Chapter 21 Notes.docx

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University of Toronto Scarborough
Biological Sciences
Mark Fitzpatrick

Chapter 21 Viruses Viroids and Prions Infectious Biological Particles 211 What is a Virus Characteristics of Viruses Viruses are not part of the tree of life because they lack properties of life shared by all organisms so they themselves are not considered living organismsThey cant reproduce on their ownThey lack a metabolic system to provide energy for their life cyclesThey depend on host cells that they infect for assistance in these functions Thus viruses are infectious biological particles instead of organisms Structure of virus is simple that is the minimum required to transmit its genome from one host cell to anotherIt is simply one or more nucleic acid molecules surrounded by a protein coat or capsidSome capsids may be enclosed within a membrane or envelope derived from their host cells membraneBecause a virus is not a cell it does not have a cytoplasm enclosed by a plasma membraneLook at Figure 212 on pg 485 for various viruses and their structureNucleic acid genome of a virus may be either DNA or RNA and can be composed of either a single strand or double of the DNA or RNA Viral genomes range from few genes to over a hundred but all viruses have genes that encode at least their coat proteins as well as the proteins involved in transcription genes required for synthesis of envelope proteins only in enveloped viruses and genes required to create enzymes for nucleic acid replication Viruses are two basic structural formsHelical virusesprotein subunits assemble in a rodlike spiral around the genome infect plant cellsPolyhedral virusesthe coat proteins form triangular units that fit together infect animals plants and bacteriaIn some there are protein spikes from the corners to provide host cell recognitionBoth can be enveloped in a membrane derived from the hosts membrane In these enveloped viruses proteins synthesized from the viral genome in the host cell are transported to and embedded in the membrane before the virus particle buds through the host cell Allows virus to recognize and bind to host cells Viruses are classified into orders families genera and species using their size structure and how their nucleic acid is replicated 4000 species into 80 families Family names end in virdae Some bacteria named for the disease they cause Each virus made of strains based by their virulenceEvery living organism is likely to be permanently infected by one or more kinds of virusesUsually virus only affects one virus or closely related viruses some even only affect one organ or tissueSome are able to infect unrelated species both naturally and after mutationOf 80viral families 21 cause human diseaseViruses can also affect animals and plants and the effect can be from undetectable to lethalNot all viruses are pathogens some benefit the hostProtective viruses interfere with replication or other functions of pathogenic viruses or they defend the host Bacteriophages are viruses that destroy other viruses in incredibly huge numbers Viruses are vital components of ecosystems and may be dominant entity in certain ones For example they affect nutrient cycling through their effect on prokaryotic organismsExample Cyanobacteria dominante the marin phytoplankton helping global photosynthesis but bacteriophages infected the cyanobacteria killing them and releasing nutrients from bacterial cells When the bacteriophage infects cyanobacteria the rate of damage exceed rate of repair but the rate of photosynthesis is maintained instead of dropping based on viruss genome which allow it to keep up with the damage Done so the host lives long enough to keep the virus cycle going but the outcome is carbon fixed on Earth212 Viruses Infect Bacterial Animal and Plant Cells by Similar Pathways Virus particles move by random molecular motions until they contact the surface of a host cellFor an infection to occur the virus the genome must enter the cell so the viral genes can be expressed leading to replication of genome and assembly of progeny viruses
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