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Virus Structure and Method of Invasion • http://videos.howstuffworks.com/tlc/29852understanding-viruses-video.htm • Viruses called bacteriophages – Can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli Figure 18.1 0.5 m • Bacteriophages, also called phages – Have the most complex capsids found among viruses Head DNA Tail sheath Tail fiber 80 Figure 18.4d 225 nm 50 nm (d) Bacteriophage T4 • Recall that bacteria are prokaryotes – With cells much smaller and more simply organized than those of eukaryotes • Viruses Are smaller and simpler still Virus Bacterium Animal cell Animal cell nucleus 0.25 m Figure 18.2 • Obligate parasites --A virus has a genome but can reproduce only within a host cell • Reproduction is their only true characteristic of being alive • Specific to type of cells they target -poliomylelitis virus attacks nerve cells -hepatitis virus attacks liver cells Viral Structure • Not a cell! • Hijacks biochemical machinery of host cell to carry out processes necessary to reproduce • Obligate intracellular parasite Viral Genomes • Viral genomes may consist of – Double- or single-stranded DNA – Double- or single-stranded RNA • Classes of animal viruses Table 18.1 Capsids and Envelopes • A capsid – Is the protein shell that encloses the viral genome – Can have various structures Capsomere of capsid RNA Capsomere DNA Glycoprotein 70–90 nm (diameter) 18 250 mm 20 nm 50 nm Figure 18.4a, b (a) Tobacco mosaic virus (b) Adenoviruses • Some viruses have envelopes – Which are membranous coverings derived from the membrane of the host cell Membranous envelope Capsid RNA Glycoprotein 80–200 nm (diameter) Figure 18.4c 50 nm (c) Influenza viruses Bacteriophage Invasion of a cell by a virus • Virus can lie dormant for many years until it comes into contact with a suitable host cell • Binds with molecules on surface of host cell • Herpes -whole virus enters cell • Bacteriophage -viral DNA injected via hollow tail Alteration of Cell Instruction • Virus takes control of cell machinery • Depends on host for -ATP -supply of free nucleotides • Virus suppresses cell’s normal nucleic acid replication and protein synthesis • Manufactures many identical copies of viral nucleic acid and protein coats The Lytic Cycle • The lytic cycle – Is a phage reproductive cycle that culminates in the death of the host – Produces new phages and digests the host’s cell wall, releasing the progeny viruses • The lytic cycle of phage T4, a virulent phage 1 Attachment. The T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. coli cell. 5 Release. The phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. The cell swells and finally bursts, releasing 100 to 200 phage particles. 2 Entry of phage DNA and degradation of host DNA. The sheath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. The cell’s DNA is hydrolyzed. Phage assembly 4 Assembly. Three separate sets of proteins self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms. Figure 18.6 Head Tails Tail fibers 3 Synthesis of viral genomes and proteins. The phage DNA directs production of phage proteins and copies of the phage genome by host enzymes, using components within the cell. The Lysogenic Cycle • The lysogenic cycle – Replicates the phage genome without destroying the host • Temperate phages – Are capable of using both the lytic and lysogenic cycles of reproduction • The lytic and lysogenic cycles of phage , a temperate phage Phage DNA The phage attaches to a host cell and injects its DNA. Phage DNA circularizes Phage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Bacterial chromosome Lytic cycle The cell lyses, releasing phages. Lysogenic cycle Certain factors determine whether Lytic cycle is induced Figure 18.7 Many cell divisions produce a large population of bacteria infected with the prophage. New phage DNA and proteins are synthesized and assembled into phages. or Lysogenic cycle is entered Prophage The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage. Retrovirus • Contains RNA • No DNA to transcribe into mRNA • Reverse transcriptase injected into cell by virus with RNA -enzyme reverses normal transcription -Produces viral DNA from RNA -Virus uses DNA to replicate Some video clips • Bacteriophage entering a cell • HIV replication • http://www.youtube.com/watch?v=HhhRQ 4t95OI Emerging Viruses • Emerging viruses – Are those that appear suddenly or suddenly come to the attention of medical scientists • Severe acute respiratory syndrome (SARS) – Recently appeared in China (b) The SARS-causing agent is a coronavirus (a) Young ballet students in Hong Kong like this one (colorized TEM), so named for the wear face masks to protect themselves “corona” of glycoprotein spikes protruding from from the virus causing SARS. the envelope. Figure 18.11 A, B • Outbreaks of “new” viral diseases in humans – Are usually caused by existing viruses that expand their host territory Viral Diseases in Plants • More than 2,000 types of viral diseases of plants are known • Common symptoms of viral infection include – Spots on leaves and fruits, stunted growth, and damaged flowers or roots Figure 18.12 • Plant viruses spread disease in two major modes – Horizontal transmission, entering through damaged cell walls – Vertical transmission, inheriting the virus from a parent Viroids and Prions: The Simplest Infectious Agents • Viroids – Are circular RNA molecules that infect plants and disrupt their growth • Prions – Are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals – Propagate by converting normal proteins into the prion version Prion Original prion Many prions Normal protein Figure 18.13 New prion • Concept 18.3: Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria • Bacteria allow researchers – To investigate molecular genetics in the simplest true organisms The Bacterial Genome and Its Replication • The bacterial chromosome – Is usually a circular DNA molecule with few associated proteins • In addition to the chromosome – Many bacteria have plasmids, smaller circular DNA molecules that can replicate independently of the bacterial chromosome Operons: The Basic Concept • In bacteria, genes are often clustered into operons, composed of – An operator, an “on-off” switch – A promoter – Genes for metabolic enzymes • Bacterial cells divide by binary fission – Which is preceded by replication of the bacterial chromosome Replication fork Origin of replication Termination of replication Figure 18.14 • Mutation and Genetic Recombination as Sources of Genetic Variation Since bacteria can reproduce rapidly – New mutations can quickly increase a population’s genetic diversity • An operon – Is usually turned “on” – Can be switched off by a protein called a repressor • The trp operon: regulated synthesis of repressible enzymes trp operon Promoter DNA Promoter Genes of operon trpD trpC trpE trpR trpB trpA Operator Regulatory gene mRNA 5 3 RNA polymerase Start codon Stop codon mRNA 5 E Protein Inactive repressor D C B A Polypeptides that make up enzymes for tryptophan synthesis (a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon’s genes. Figure 18.21a DNA No RNA made mRNA Protein Active repressor Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off. As tryptophan accumulates, it inhibits its own production by activating the repressor protein. Figure 18.21b Repressible and Inducible Operons: Two Types of Negative Gene Regulation • In a repressible operon – Binding of a specific repressor protein to the operator shuts off transcription • In an inducible operon – Binding of an inducer to an innately inactive repressor inactivates the repressor and turns on transcription • The lac operon: regulated synthesis of inducible enzymes Promoter Regulatory gene DNA Operator lacl lacZ 3 mRNA Protein No RNA made RNA polymerase 5 Active repressor (a) Lactose absent, repressor active, operon off. The lac repressor is innately active, and in the absence of lactose it switches off the operon by binding to the operator. Figure 18.22a lac operon DNA lacl lacz 3 mRNA 5 lacA RNA polymerase mRNA mRNA 55' -Galactosidase Protein Allolactose (inducer) lacY Permease Transacetylase Inactive repressor (b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced. Figure 18.22b • Inducible enzymes – Usually function in catabolic pathways • Repressible enzymes – Usually function in anabolic pathways • Regulation of both the trp and lac operons – Involves the negative control of genes, because the operons are switched off by the active form of the repressor protein Positive Gene Regulation • Some operons are also subject to positive control – Via a stimulatory activator protein, such as catabolite activator protein (CAP) • In E. coli, when glucose, a preferred food source, is scarce – The lac operon is activated by the binding of a regulatory protein, catabolite activator protein (CAP) Promoter DNA lacl lacZ CAP-binding site cAMP Inactive CAP RNA Operator polymerase can bind Active and transcribe CAP Inactive lac repressor (a) Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized. If glucose is scarce, the high level of cAMP activates CAP, and the lac operon produces Figure 18.23a large amounts of mRNA for the lactose pathway. • When glucose levels in an E. coli cell increase – CAP detaches from the lac operon, turning it off Promoter DNA lacl lacZ CAP-binding site Operator RNA polymerase can’t bind Inactive CAP Inactive lac repressor (b) Lactose present, glucose present (cAMP level low): little lac mRNA synthesized. When glucose is present, cAMP is scarce, and CAP is unable to stimulate transcription. Figure 18.23b