Chapter 13: Characterizing and Classifying Viruses, Viroids, and PrionsWhy is this important?• Although viruses are not living entities, they do have shape and structure• They aren’t cells – not classified as prokaryotic or eukaryotic• Although they aren’t cells they have a structure• In order to understand viral diseases, we need to have an understanding of viral structureCharacteristics of Viruses• Virus: obligate intercellular parasite• Many infections of humans, animals, and plants, and even of bacteria, are caused by acellular (non-cellular) agents, including viruses• Viruses that infect every thing on earth• Bacteriophage: virus that infects bacteria• Viruses infect very specific cells• Like some eukaryotic and prokaryotic microbes in that they cause disease when they invade susceptible cells• Simple compared to a cell; no cell structure• Recall from Chapter 3 that viruses lack most of the characteristics of lifeo Cannot carry out any metabolic pathwayo Can neither grow nor respond to the environmento Cannot reproduce independentlyo Recruit the cell’s metabolic pathways to increase their numbers- Cause most of the diseases that plague the industrialized world (colds, herpes, AIDS, and flu)o Characteristics of viruses and the means by which they attack their hosts make cures for viral diseases elusive- All have one or several pieces of either DNA or RNAo Never both, one or the other- Do not have a cytoplasmic membrane, cytosol, or functional organelles- Do not undergo metabolic activity on their own- Have extracellular and intracellular stateo Extracellular State (virion)• Protein coat (capsid) surrounding nucleic acid core• Nucleic acid and capsid also called nucleocapsid• Some have phospholipid membrane called an envelope surrounding the nucleocapsid• Outermost layer provides protection and recognition sites that bind to complementary chemicals on the surfaces of cellso Intracellular State• Capsid removed• Virus exists as nucleic acid- Viruses differ in the type of genetic material they contain- Different viruses attack different kinds of cells- Viruses differ in size- The nature of the capsid coat is different from virus to virus- Viruses have different shapes- Viruses may or may not have an envelopeGenetic Material of Viruses• Show more variety in nature of their genomes than do cellso May be DNA or RNA, but never botho Primary way scientists categorize and classify viruses• Can be dsDNA, ssDNA, dsRNA, ssRNAo SsDNA and dsRNA are almost nonexistent in cells• May be linear and composed of several segments or single and circularo Much variation in the nucleic acid of cells• Much smaller than genomes of cellso Ebola virus encodes 7 genesHosts of Viruses• Most viruses infect only particular host’s cellso Due to attraction of viral surface proteins or glycoproteins for complementary proteins or glycoproteins on host cell surfaceo Viruses adapt to specific cells to allow them to infect certain cells• Cells don’t have viral recognition o They “pick” the lock and enter the cello May be so specific they only infect particular kind of cell in a particularhosto Generalists – infect many kinds of cells in many different hosts• All cells are susceptible to some sort of viral infection– Cells infected include archaea, bacteria, fungi, plants, protozoa, and vertebrates– Bacteriophage/phage – a virus that infects bacteria– Scientists have determined that bacteriophages outnumber all bacteria, archaea, and eukaryotes put togetherSizes of VirusesRed blood cells are the smallest cells in the human body because they have no nucleiCapsid Morphology• Capsids – protein coats that provide protection for viral nucleic acid and means of attachment to host’s cells• Capsid composed of proteinaceous subunits called capsomereso Some capsomeres are composed of only a single type of protein, whereas others are composed of several different kinds of proteins• Characteristic of the virusViral ShapesThe Viral Envelope• Some viruses, particularly animal viruses, have a membrane similar in composition to a cytoplasmic membrane surrounding their capsids– Matrix proteins fill the region between capsid and envelope– Enveloped virion versus non-enveloped (naked) virion• Most animal viruses contain envelopes• Most bacterial viruses don’t contain envelopes• Enveloped virus acquires its envelope from its host cell during viral replication or release– Envelope is a portion of the membrane system of a host cell• Viral envelope is composed of phospholipid bilayer and proteins– Some proteins are virally encoded glycoproteins, which appear as spikes protruding outward from the envelope’s surface– Other proteins are host proteins– Lipids are easily dissolved by chemicals so the membrane is easily dissolved• Naked viruses can better combat chemicals than envelope viruses• Envelope’s proteins and glycoproteins often play role in host recognition• Non-enveloped viruses are more easily recognized as foreign– Enveloped viruses have a host envelope making them harder to distinguish as foreign Glycoproteins: Come down and recognize a new host cell and easier infect a hostEnvelope= hostGlycoproteins=virusViral Replication• Viruses lack the genes for all the enzymes necessary for replication and do not possess functional ribosomes for protein synthesis• Viruses are dependent on their hosts’ enzymes and organelles to produce new virions• Viral replication cycle usually results in the death and lysis of the host cell• Lytic replication cycle (example – T4 phage)– Attachment– Entry– Synthesis– AssemblyVirus pokes itself through the cytoplasmic membrane-Blue balls are virus– ReleaseLytic Replication Cycle of Bacteriophages – Attachment • Random collision• Virus specifically binds to host cell• It’s random whether they come in contact with each other though• Tail fibers are responsible for attachment• Chemical attraction (not random) and precise fit to receptor sites• Due to attachment proteins on tail fibers and complementary receptor proteins on the surface of the host’s cell wall (for other viruses, the flagella orpili can also be attachment sites)• Very specificLytic Replication Cycle of Bacteriophages – EntryLegs specifically bind to the phage receptor• Virus must overcome barrier posed by the cell wall and cytoplasmic membrane• Nucleic acid has to get through