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Lecture 20

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Biological Sciences
Mark Fitzpatrick

BIOA01H3 – Lecture 20 th Week of Oct. 28 : Tree of Life; Viruses & Prions Chapter 21 21.1 What Is a Virus? Characteristics of Viruses  Viruses lack many of the properties of the life that is shared by all organisms o I.e. viruses cannot reproduce on their own and lack a metabolic system to provide energy for their life cycles, they depend on host cells that they infect for these mechanisms  Viruses are infectious biological particles rather than organisms  Structure of a virus is reduced to minimum necessary to transmit its genome from one host cell to another  Virus simply one or more nucleic acid molecules surrounded by protein coat called capsid o Some may be enclosed within a membrane derived from host cell’s membrane  Virus is not a cell – doesn’t have cytoplasm enclosed by plasma membrane, as do all known living organisms  Nucleic acid genome of virus may be either DNA or RNA and can’t be composed of either o Viral genomes range from just a few genes to over a hundred genes; all viruses have genes that encode at least their coat proteins as proteins involved in regulation of transcription  Most viruses take on of two basic forms: helical and polyhedral o Helical viruses  the protein subunits assemble in a rodlike spiral around the genome o Number of viruses that infect plant cells are helical  Polyhedral viruses  coat proteins form triangular units that fit together like the parts of a soccer ball  Infect animals, plants, and bacteria  In some polyhedral viruses, protein spikes that provide host cell recognition extend from the corners, where facets fit together  Both helical and polyhedral viruses can be enveloped in membrane derived from host cell’s  In enveloped viruses, proteins synthesized from viral genome in host cell transported to and embedded in membrane before virus particle buds through host cell  These proteins allow virus to recognize and bind to host cell  Although not alive, viruses classified into orders, families, genera, and species using criteria incl. size and structure, genome structure (RNA or DNA, single stranded or double), and how nucleic acid replicated  Family names end in –viridae and may refer to geographic region where virus first discovered or structure of virus  Every living organism likely permanently infected by one or more kinds of viruses  Usually, virus infects single species or few closely related ones  May even infect only one organ system or single tissue  Some viruses able to infect unrelated species, either naturally or after mutating  Not all viruses are pathogens, infection of some viruses benefit their hosts i.e. infection by nonpathogenic viruses protects human hosts against pathogenic viruses o These protective viruses interfere with the replication of the pathogenic viruses i.e. reason why bacteria doesn’t run the planet is because of bacteriophages 21.2 Viruses Infect Bacterial, Animal, and Plant Cells by Similar Pathways  Viral particles moves by random molecular motions until contact surface of host cell 22.2a Bacteriophages Are Viruses That Infect Bacteria Virulent bacteriophages  kill their host cells during each cycle of infection. Temperate bacteriophages  enter an inactive phase inside host cell and can be passed on to several generations of daughter cells before becoming active and killing their host. Virulent Bacteriophages  Among bacteriophages infecting E. coli, T-even bacteriophages T2, T4, and T6 have been most valuable in genetic studies  Coats of these phages divided into head and tail  Double stranded linear molecule of DNA packed into head  Tail, assembled from several diff. proteins, has recognition proteins at tips that an bind to surface of host cell  Once tail attached, it functions as sort of a syringe that injects DNA genome into cell  Infection begins when T-even phage collides randomly w/ surface of E. coli cell and tail attaches to host cell well o Enzyme present in viral coat, lysozyme, then digests hole in cell wall through which tail injects DNA of phage o Proteins of viral coat remain outside o Throughout life cycle within bacterial cell, phage uses host cell machinery to express genes o One of proteins produces early in infection is an enzyme that breaks down bacterial chromosome o Phage gene for a DNA polymerase replicates phage’s DNA also expressed early on o Eventually, 100-200 new viral DNA molecules synthesized o Later in infection, host cell machinery transcribes phage genes for viral coat proteins o As head and tail protein assemble, replicated viral DNA packed into heads o When viral assembly complete, cell synthesizes phage-encoded lysozyme that lyses bacterial cell wall, releasing viral particles o This whole series of events called lytic cycle  Some may package fragments of host cell’s DNA in heads as viral particles assemble  known as transduction A Scientist’s Favourite Temperate E. coli
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