Lecture 5: Viruses PDF

Lecture 5 Viruses 1  Viruses A virus is a biological agent that reproduces inside the cells of living hosts. When infected by a virus, a host cell is forced to produce thousands of identical copies of the original virus Major cause of disease...

Lecture 5 Viruses 1  Viruses A virus is a biological agent that reproduces inside the cells of living hosts. When infected by a virus, a host cell is forced to produce thousands of identical copies of the original virus Major cause of disease – also importance as a new source of therapy Virotherapy is a treatment using biotechnology to convert viruses into therapeutic agents by reprogramming viruses to treat diseases. – new viruses are emerging (SARS-CoV-2 (covid- 19),H1N1) Important in evolution – transfer genes between bacteria, others 2  General Properties of Viruses Virion – complete virus particle – consists of ≥1 molecule of DNA or RNA enclosed in coat of protein – may have additional layers – Viewed under electron microscope – cannot reproduce independent of living cells nor carry out cell division but can exist extracellularly 3 Intracellular and Extracellular Viral Form Viruses exist in one of two states; extracellular and intracellular. Extracellular State: Before it invades a host cell, a virus is in the ‘extracellular state’ called virion Intracellular State: Once the virus invades a host cell it is in an ‘intracellular state.’ In this state, the capsid is removed and the virus exists as only as nucleic acid (genetic material). 4  Virions Infect All Cell Types Bacterial viruses called bacteriophages (phages) Most are eukaryotic viruses – plants, animals, protists, and fungi Classified into families based on – genome structure, life cycle, morphology, genetic relatedness 5  The Structure of Viruses Virion size range is ~10–400 nm in diameter and most viruses must be viewed with an electron microscope All virions contain a nucleocapsid which is composed of nucleic acid (DNA or RNA) and a protein coat (capsid) – some viruses consist only of a nucleocapsid, others have additional components Enveloped viruses/non enveloped viruses 6 7 Viral Shapes and Sizes 9  Capsids Large macromolecular structures which serve as protein coat of virus Protect viral genetic material and aids in its transfer between host cells Made of protein subunits called protomers Capsids are helical, icosahedral, or complex 10 a) T4 bacteriophage virion that attacks E.coli and b) micrograph of virion before injection of DNA 11  Viral Envelopes and Enzymes Many viruses are bound by an outer, flexible, membranous layer called the envelope Animal virus envelopes (lipids and carbohydrates) usually arise from host cell plasma or nuclear membranes 12  Viral Envelope Proteins Envelope proteins, which are viral encoded, may project from the envelope surface as spikes or peplomers – involved in viral attachment to host cell e.g., hemagglutinin of influenza virus – used for identification of virus – may have enzymatic or other activity e.g., neuraminidase of influenza virus – may play a role in nucleic acid replication 13  Virion Enzymes It was first erroneously thought that all virions lacked enzymes Now accepted that a variety of virions have enzymes – some are associated with the envelope or capsid but most are within the capsid 14  Viral Genome Diverse nature of genomes A virus may have single or double stranded DNA or RNA The length of the nucleic acid also varies from virus to virus Genomes can be segmented or circular 15 Viral Multiplication Mechanism used depends on viral structure and genome Steps are similar – attachment to host cell – entry – uncoating of genome – synthesis – assembly – release 16  Attachment (Adsorption) Specific receptor attachment Receptor determines host preference – may be specific tissue (tropism) – may be more than one host – may be more than one receptor – may be in lipid rafts providing entry of virus 17  Viral Entry and Uncoating Entire genome or nucleocapsid Varies between naked or enveloped virus Three methods used – fusion of the viral envelope with host membrane; nucleocapsid enters – endocytosis in vesicle; endosome aids in viral uncoating – injection of nucleic acid 18 19  Synthesis Stage Genome dictates the events ds DNA typical flow RNA viruses – virus must carry in or synthesize the proteins necessary to complete synthesis 20  Assembly Late proteins are important in assembly Assembly is complicated but varies – bacteriophages – stages – some are assembled in nucleus – some are assembled in cytoplasm – may be seen as paracrystalline structures in cell 21  Virion Release Nonenveloped viruses lyse the host cell 22  Virion Release Enveloped viruses use budding – viral proteins are placed into host membrane – nucleocapsid may bind to viral proteins – envelope derived from host cell membrane, but may be Golgi, ER, or other – virus may use host actin tails to propel through host membrane 23 Fighting Bacteria with Viruses 24 Thank you 25