BIOL 2013 Slides for Exam 2 Characteristics of Viruses Contain a single type of nucleic acid either DNA or RNA but not both Contain a protein coat sometimes itself enclosed by an envelope of lipids proteins carbohydrates that surrounds the nucleic acid Multiply inside living cells by using the synthesizing machinery of the cell Cause the synthesis of specialized structures that can transfer the viral nucleic acid to other cells Viruses Non living Obligate Intracellular Parasites they absolutely require living host cells in order to multiply Only purpose is to make more viruses replication Why aren t viruses living Not cellular Can only replicate inside of a host cell No ATP generating system No ribosomes Contain only one nucleic acid No metabolism have few or no enzymes of their own for metabolism for example they lack enzymes for protein synthesis and ATP generation To multiply viruses must take over the metabolic machinery of the host cell Sizes and Shapes different viruses vary considerably in size most are quite a bit smaller than bacteria but some of the larger viruses are about the same size as some of the smallest bacteria Viruses range from 20 to 1000 nm in length Figure 13 1 page 372 Virus Size the sizes of several viruses and bacteria are compared with a human red blood cell The red blood cell is huge compared to the bacteria and the bacteria is huge compared to the viruses What are viruses made of Protein Coat Capsid Made of capsomeres Protects the nucleic acid Responsible for shape accounts for most of the mass of the virus Nucleic Acid viruses can have either DNA or RNA but never both DNA can be single stranded or double stranded Nucleic acid can also be linear or circular ds DNA e g Herpes ss DNA e g B19 fifth disease ds RNA e g rotavirus ss RNA e g rhinovirus flu virus rabies All viruses have protein coats and one nucleic acid Some also have an Envelope consists of some combination of lipids proteins and carbohydrates In many cases the envelope contains proteins determined by the viral nucleic acid and material derived from normal host cell components Nonenveloped viruses are viruses whose capsids are NOT covered by an envelope In this case the capsid protects the nucleic acid from nuclease enzymes in biological fluids and promotes the virus s attachment to susceptible host cells Figure 13 2 page 372 morphology of a nonenveloped polyhedral virus Figure 13 3 page 373 morphology of an enveloped helical virus has a halo of spikes projecting from the outer surface of each envelope spikes carbohydrate protein complexes that project from the surface of the envelope Some viruses attach to host cells by means of spikes They can also be used for means of identification Virus Specificity Animal Viruses receptor sites are on the plasma membranes of the host cell Plant Viruses Bacterial Viruses viruses that infect bacteria are called bacteriophages or phages For the virus to infect the host cell the outer surface of the virus must chemically interact with specific receptor sites on the surface of the cell Figure 13 5a page 376 morphology of complex viruses Viral Replication Figure 13 15 page 387 Replication of a DNA Containing Animal Virus 1 Attachment virion complete fully developed infectious viral particle composed of nucleic acid and surrounded by a protein coat outside of a host cell and is a vehicle of transmission from one host cell to another attaches to host cell 2 Entry and Uncoating Virion enters the cell and its DNA is uncoated viral DNA is released into the nucleus of the host cell 3 A portion of viral DNA is transcribed producing mRNA that encodes early viral proteins 4 Biosynthesis viral DNA is replicated and some viral proteins are made Late translation capsid proteins are synthesized late proteins include capsid and other structural proteins 5 This leads to the synthesis of capsid proteins which occurs in the cytoplasm of the host cell 6 Maturation virions mature the viral DNA and capsid proteins assemble to form complete viruses 7 Release virions are released from the host cell Figure 13 14 page 386 entry of viruses into host cells receptor mediated endocytosis virus rolls along the outside of host cell until it gets sucked in by the plasma membrane Fusion virus rolls along plasma membrane until viral envelope and plasma membrane fuse forming a vesicle inside the plasma membrane which then releases the virus from its capsid into the cytoplasm of the cell Figure 13 20a page 392 Diagram of budding process budding where the envelope develops around the capsid releasing a portion of the plasma membrane which adheres to the virus Figure 13 11 page 382 Lytic cycle of bacteriophages Attachment phage attaches to host cell Penetration phage penetrates host cell and injects its DNA Biosynthesis phage DNA directs synthesis of viral components by the host cell Maturation viral components are assembled into virions Release host cell lyses and new virions are released Lysogeny phage remains inactive Lambda bacterial virus of e coli Lytic Cycle Lysogenic Cycle Figure 13 12 page 383 Lysogenic Cycle Phage remains inactive instead DNA replicates and cells divide Lytic Cycle Upon penetration into an E coli cell the originally linear phage DNA forms a circle This circle can multiply and be transcribed leading to the production of new phage and to cell lysis the lytic cycle releasing phage virions ENDS WITH LYSIS AND DEATH OF THE HOST CELL Lysogenic Cycle phage attaches to host cell and injects DNA The phage DNA circularizes and enters the lysogenic cycle Phage DNA integrates within the bacterial chromosome by recombination becoming a prophage Lysogenic bacterium then reproduces normally Occasionally the prophage may excise from the bacterial chromosome by another recombination event initiating a lytic cycle HOST CELL REMAINS ALIVE Lysogenic Conversion Imparting new characteristic to lysogenic cell Host cell may exhibit new properties e g Bacteria that release botulinum toxin scarlet fever toxin and diphtheria toxin must be lysogenized in order to be able to produce the toxins VIRUSES DO NOT PRODUCE TOXINS What is an oncogenic virus Causes cancer Similarity to lysogeny Nucleic acid becomes a part of the host cell That s how it causes the cancer not lysogeny but similar How do we control viral infections Antivirals might help treat an existing virus Vaccines prevent viral infections Viroids short pieces of naked RNA only 300 400 nucleotides long with no protein coat Pathogens