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Chapter 18: The Genetics of Viruses and Bacteria Overview: Microbial Model Systems Bacteria are prokaryotes with cells much smaller and more simply organized than those of eukaryotes Viruses are smaller and simpler than bacteria Structure of Viruses Viruses are not cells Viruses are very small infectious particles consisting of Nucleic acid enclosed in a protein coat A capsid is the protein shell that encloses the viral genome A capsid can have various structures In some cases, a membranous envelope Viral Genomes Viral genomes may consist of o Double- or single-stranded DNA o Double- or single-stranded RNA Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus Some viruses have membranous envelopes that help them infect hosts o Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and host cell molecules Viral envelopes surround the capsids of many viruses found in animals Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell Bacteriophages, also called phages, are viruses that infect bacteria o Have the most complex capsids Have an elongated capsid head that encloses their DNA A protein tailpiece attaches the phage to the host and injects the phage DNA inside General Features of Viral Reproductive Cycles Viruses are obligate intracellular parasites o Can reproduce only within a host cell o Host range: limited number of host cells that it can infect, virus specific Viruses use enzymes, ribosomes, and small host molecules to synthesize progeny viruses Reproductive Cycles of Phages Phages have two reproductive mechanisms o Lytic cycle o Lysogenic cycle The Lytic Cycle Lytic cycle: phage reproductive cycle that culminates in death of the host cell o Known as a virulent phage o Produces new phages o Digests the host’s cell wall, releasing the progeny viruses Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage DNA The Lysogenic Cycle Lysogenic cycle: phage reproductive cycle that replicates the phage genome without destroying the host o The viral DNA molecule is incorporated by genetic recombination into the host cell’s chromosome o 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 Phages that use both the lytic and lysogenic cycles are called temperate phages RNA as Viral Genetic Material The broadest variety of RNA genomes is found in viruses that infect animals Retroviruses use reverse transcriptase to copy their RNA genome into DNA o This violates the dogma of DNA > RNA > protein o HIV is the retrovirus that causes AIDS The viral DNA that is integrated into the host genome is called a provirus o Unlike a prophage, a provirus remains a permanent resident of the host cell The host’s RNA polymerase transcribes the proviral DNA into RNA molecules The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell Viral Diseases in Animals Viruses may damage or kill cells by causing the release of hydrolytic enzymes from lysosomes Some viruses cause infected cells to produce toxins that lead to disease symptoms Vaccines are (usually) harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen Vaccines can prevent certain viral illnesses Viral Diseases in Plants More than 2,000 types of viral diseases of plants are known Some symptoms are spots on leaves and fruits, stunted growth, and damaged flowers or roots Spread disease in two major modes: o Horizontal transmission, entering through damaged cell walls o Vertical transmission, inheriting the virus from a parent Viruses, viroids, and prions are formidable pathogens in animals and plants Smaller, less complex entities called viroids and prions also cause disease in plants and animals Viroids: circular RNA molecules that infect plants and disrupt their growth Prions: slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals o Prions propagate by converting normal proteins into the prion version Mechanisms of Gene Transfer and Genetic Recombination in Bacteria Three processes bring bacterial DNA from different individuals together: o Transformation o Transduction o Conjugation Transformation Transformation is the alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment For example, harmless Streptococcus pneumoniae bacteria can be transformed to pneumonia-causing cells Transduction In the process known as transduction, phages carry bacterial genes from one host cell to another Conjugation and Plasmids Conjugation is the direct transfer of genetic material between bacterial cells that are temporarily joined The transfer is one-way: o One cell (“male”) donates DNA, and its “mate” (“female”) receives the genes “Maleness,” the ability to form a sex pilus and donate DNA, results from an F (for fertility) factor as part of the chromosome or as a plasmid Plasmids, including the F plasmid, are small, circular, self-replicating DNA molecules R plasmids and Antibiotic Resistance R plasmids confer resistance to various antibiotics When a bacterial population is exposed to an antibiotic, individuals with the R plasmid will survive and increase in the overall population o Ampicillin o Kyntamycin Operons: The Basic Concept In bacteria, genes are often clustered into operons, composed of o An operator, an “on-off” switch o A promoter o Genes for metabolic enzymes Repressor: switch operon on/off Corepressor: cooperates with a repressor to switch an operon off Repressible and Inducible Operons: Two Types of Negative Gene Regulation A repressible operon is one that is usually on o Binding of a repressor to the operator shuts off transcription o The trp operon is a repressible operon An inducible operon is one that is usually off o Molecule called an inducer inactivates the repressor and turns on transcription o The Lac Z operon is an inducible operon