VIRUSES DIFFERENCES BETWEEN VIRUS AND CELL The Discovery of Viruses: Scientific Inquiry • Tobacco mosaic disease stunts growth of tobacco plants and gives their leaves a mosaic coloration • In the late 1800s, researchers hypothesized that a particle smaller than bacteria caused the disease • In 1935, Wendell Stanley confirmed this hypothesis by crystallizing the infectious particle, now known as tobacco mosaic virus (TMV) TRANSITION BETWEEN LIVING AND NONLIVING THINGS ACELLULAR (JUST HAVE GENETIC MATERIAL AND CAPSID FROM PROTEIN) PARASITE INTRACELLULAR OBLIGATE REPRODUCE WITHIN HOST CELL THEY HAVE ONLY RNA / DNA IN CRYSTAL SHAPED IF OUTSIDE CELL CHARACTERISTIC OF VIRUS Structure of Viruses • Viruses are not cells RNA Viral genomes (bringing genetic code) DNA structure Capsid (protect the genetic materials) Capsids are built from protein subunits called capsomeres Envelope From glycoprotein become a reseptor ZXX SPHERICAL / ISOMETRIC >> HIV, INFLUENZA ROD-SHAPED/ HELICAL >> TMV POLYHEDRAL/ ICOSAHEDRON >> ADENOVIRUS OVAL/CAPSULE SHAPED >> RABIES T-SHAPED >> BACTERIOPHAGE FILAMENT >> EBOLA VIRUS SHAPE REPLICATIVE CYCLES OF PHAGES Phages are the best understood of all viruses Phages have two reproductive mechanisms: the lytic cycle and the lysogenic cycle THE LYTIC CYCLE The lytic cycle is a phage replicative cycle that culminates in the death of the host cell The lytic cycle produces new phages and lyses (breaks open) the host’s cell wall, releasing the progeny viruses A phage that reproduces only by the lytic cycle is called a virulent phage Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA FIGURE 19.5-1 1 Attachment FIGURE 19.5-2 1 Attachment 2 Entry of phage DNA and degradation of host DNA FIGURE 19.5-3 1 Attachment 2 Entry of phage DNA and degradation of host DNA 3 Synthesis of viral genomes and proteins FIGURE 19.5-4 1 Attachment 2 Entry of phage DNA and degradation of host DNA Phage assembly 4 Assembly Head Tail Tail fibers 3 Synthesis of viral genomes and proteins FIGURE 19.5-5 1 Attachment 2 Entry of phage DNA and degradation of host DNA 5 Release Phage assembly 4 Assembly Head Tail Tail fibers 3 Synthesis of viral genomes and proteins LYTIC CYCLE STEPS DESCRIPTION 1. ATTACHMENT Virus particle (virion) attaches to a cell 2. PENETRATION/ INJECTION Nucleic acid enters or penetrates the host’s cytoplasm 3. REPLICATION & SYNTHESIS Virus multiplies itself by controlling or manipulating the host to produce viral components, i.e. nucleic acid and protein for capsid 4. ASSEMBLY Nucleic acid and capsid are assembled to become intact viruses 5. RELEASE/ LYSIS Newly assembled viruses are released from the host cell by lysis THE LYSOGENIC CYCLE The lysogenic cycle replicates the phage genome without destroying the host The viral DNA molecule is incorporated into the host cell’s chromosome This integrated viral DNA is known as a prophage Every time the host divides, it copies the phage DNA and passes the copies to daughter cells LYSOGENIC CYCLE STEPS DESCRIPTION 1. ATTACHMENT Virus particle (virion) attaches to a cell 2. PENETRATION/ INJECTION Nucleic acid enters or penetrates the host’s cytoplasm 3. INTEGRATION The virus DNA/RNA is inserted into host chromosomes becoming a prophage 4. DIVISION The modified chromosomes undergo replication and reproduce normally creating many new daughter cells with the infected genetic materials An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode Phages that use both the lytic and lysogenic cycles are called temperate phages BENEFICIAL VIRUSES HUMAN DISADVANTAGEOUS BECAUSE CAUSE DISEASES ANIMAL PLANT BACTERIA HIV caused AIDS RSV TMV (tobacco mosaic virus) ADENOVIRUS FOOT AND MOUTH DISEASE VIRUS TUNGRO VIRUS RABIES NEWCASTLE DISEASE VIRUS CVPD (citrus vein phloem degeneration) POLIO RABIES HERPES EBOLA INFLUENZA BACTERIOPHAGE BENEFICIAL VIRUSES ADVANTAGEOUS GENE THERAPY (CURE GENETIC DISORDERS) Bacteriophage for killing bacteria cause disease (in human body)
