Microbiology 1 - Medical Virology PDF

Microbiology Medical Virology Microbiology| Medical Virology Contents : Definition 3 History 4 Properties 6 Methods of Analysis 10 Size of Viruses 11 Origin of Viruses 14 Microbiology| Medical Virology Definition : Virology is the bioscience for study of viral nature, and the relationship between vi...

Microbiology Medical Virology Microbiology| Medical Virology Contents : Definition 3 History 4 Properties 6 Methods of Analysis 10 Size of Viruses 11 Origin of Viruses 14 Microbiology| Medical Virology Definition : Virology is the bioscience for study of viral nature, and the relationship between viruses and hosts. Viruses often cause serious diseases, relate to some cancers and congenital deformities, also can be used as tool for genetic engineering. Microbiology| Medical Virology History : Originated 1000 yrs. BC in China when dried crusts from small box lesions isolated from cured patients where inhaled or inoculated of the pus into a scratch in the arm of a child. They coined the first concept of virulation. Microbiology| Medical Virology Viral infections have been recorded unknowingly from the beginning of recorded history. Plant scientists had isolated material that passed through a low pore filter that was infectious to tobacco plants. The twentieth century saw the discovery of bacteriophage, viruses that attack bacteria, and the use of such bacteriophage to launch studies of molecular biology, and DNA and RNA structure. Dmitri Ivanovsky the discoverer of viruses (1892) Microbiology| Medical Virology Viral Properties : Viruses are inert (nucleoprotein ) filterable Agents. Viruses are obligate intracellular parasites Viruses cannot make energy or proteins independent of a host cell. Viral genome are RNA or DNA but not both. Viruses have a naked capsid or envelope with attached proteins. Viruses do not have the genetic capability to multiply by division. Viruses are non-living entities. Microbiology| Medical Virology Viruses must be infectious to endure in nature. Viruses must be able to use host cell processes to produce their components (viral messenger RNA, protein, and identical copies of the genome). Viruses must encode any required processes not provided by the cell. Viral components must self-assemble. Microbiology| Medical Virology The universe of viruses is rich in diversity. Viruses vary greatly in structure, genome organization and expression, and strategies of replication and transmission. The host range for a given virus may be broad or extremely limited. Microbiology| Medical Virology Viruses are known to infect unicellular organisms such as mycoplasmas, bacteria, and algae and all higher plants and animals. Viruses do not respire, nor do they display irritability, they do not move, they do not grow, they do most certainly reproduce, and may adapt to new hosts. Microbiology| Medical Virology Methods of Analysis : Measuring the Sizes of Viruses: Small size and the ability to pass through filters that hold back bacteria are classic. Some bacteria may be smaller than the largest viruses. 1. Electron microscopy: The resolution is 5nm (1nm = 10-9 m). 2. X-ray crystallography. Microbiology| Medical Virology Size of Viruses : A small virus has a diameter of about 18-26 nm. Having only 30 capsomeres. E.g. Parvovirus: A large virus have a diameter of up to 230x400nm. (130-375 NA) kb Poxviruses. Microbiology| Medical Virology Shape of Viruses : Spherical Rod-shaped (Rudivirus) Brick-shaped (Poxviridae) Microbiology| Medical Virology Tadpole-shaped Bullet-shaped Filament (Filoviridae), (Rhabdoviridae), rabies Ebola virus Microbiology| Medical Virology Origin of Viruses : Regressive hypothesis Cellular origin hypothesis Coevolution hypothesis Microbiology| Medical Virology Regressive hypothesis : Viruses may have once been small cells that parasitized larger cells. Over time, genes not required by their parasitism were lost. Microbiology| Medical Virology The bacteria rickettsia and chlamydia are living cells that, like viruses, can reproduce only inside host cells. They lend support to this hypothesis, as their dependence on parasitism is likely to have caused the loss of genes that enabled them to survive outside a cell. This is also called the degeneracy hypothesis, or reduction hypothesis. Microbiology| Medical Virology Cellular origin hypothesis : Some viruses may have evolved from bits of DNA or RNA that "escaped" from the genes of a larger organism. Microbiology| Medical Virology The escaped DNA could have come from: 1. plasmids (pieces of naked DNA that can move between cells). 2. Transposons (molecules of DNA that replicate and move around to different positions within the genes of the cell). Once called "jumping genes", transposons are examples of mobile genetic elements and could be the origin of some viruses. They were discovered by Barbara McClintock in 1950. This is sometimes called the vagrancy hypothesis, or the escape hypothesis. Microbiology| Medical Virology Coevolution hypothesis : This is also called the virus-first hypothesis and proposes that viruses may have evolved from complex molecules of protein and nucleic acid at the same time as cells first appeared on Earth and would have been dependent on cellular life for billions of years. Microbiology| Medical Virology Viroids are important pathogens of plants. They do not code for proteins but interact with the host cell and use the host machinery for their replication. satellites virus may represent evolutionary intermediates of viroids and viruses. Microbiology| Medical Virology The regressive hypothesis did not explain why even the smallest of cellular parasites do not resemble viruses in any way. The escape hypothesis did not explain the complex capsids and other structures on virus particles. The virus-first hypothesis contravened the definition of viruses in that they require host cells. Microbiology| Medical Virology Viruses are now recognized as ancient and as having origins that pre-date the divergence of life into the three domains. This discovery has led modern virologists to reconsider and re-evaluate these three classical hypotheses. Prions are infectious protein molecules that do not contain DNA or RNA. Microbiology| Medical Virology They can cause infections such as scrapie in sheep, bovine spongiform encephalopathy ("mad cow" disease) in cattle, in humans, prionic diseases include Kuru, Creutzfeldt–Jakob disease. Although prions are fundamentally different from viruses and viroids, their discovery gives credence to the theory that viruses could have evolved from self-replicating molecules.