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THE GENETICS OF VIRUSES Chapter 19 CHARACTERISTICS OF VIRUSES Small (20nm) Composed of RNA or DNA and protein Capsid- protein coat that encloses the viral genome Viral envelopes – a membrane that encloses capsid on some viruses (derived from host) Comparing the size of a virus, a bacterium, and a eukaryotic cell Viral structures Adenovirus Bacteriophage – a virus that infects bacteria Viruses are obligate parasites – can only reproduce within a host cell Host Range – virus can only infect a limited number of host cells – Ex. HIV only attacks T-cells Considered nonliving Viruses infect all life on the planet Phages VIRAL INFECTION Virus injects its genome (DNA or RNA) into host cell Two major reproduction pathways for phages: –1. Lytic –2. Lysogenic A simplified viral reproductive cycle The lysogenic and lytic reproductive cycles of phage , a temperate phage THE LYTIC CYCLE Ends in death of host cell Viruses called virulent Viral DNA inserted into host Host’s DNA hydrolyzed Viral DNA makes proteins and more viral DNA New viruses released by bursting out of host cell THE LYSOGENIC CYCLE Does not kill host cell Called temperate viruses Viral DNA is inserted into host cell Viral DNA is inserted into host cell’s DNA (called a prophage) When host cell replicates it also replicates the viral DNA section Prophage genes are mostly inactive Some can make harmful toxins (ex. diphtheria and scarlet fever) Classes of Animal Viruses, Grouped by Type of Nucleic Acid Classes of Animal Viruses, Grouped by Type of Nucleic Acid Smallpox Measles Polio Influenza epidemic (Killed 40 million people in 1918-19) Herpes RNA VIRUSES Can be transcribed to make protein Can make more RNA with special enzymes (within capsid) Retrovirus – contain reverse transcriptase enzyme which transcribes DNA from RNA. (reverse) – Ex. HIV Lack error checking ability during RNA replication so higher rates of mutation HIV, a retrovirus HIV infection VACCINES Harmless variants, dead or derivatives of viruses that allow us to build an immunity to the real thing Antibiotics are powerless against viruses EMERGING VIRUSES Viruses have high mutation rates Dissemination of virus from small population to larger (airplanes) Spread of existing viruses from other animals Cancer Causing Viruses Some viruses can cause cancer –Ex. Hepatitis B – liver cancer –Ex. Papilloma – cervix cancer Hepatitis INFLUENZA Three types – Type A: can cause epidemics and found in many animals including humans – Types B and C: only in humans and no epidemics Type A contain two proteins on capsid – H = hemagglutinin (helps virus attach to host) – N = neuroaminidase (helps release new viruses from infected cell) H1N1 (swine flu) and H5N1 (avian flu) H1N1 – Caused both flu pandemic in 1918 and in 2009 – Probably mutated in pigs and moved to humans – 79% people infected were under 30 in 2009 H5N1 – Expanding host range and 50% mutation rate – Greater threat – Human to human transmission rare (so far) PRIONS Prions are infectious proteins – Diseases caused by prions: Mad cow, Creutzfeldt-Jacob, Kuru and maybe Alzheimer’s – Misfolded forms of proteins – Associated with eating infected meat – Incubate very slowly – No cure and always deadly A hypothesis to explain how prions propagate Bacteria and Archaea CHAPTER 27 Bacteria on the point of a pin THREE MAIN LINEAGES OF LIFE Prokaryotes Unicellular Contain cell wall, plasma membrane, ribosomes, DNA, and cytoplasm First organisms to inhabit earth Some are autotrophs and others are heterotrophs STRUCTURE AND FUNTION Three most common shapes – Cocci – round – Bacilli – rods – Helices – spiral Usually small (1-5μm) Figure 27.3 The most common shapes of prokaryotes Prokaryotic cell walls – Most walls contain peptidoglycan (sugars cross-linked with polypeptides) except archaea – Gram positive – large amounts of peptidoglycan (stain violet) – Gram negative – small amounts of peptidoglycan (stain red) • Often more threatening due to lipopolysaccharides on cell walls that are often toxic – Antibiotics often inhibit synthesis of cross-links of peptidoglycan Figure 27.5 Gram-positive and gram-negative bacteria Figure 27.5x Gram-positive and gram-negative bacteria Some have pilli (some for “sex”) Some have flagella (not covered by membrane) Some capable of taxis – movement away from or toward stimuli Prokaryotic Genome – 1/1000 as much DNA – One circular chromosome (may be concentrated in a nucleoid region) – Plasmids – smaller, rings of DNA; may carry resistance genes, conjugation genes Figure 27.6 Pili REPRODUCTION Binary fission – cell division; requires copying the one chromosome and then cell divides (can happen in 20 minutes) Genetic recombination – ways that bacteria can get genes from other organisms – Conjugation (bacterial sex) – Transformation (bacteria grab foreign DNA from environment) – Transduction (viruses infect bacteria with foreign DNA) Figure 27.x1 Prokaryotic conjugation Mutation is the main source of variation in prokaryotes!! Figure 27.11x1 Cyanobacteria: Gloeothece (top left), Nostoc (top right), Calothrix (bottom left), Fischerella (bottom right) DOMAIN ARCHAEA Prokaryotes – Methogens – use H2 to reduce CO2 to CH4; poisoned by O2, live in swamps, decompose sewage, in guts of animals (cows and termites) help digest cellulose – Extreme halophiles – like salt, purple-red scum due to bacteriorhodopsin – Extreme thermophiles – in hot springs, 60° – 80° C, deep sea vents at 150° C Figure 27.14 Extreme halophiles Figure 27.14x1 Hot springs, home of thermophiles Figure 27.14x2 Beggiatoa, sulfur-eating bacteria Figure 27.1 “Heat-loving” prokaryotes