Viral structure م.م رنا مشعل
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.) 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.
General characteristics of viruses 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. 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.
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 Absent 1 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.
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
Viral Nucleic Acids The viral nucleic acid (genome) is located internally and can be either Single- or double-stranded DNA or Single- or double-stranded RNA The nucleic acid can be either linear or circular. The DNA is always a single molecule; the RNA can exist either as a single molecule or in several pieces. For example, both influenza virus have a segmented RNA genome. Viral Capsid & Symmetry The nucleic acid is surrounded by a protein coat called a capsid, made up of subunits called capsomers. The structure composed of the nucleic acid genome and the capsid proteins is called the nucleocapsid. The arrangement of capsomers gives the virus structure its geometric symmetry.
Viral Proteins Functions of viral proteins The outer capsid proteins protect the genetic material and mediate the attachment of the virus to specific receptors on the host cell surface. Outer viral proteins are also important antigens that induce neutralizing antibody and activate T cells to kill virus-infected cells. Induce immune responses following both the natural infection and immunization. If a virus has an envelope, then a matrix protein that mediates the interaction between the capsid proteins and the envelope proteins is present.
Viral Envelope 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 or nuclear membrane. and protein that is virus-specific. The viral envelope is acquired as the virus exits from the cell in a process called "budding". there are frequently glycoproteins in the form of spike like projections on the surface, which attach to host cell receptors during the entry of the virus into the cell. Enveloped viruses are more sensitive to heat, drying, detergents, and lipid solvents such as alcohol and ether than are nonenveloped (nucleocapsid) viruses, which are composed only of nucleic acid and capsid proteins. An interesting clinical correlate of this observation is that virtually all viruses that are transmitted by the fecal oral route (those that have to survive in the environment) do not have an envelope, that is, they are naked nucleocapsid viruses., These include viruses such as hepatitis A virus, poliovirus
Steps in Viral Replication A. Attachment. This is the first step in viral replication. Surface proteins of the virus interact with specific receptors on the target cell surface. These may be specialized proteins with limited distribution or molecules that are more widely distributed on tissues throughout the body. The presence of a virus-specific receptor is necessary but not sufficient for viruses to infect cells and complete the replicative cycle. B. Penetration. Enveloped viruses (e.g., HIV, influenza virus) penetrate cells through fusion of the viral envelope with the host cell membrane. Non-enveloped viruses penetrate cells by translocation of the virion across the host cell membrane or receptor mediated endocytosis of the virion in clathrin coated pits with accumulation of viruses in cytoplasmic vesicles. C. Uncoating (disassembly). A complex process which differs bytaxonomic class and is not fully understood for many agents. Thisprocess makes the nucleic acid available for transcription to permitmultiplication of the virus. D. Transcription and Translation. The key to understanding thegenomic expression of viruses is noting the fact that viruses must use host cellular machinery to replicate and make functional and structuralproteins. Strategies for genomic expression for different taxonomicgroupings of viruses are described below (section II). F. Assembly and Release. The process of virion assembly involvesbringing together newly formed viral nucleic acid and the structuralproteins to form the nucleocapsid of the virus.
Morphologic and Structural Effects 1- The cytopathic effect, or necrosis of cells in the tissue culture. 2- The inhibition of cellular metabolism, or failure of virus-infected cells to produce acid). 3 -Genotoxic Effects (Chromosome damage may be caused directly by the virus particle or indirectly by events occurring during synthesis of new viral macromolecules (RNA, DNA, pro tein).
Prions Prions (Proteinaceous infectious particle):-it is a protein molecule that is misfolded and can cause misfolding of normal proteins-results in spongiform encephalopathiese.g. Mad Cow disease, Sheep scrapie -Jacob Disease -prion protein in the brain convertsnormal proteins PrPC into prion proteins PrPSc-prion proteins cause plaques and holesin neural tissue resulting inprogressive loss of function and eventual death
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 (amyloidlike) Icosahedral or helical symmetry Infection induces antibody No Yes Infection induces inflammation No Yes