruses range in size from that of large proteins (~20 nm) to that of the smallest cells (~300 nm). Most viruses appear as spheres or rods in the electron microscope. -  Viruses contain either DNA or RNA, but not both.  All viruses have a protein coat called a capsid that- covers the genome. The capsid is composed of repeating subunits called capsomers. Each capsomer, consisting of one or several proteins, can be seen in the electron microscope as a spherical particle, - sometimes with a central hole.  In some viruses, the capsid is the outer surface, but in other viruses the capsid is covered with a lipoprotein envelope that becomes the outer surface. The structure composed of the nucleic acid genome and the capsid proteins is called the nucleocapsid.  The repeating subunits of the capsid give the virus a symmetric appearance that is useful for classification purposes. Some viral nucleocapsids have spherical (icosahedral) symmetry, whereas others have helical symmetry.  All human viruses that have a helical nucleocapsid are enveloped, i.e., there are no naked helical viruses that infect humans. Viruses that have an icosahedral nucleocapsid can be either enveloped or naked. lecture one virology O Dr.Maryam  Function of capsid: One the function of the outer shells of a virion is to protect the fragile nucleic acid genome from Nucleases))physical, chemical, or enzymatic O damage.The outer surface of the virus is also responsible for recognition of & the first interaction with the host cell Initially, this takes the form of binding of a specific virus- attachment protein to a cellular receptor molecule.The capsid also has a role to play in 8 initiating infection by delivering the genome in a form in which it can interact with the host cell. Viral Nucleic Acids  The genome of some viruses is DNA, whereas the genome of others is RNA. These DNA and RNA genomes can be either single-stranded or double-stranded.  Some RNA viruses, such as influenza virus and rotavirus, have a segmented genome, i.e., the genome is in several pieces.  lecture one virology Dr.Maryam  All viruses have one copy of their genome (haploid) except retroviruses, which have two copies (diploid). Viral Proteins Viral proteins serve several important functions.. 1. The outer capsid proteins protect the genetic material and mediate the attachment of the virus to specific receptors on the host cell surface. This interaction of the viral proteins with the cell receptor is the major determinant of species and organ specificity. Outer viral proteins are also important antigens that induce neutralizing antibody and activate cytotoxic T cells to kill virus-infected cells. These outer viral proteins not only induce antibodies but are also the target of antibodies, i.e., antibodies bind to these viral proteins and prevent ("neutralize") the virus from entering the cell and replicating. Some viruses produce antigenic variants of their surface proteins that allow the viruses to evade our host defenses. Antibody against one antigenic variant (serotype) will not neutralize a different serotype. Some viruses have one serotype; others have multiple serotypes. - 2. Some of the internal viral proteins are structural (e.g., the capsid proteins of the enveloped -m - viruses), whereas others are enzymes (e.g., the polymerases that synthesize the viral mRNA). The internal viral proteins vary depending on the virus. Some viruses have a DNA or RNA polymerase attached to the genome; others do not. 3. Some viruses produce proteins that act as "superantigens" similar in their action to the superantigens produced by bacteria, such as the toxic shock syndrome toxin of &Staphylococcus aureus. Viruses known to produce superantigens include two& members of the herpesvirus family, namely, Epstein-Barr virus and cytomegalovirus, and the retrovirus - mouse mammary tumor virus. 4. Some viruses contain regulatory proteins in the virion in a structure called the tegument, - which is located between the nucleocapsid and the envelope. These regulatory proteins include transcription and translation factors that control either viral or cellular processes. Members of the herpesvirus family, such as herpes simplex virus and cytomegalovirus, have a prominent, well-characterized tegument. 5. In addition to the capsid and internal proteins, there are two other types of proteins, both of which are associated with the envelope. The envelope is a lipoprotein membrane composed of lipid derived from the host cell membrane and protein that is virus-specific. Another protein, the matrix protein, mediates the interaction between the capsid proteins and the envelope. lecture one virology Dr.Maryam Viral Envelope  The viral envelope consists of a membrane that contains lipid derived from the host cell and proteins encoded by the virus. Typically, the envelope is acquired as the virus exits from the cell in a process called budding.  Viruses with an envelope are less stable, i.e., they are more easily inactivated, than naked viruses (those without an envelope). In general, enveloped viruses are transmitted by direct contact via blood and body fluids, whereas naked viruses can survive longer in the environment and can be transmitted by indirect means such as the fecal–oral route. Properties of naked capsid viruses Capsid is resistant to – Drying – Heat – Detergents – Acids – Proteases Consequences – Can survive in the gastrointestinal tract – Retain infectivity on drying – Survive well on environmental surfaces – Spread easily via fomitesjgs] – Must kill host cells for release of mature virus particles – Humoral antibody response may be sufficient to neutralize Infection Properties of enveloped viruses Envelope is sensitive to – Drying – Heat – Detergents – Acid lecture one virology Dr.Maryam Consequences – Must stay wet during transmission – Transmission in large droplets and secretions – Cannot survive in the gastrointestinal tract – Do not need to kill cells in order to spread – May require both a humoral and a cellular immune response Atypical Virus-like Agents There are four exceptions to the typical virus as described above: 1. Defective viruses are composed of viral nucleic acid and proteins but cannot replicate without a "helper" virus, which provides the missing function. Defective viruses usually have a mutation or a deletion of part of their genetic material. During the growth of most human viruses, many more defective than infectious virus particles are produced. 2. Pseudovirions contain host cell DNA instead of viral DNA within the capsid. They are formed during infection with certain viruses when the host cell DNA is fragmented and pieces of it are incorporated within the capsid protein. Pseudovirions can infect cells, but they do not replicate. 3. Viroids consist solely of a single molecule of circular RNA without a protein coat or envelope. There is extensive homology between bases in the viroid RNA, leading to large double-stranded regions. They cause several plant diseases but are not implicated in any human disease. > 4. Prions are infectious particles that are composed solely of protein, i.e., they contain no detectable nucleic acid. They are implicated as the cause of certain "slow" diseases called transmissible spongiform encephalopathies, which include such diseases as Creutzfeldt-Jakob disease in humans and scrapie in sheep. Prions are much more resistant to inactivation by ultraviolet light and heat than are viruses. They are remarkably resistant to formaldehyde and nucleases. Table –2 Comparison of Prions and Conventional Viruses Feature Prions Conventional Viruses Particle contains nucleic acid No Yes Particle contains protein Yes, encoded by cellular genes Yes, encoded by viral genes Inactivated rapidly by UV light or heat No Yes Appearance in electron microscope Filamentous rods (amyloid-like) Icosahedral or helical symmetry Infection induces antibody No Yes Infection induces inflammation No Yes lecture one virology Dr.Maryam EVOLUTIONARY ORIGIN OF The origin of viruses is not known.Two theories of viral origin can be summarized as follows: 1. Viruses may be derived from DNA or RNA nucleic acid components of host cells that became able to replicate autonomously and evolve independently. They resemble genes that have acquired the capacity to exist independently of the cell. Some viral sequences are related to portions of cellular genes encoding protein functional domains. It seems likely that at least some viruses evolved in this fashion. 2. Viruses may be degenerate forms of intracellular parasites. There is no evidence that viruses evolved from bacteria, although other obligately intracellular organisms (eg, rickettsiae and chlamydiae) presumably did so. However, poxviruses are so large and complex that they might represent evolutionary products of some cellular ancestor