The viroid consists of about 350 nucleotides. Prion because pr oteinacious in fectious particles.
Viruses have several features that distinguish them from cells. They lack the cellular components necessary to generate energy and synthesize protein (for example, ribosome). Because they contain so little nucleic acid, they do not have all of the genes necessary for the synthesis of many different proteins. As consequence, viruses can only multiply inside living cells, where they use cell structures and enzymes to support their own multiplication.
Viruses have ultramicroscopic size. This term means that most of them are so minute (less than 200 nm) that an electron microscope is necessary to detect them of to examine their structure. More than 2000 bacterial viruses could fit into an average bacterial cell, and more than 50 million polioviruses could be accommodated by an average human cell. Animal viruses range in size from the small parvoviruses (around 20 nm in diameter) to poxviruses that are as large as the smallest bacterial cell (about 300 nm in diameter).
Most viruses are too small to be seen under the light microscope. Observing the structure of a virus generally required the use of an electron microscope.
Many purified viruses can form large aggregates or crystals.
Viruses contain only those parts needed to invade and control a host cell: an external coating and a core containing nucleic acid strand.
Each virus particle is called virion . All viruses consist of an RNA or DNA core genome surrounded by a protein coat capsid. The combined viral genome and capsid is called the nucleocapsid .
The capsid symmetry is an important taxonomic criterion used in classifying viruses.
T-even bacteriophages are composed of a polyhedral head containing double-stranded DNA and helical tail.
Tobacco mosaic virus is one of the simplest viruses that consist of one strand RNA molecule and one type of protein. Each capsid is composed of many identical protein subunits called capsomeres .
Unlike cells, which always contain both DNA and RNA, viruses contain either DNA or RNA. Reoviruses, for example, is a double-stranded RNA virus, with a segmented genome consisting of 10 separate RNA molecules, influenza viruses have single-stranded segmented RNA genome. As a rule, one segment is a one gene and codes one protein.
Because viruses must pack into a tiny space all of the genes necessary to instruct the host cell to make new viruses, the size of a viral genome is quite small compared with that of a cell. It varies from four genes in hepatitis B virus to hundreds of genes in some herpesviruses. By comparison, the E.coli has approximately 4000 genes.
The capsid can be composed of a single type of protein (TMV) or may contain several different proteins.
Enveloped viruses are more sensitive to heat, detergents, and lipid solvents such as alcohol and ether than are nonenveloped (naked) viruses.
For example, the capsids of enteric viruses such as polioviruses and hepatitis A are resistant to the acid- and protein-digesting enzymes of the gastrointestinal tract. 3. so, they have antigenic properties.
Enzymes that are used by viruses for their reproduction can be divide into 3 gropes:
In an informal and general way, we have already begun classifying viruses—as animal, plant, or bacterial viruses; enveloped or naked viruses; DNA or RNA viruses; and helical or icosahedral viruses The main criteria presently used to group viruses are structure, chemical composition, and similarities in genetic makeup. A widely used scheme for classifying animal viruses first assigns them to one of two superfamilies, either those containing DNA or those containing RNA. DNA viruses can be further subdivided into six families, and RNA viruses into 13 families, for a total of 19 families of animal viruses
Characteristics used for placement in a particular family include type of capsid, nucleic acid strand number, presence and type of envelope, overall viral size, and area of the host cell in which the virus multiplies.
Some virus families are named for their microscopic appearance (shape and size). Examples include rhabdoviruses,* which have a bullet-shaped envelope, and togaviruses, which have a cloaklike envelope. Anatomical or geographic areas have also been used in naming. For instance, adenoviruses were first discovered in adenoides, and bunyaviruses were originally isolated in an area in Africa called Bunyamwera.
Viruses can also be named for their effects on the host. Lentiviruses tend to cause slow, chronic infections. Acronyms made from blending several characteristics include picornaviruses,* which are tiny RNA viruses, and reoviruses (or respiratory enteric orphan viruses), which inhabit the respiratory tract and the intestine and are not yet associated with any known disease state.
Virus families are given a name composed of a Latin root followed by -viridae. Viral genera are denoted by a special Latinized root followed by the suffix -virus (for example, Enterovirus and Herpesvirus).
Bohomolets Microbiology Lecture #4
Virology Morphology, ultrastructure of viruses. Classification
Different non-cellular forms of infectious agents <ul><li>Viruses are obligate intracellular parasites that consist of own genome and proteins </li></ul><ul><li>V iroids are agents that consist solely of a single molecule of circular RNA within the proteins </li></ul><ul><li>Virusoid are viruslike agent that consisted of own nucleic acid (DNA or RNA) and capsid of virus-”helper”. </li></ul><ul><li>Prions are infectious protein particles that are composed solely of protein, they contain no nucleic acid. They cause “slow” diseases such as Crautzfeldt-Jakob disease in human and scrapie in sheep . </li></ul><ul><li>Defective viruses are composed of viral nucleic acid and proteins but cannot replicate without a “helper” virus, which provides the missing function. They usually have a mutation or a deletion of part of their genetic material. </li></ul><ul><li>Similar features : </li></ul><ul><li>non-cellular structure, </li></ul><ul><li>obligate intracellular parasitism, </li></ul><ul><li>absence own metabolism, </li></ul><ul><li>dependency on metabolic processes of cell-host, </li></ul><ul><li>possibility to cause infectious process. </li></ul>
D mitri Ivanovsky (1864-1920) The first virus was unclosed by Dmitri Ivanovsky , Russian scientist. He studied cause of disease of tobacco. Ivanovsky showed that a disease of tobacco plant was caused by a virus (tobacco mosaic virus).
Definition Viruses are non-cellular life forms that have their own genome and can multiply only in host cell, using its metabolic processes
Comparison of viruses and cells Most are freeliving Only within cells Growth Yes No Multiplication by binary fission or mitosis Many None or few Enzymes Present Absent Ribosome Present in all cells Envelope present in some viruses Lipoprotein membrane Many Few Proteins Always contain DNA and RNA DNA or RNA but not both Type of nucleic acid Cells Viruses Property
The history of discovered in virology <ul><li>1892 – Dmitri Ivanovsky discovered first virus - tobacco mosaic virus that caused disease in plant </li></ul><ul><li>1898 – Friedrich Loeffler and Paul Frosch unclosed first virus of animals </li></ul><ul><li>1900 – Walter Reed showed that viruses may cause disease in human (yellow fever). Their work demonstrated that viruses could infect more than one animal species and that viral disease could be transmitted to human with arthropods. </li></ul><ul><li>1911 – Peyton Rous demonstrated that viruses could cause malignant growths in animals, showing that some viruses cause cancer (chicken Rous’ sarcoma virus). </li></ul><ul><li>1915 – Frederick Twort and Felix d’Herelle discovered viruses that infect bacteria (bacteriophages). </li></ul>
Properties of viruses <ul><li>Ultramicroscopic size, ranging from 20 nm up to 450 nm. </li></ul><ul><li>Can pass through bacterial filters. </li></ul><ul><li>They are not cells, structure is very compact and economical. </li></ul><ul><li>Do not independently fulfill the characteristics of life. </li></ul><ul><li>Are inactive outside of the host cell and active only inside host cell. </li></ul><ul><li>Are geometric; can form crystal-like masses. </li></ul><ul><li>Basic structure consists of protein capsid and nucleic acid. </li></ul><ul><li>Nucleic acid can be either DNA or RNA but not both. </li></ul><ul><li>Molecules on virus surface impart high specificity for attachment to host cell. </li></ul><ul><li>Lack metabolic processes. </li></ul><ul><li>Lack machinery for synthesizing proteins. </li></ul><ul><li>Disjunctive method of reproduction. </li></ul><ul><li>Like alive organisms : </li></ul><ul><li>capability for reproduce </li></ul><ul><li>heredity </li></ul><ul><li>variability, </li></ul><ul><li>capacity for evolution </li></ul>
The electron microscope on microbiology, virology and immunology department of NMU.
The crystalline nature of viruses Light microscope magnification (l,200x) of purified poliovirus crystals Highly magnified (150,000x) electron micrograph of the capsids of this same virus, demonstrating their highly geometric nature
Scheme of viral structure Capsid Envelope (not found in all viruses) Various proteins (enzymes) Virus particle Nucleic acid molecule (DNA or RNA) Covering Central core
Generalized structure of viruses a) An simple virus is a naked virus ( nucleocapsid ) consist of a geometrical capsid assembled around a nucleic acid molecule b) An enveloped virus is composed of a nucleocapsid surrounded by a flexible membrane called an envelope or supercapsid .
Morphology of viruses - - - a) b) c) d) i) f) g) h)
Structure of viruses <ul><li>Model tobacco mosaic virus that has a helical symmetry with capsid surrounding an RNS genome. </li></ul><ul><li>Many viruses that infect bacteria, such as the T-even bacteriophages, have complex capsid with DNA contained with a head structure. </li></ul><ul><li>Model adenovirus that has isometric (cubical) symmetry and is naked virus. </li></ul><ul><li>Model coronavirus that complex capsid and envelope </li></ul>
Viral nucleic acid. Unique properties <ul><li>Viruses contain either RNA of DNA, never both </li></ul><ul><li>Viral genome may consist of linear or circular double-stranded DNA, single-stranded DNA, single-stranded linear RNA, or double-stranded linear RNA </li></ul><ul><li>Some RNA viruses have segmented genome that consist of several molecules of RNA. </li></ul><ul><li>Viruses with single-stranded RNA can have positive ore negative genome. </li></ul>
Size and genome content of viruses and bacteria 150-200 genes 300 nm in diameter Largest virus 4000 genes 1000 nm in diameter E.coli cell 4-5 genes 20 nm in diameter Smallest virus Genome content Size
Relationship of viral size to level of dependency on host enzymes for DNA replication Totally independent of host cell enzymes Poxvirus Virus codes for proteins involved in initiation of DNA synthesis and also DNA polymerase Adenovirus Virus codes for a protein that is involved in start of DNA synthesis, rest depends on host enzymes Polyoma virus Depend totally in host cell enzymes Parvovirus Dependence on host cell DNA synthetic enzymes Increa-sing size Virus
Polarity of viral RNA <ul><li>Viruses with negative genome has single-stranded RNA with negative polarity . They use molecule RNA just as its genetic material. An mRNA must be transctibed by using the negative strand as a template. </li></ul><ul><li>For instance, influenza virus </li></ul><ul><li>2. Viruses with positive genome has single-stranded RNA with positive polarity . They use molecule RNA as its genetic material and as mRNA that can connect with ribosomes. </li></ul>
Nucleocapsid of Adenovirus Naked viruses consist of nucleic acid and proteins. The viral coat structure surrounded the nucleic acid genome of a virus is called the capsid .
Virions of herpesviruses with supercapsid The envelope is lipoprotein membrane composed of lipid derived from the host cell membrane, protein that is virus-specific, and glycoprotein in the form of spikes on the surface. The matrix protein, mediates the interaction between the capsid proteins and the envelope Enveloped viruses that have supercapsid consist of nucleic acid, proteins, lipids and glycoproteins.
Functions of the viral capsid / envelope <ul><li>Protection of the nucleic acid from the effects of various enzymes and chemicals when virus is outside the host cell. </li></ul><ul><li>Capsid and supercapsid are responsible for helping to introduce the viral DNA or RNA into a suitable host cell, first by binding to the cell surface and then by assisting in penetration of the viral nucleic acid. </li></ul><ul><li>Parts of the viral capsid and envelope stimulate the immune system to produce antibodies that can neutralize viruses and protect the host organism against future infections. </li></ul>
Functions of viral proteins <ul><li>Defence of viral genome </li></ul><ul><li>Receptors that define spectrum of host cell, which can be affected by the viruses </li></ul><ul><li>Attachment on host cell </li></ul><ul><li>Stabilization of nucleic acid molecules (like histone proteins in eukaryotic cells) </li></ul><ul><li>Contraction of bacteriophages tails </li></ul><ul><li>Enzymes </li></ul><ul><li>Antigens </li></ul>
Enzymes for viral replication <ul><li>Virion enzymes that are constituents of viral particle. There are DNA and RNA polymerases, nuclease, neuraminidase, RNA-dependent DNA polymerase (reverse transcriptase) </li></ul><ul><li>Viral-induced enzymes that are coded in viral nucleic acid but are not constituents of virion. There are DNA polymerase of poxviruses and herpesviruses, RNA polymerase of polioviruses. </li></ul><ul><li>Host cell enzymes . There are DNA polymerase of parvoviruses </li></ul>
Morphology of bacteriophages Grope C. Tail is very short and noncontrative Grope B. Tail is long and noncontrative Grope A. The tails of the phage are long and contractile DNA genome, single-stranded. Rod-shaped virion, helical symmetry (M13). Filamentous BF DNA of RNA genome, icosahedral virion, cubic symmetry, enveloped or naked ( X174). Cubic BF DNA genome, double-stranded. Virion complex shaped, binary symmetry, variable number of capsomers . T-even coliphages Tailed BF Description and examples Shape
Features that used for classification of viruses <ul><li>Type and characteristic of viral nucleic acid </li></ul><ul><li>Size and shape of virion </li></ul><ul><li>Presence or absence of envelope </li></ul><ul><li>Type of nucleocapsid symmetry </li></ul><ul><li>Strategy of virus genome </li></ul><ul><li>Antigenic characteristic of virion </li></ul><ul><li>Tissue that affected by virus </li></ul><ul><li>Disease that virus causes </li></ul><ul><li>Geographic areas where virus was obtained firstly </li></ul>
Animal DNA viruses families Parvovirus B19 - 18-26 Icosahedral Single Parvo-viridae Hepatitis B virus + 42 Icosahedral double Hepadna-viridae Human papillomavirus - 45-55 Icosahedral Double Papova-viridae Human adenoviruses - 70-90 Icosahedral Double Adeno-viridae Herpes simplex virus, Varicella zoster virus, Epstein-Barr virus + 150-200 Icosahedral Double Herpes-viridae Smallpox virus; complex virus; brick-shaped + 130-300 None Double Poxviridae Common Name of Important Members Envelope Size Capsid type Strand type Family
Animal RNA viruses families (1) Parainfluenza virus, mumps virus, measles virus + 125-250 Helical Single Paramyxo-viridae Yellow fever virus, Japanese encephalitis virus + 40-70 Icosahedral Single Flaviviridae Rubella virus, western equine encephalitis + 45-70 Icosahedral Single Togaviridae Norwalk virus - 35-40 Icosahedral Single Calciviridae Hepatitis A virus, poliovirus, coxsackieviruses, rhinoviruses - 20-30 Icosahedral Single Picorna-viridae Common Name of Important Members Envelope Size (nm) Capsid Type Strand type Family
Animal RNA viruses families (2) Influenza viruses + 80-120 Helical Single Orthomyxo-viridae Human rotavirus, Colorado tick fever virus - 60-80 Icosahedral Double Reoviridae Bunyamwera virus, Hanta virus + 90-100 Helical Single Bunyaviridae Ebola and Marburg viruses + 790-970 Helical Single Filoviridae Common Name of Important Members Envelope Size (nm) Capsid Type Strand type Family
Animal RNA viruses families (3) Human infectious bronchitis and corona viruses + 80-130 Helical Single Corona-viridae Lassa virus; lymphocytic choriomeningitis virus + 50-300 ? Single Arenaviridae Human immunodeficiency virus (AIDS), oncoviruses + 100 Icosahedral Single Retroviridae Rabies virus + 60-75 Helical Single Rhabdo-viridae Common Name of Important Members Envelope Size (nm) Capsid Type Strand type Family
General taxonomic categories ( for herpesviruses ) 1, 2 Type simplex Species Alphaherpes virus Genus Alphaherpes virinae Underfamily Herpes viridae Family Vira Kingdom