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Viruses are microscopic particles that infect cells and rely on host cells to replicate. They contain genetic material in the form of DNA or RNA and have a protein coat. Viruses infect both eukaryotic and prokaryotic cells and differ from other organisms in their structure, biology, and reproduction. Viruses are classified into groups based on their structure and method of replication, with the Baltimore system categorizing viruses into 7 groups depending on their nucleic acid and method of replication.

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Viruses. Structure and classification. Features of Coronavirus. Reproduction of viruses. Bacteriophages
• Viruses are microscopic particles that infect cells of other organisms. They differ from other microorganisms in their structure, biology, and reproduction. Viruses do carry conventional genetic material in the form of DNA or RNA, but they cannot reproduce on their own because they lack the biochemical machinery necessary for replication. Viruses therefore are obligate intracellular parasites capable of infecting both eukaryotic and prokaryotic organisms. A virus that infects bacteria is referred to as a bacteriophage or simply phage. For a virus to reproduce, the individual components of the virus must be synthesized by the host cell and then assembled within that cell. In other words, a virus uses the biochemical machinery of its host cell in order to replicate.
- Viruses have a host range. That is, viruses infect specific cells or tissues of specific hosts, or specific bacteria, or specific plants. - Viral specificity refers to the specific kinds of cells a virus can infect. It is regulated by the specificities of attachment, penetration and replication of the virus
Baltimore system of Classifications of Viruses I. Double-stranded DNA viruses • Some replicate in the nucleus e.g adenoviruses using cellular proteins. Poxviruses replicate in the cytoplasm and make their own enzymes for nucleic acid replication. e.g. Adenoviruses; Herpesviruses; Poxviruses, etc II. Single-stranded (+) sense DNA viruses • Replication occurs in the nucleus, involving the formation of a (-) sense strand, which serves as a template for (+) strand RNA and DNA synthesis. e.g., Parvoviruses
III. Double-stranded RNA viruses • These viruses have segmented genomes. Each genome segment is transcribed separately to produce monocistronic mRNAs. e.g., Reoviruses IV. Single-stranded (+) sense RNA viruses (Picornaviruses; Togaviruses, etc) • Polycistronic mRNA: Genome RNA = mRNA. Since the RNA is the same sense as mRNA, the RNA alone is infectious, no virion particle associated polymerase. Translation results in the formation of a polyprotein product, which is subsequently cleaved to form the mature proteins. e.g. Picornaviruses (poliovirus, rhinovirus); Hepatitis A virus • Complex Transcription: Two or more rounds of translation are necessary to produce the genomic RNA. e.g. Picornaviruses; Hepatitis A.
V. Single-stranded (-) sense RNA viruses • The virion RNA is a negative sense (complementary to mRNA) and must, therefore, be copied into the complementary plus-sense mRNA to make proteins. This group of viruses must code for RNA-dependent RNA-polymerase and also carry it in the virion so that they can make mRNAs upon infecting the cell. e.g. Orthomyxoviruses, Rhabdoviruses, etc • Segmented e.g. Orthomyxoviruses. The first step in replication is a transcription of the (-) sense RNA genome by the virion RNA-dependent RNA polymerase to produce monocistronic mRNAs, which also serve as the template for genome replication. • Non-segmented e.g. Rhabdoviruses. Replication occurs as above and monocistronic mRNAs are produced. VI. Single-stranded (+) sense RNA viruses with DNA intermediate in life-cycle • RNA genome is (+) sense but unique among viruses in that it is DIPLOID, and does not serve as mRNA, but as a template for reverse transcription. e.g. Retroviruses. • Retroviruses, therefore, encodes an RNA-dependent DNA polymerase (reverse transcriptase) to make the DNA provirus which then is transcribed to genomic RNA by a host enzyme, RNA polymerase II. VII. Double-stranded DNA viruses with RNA intermediate • This group of viruses also relies on reverse transcription, but unlike the Retroviruses, this occurs inside the virus particle on maturation. On infection of a new cell, the first event to occur is a repair of the gapped genome, followed by a transcription. e.g., Hepadnaviruses
Properties of viruses Viruses are not cells, do not have nuclei or mitochondria or ribosomes or other cellular components. Viruses replicate or multiply. Viruses do not grow. Viruses replicate or multiply only within living cells. Viruses are obligate intracellular parasites. The term virus was coined by Pasteur, and is from the Latin word for poison.
Viruses Obligate Intracellular Parasites – only demonstrate characteristics of life while “inside” a host cell: Bacteria, animal, plant
Viruses Outside a host cell, inert, no enzyme or other activity Inside a host cell – viral Nucleic Acid (DNA or RNA) takes over the cell and directs the cell to produce new virus particles (replication) Size of Viruses: very tiny (picorna) to huge (pox viruses)
Components of viruses  A virion is an infectious virus particle - not all virus particles are infectious  Viruses are composed of a nucleic acid, RNA or DNA - never both.  All viruses have a protein coat or shell that surrounds and protects the nucleic acid core.  Some viruses have a lipid envelope or membrane surrounding a nucleocapsid core. The source of the envelope is from the membranes of the host cell.  Some viruses package enzymes - e.g. RNAdependent-RNA polymerase or other enzymes - some do not package enzymes
Morphology The size of viruses ranges from very small, such as parvovirus around 20 nm and poliovirus around 30 nm, to quite large, such as vaccinia virus with a wavelength of 400 nm and poxviruses, which can reach 450 nm. Most viruses cannot be visualized with light microscopy; therefore, electron microscopy is used to study their structure. However, some viruses are as large as the smallest bacteria and can be visualized at high optical magnification.
Size comparison between bacteria, viruses, viroids, and prions.
Virus Envelope Some viruses are naked (non- enveloped viruses are called naked), whereas others possess lipid- containing layers around the nucleocapsid called an envelope. Enveloped viruses contain a membrane surrounding the nucleocapsid. The viral envelope consists of a lipid bilayer, derived from the membranes of the host cell. Embedded in it are viral membrane proteins, usually, glycoproteins, coded by viral genes. Glycoprotein spikes extend from the surface of the virus and act as attachment projections or as enzymes (e.g., neuraminidases).
Viruses Basic virus particle is called a “virion” – intact and infective virus particle Components: Nucleic Acid (DNA or RNA), Protein coat (capsid) made of individual protein subunits called capsomeres. Some may have and outer envelope, a membrane, derived from the host cell. The envelope can have specific spikes of protein (H and N spikes of Influenza) that aid in attachment and makes them sensitive to chemical actions of disinfectants.
Viruses Types of viruses based on “morphology” – shape; structure Helical (like TMV or Ebola) Polyhedral (adeno and polio) Enveloped (flu) and Complex (bacteriophage)
Virus Symmetry The nucleocapsids of viruses are constructed in highly symmetric ways. Three kinds of symmetry are recognized in viruses, helical, icosahedron, and complex.
1. Helical symmetry: Rod-shaped viruses have helical symmetry. For example, tobacco mosaic virus (TMV), measles, mumps, influenza, rabies, etc. 2. Icosahedral symmetry: The icosahedron pattern is the most efficient arrangement for subunits in a closed shell. Spherical viruses have icosahedral symmetry. It is a symmetric structure roughly spherical in shape and contains 20 faces (each an equilateral triangle). There are exactly 60 identical subunits on the surface of an icosahedron. Most viruses are built with icosahedral symmetry. For example, polioviruses, adenoviruses, etc. 3. Complex symmetry: These are viruses with complex or uncertain symmetries. For example, smallpox virus has the most complex virion structure consisting of many different proteins and lipoproteins. Bacteriophages are the most complicated viruses in terms of structure as they contain icosahedral heads and helical tails.
Viruses Influenza A: Enveloped, with spikes, RNA, multisegmented genome (8 separate pieces of RNA)
Viruses Bacteriophage: Complex
1. Attachment/adsorption of bacteriophage to the bacteria 2. Penetration of phage DNA 3. Replication of phage DNA/RNA 4. Synthesis of nucleic acid and proteins 5. Assembly of phage protein and nucleic acid 6. Release of mature bacteriophage
Viruses Cultivation of viruses: need living cells, living hosts Tissue cultures, embryonated eggs, bacterial cultures
Viruses Cultivation of viruses: need living cells, living hosts Tissue cultures, embryonated eggs, bacterial cultures Bacteria grown as a “lawn” – and viruses are in the clear zones, plaques
Viruses: Viral replication in bacteria – life cycle of bacterial virus LYTIC Cycle
Viruses Viral replication in bacteria – life cycle of bacterial virus Lysogneic (latent) cycle, genome of virus incorporated into host cell genome “infected with seeds of destruction”
Viruses Animal Virus Life Cycle: Penetration, Uncoating, Biosynthesis, Assembly, Maturation, Release Can have Latent infection also.
Viruses Animal Virus Life Cycle: Penetration, Uncoating, Biosynthesis, Assembly, Maturation, Release Can have Latent infection also. Latent infection is seen in herpes type and even HIV
Viruses Multi-segmented RNA genome of Influenza: higher mutation rate, genetic shift and drift, new vaccines required
LABORATORY DIAGNOSTICS OF ENTEROVIRAL INFECTIONS • Study material - Patient's bowel movements, nasopharyngeal swabs(Coxsack viruses and ECHO), blood (hepatitis A). • Virological method - Isolation of the virus from the test materialon primary and transplantable cropscells, as well as when infecting suckling mice.Identification in PH ("Color test").For hemagglutinating viruses ECHO andCoxsackie – RTGA • Serological method - Paired serums are being investigated,an increase in the Ab titer is diagnosticnot less than 4 times.RN, RSK, RTGA are installed
VIRUS INDICATION METHODS • Cytopathic action • Identification of inclusions in tissue cells • Hemagglutination reaction • Plaque formation on a cell monolayer under agarcovering • Identification of inclusions in tissue cells • Hemadsorption of erythrocytes