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Ch. 18 Microbial models: The genetics of viruses and bacteria THE GENETICS OF VIRUSES Researchers discovered viruses by studying a plant disease • In the late 1800s, they found an infectious agent much smaller than bacteria. A virus is a genome enclosed in a protective coat • A virus is a small nucleic acid genome enclosed in a protein capsid and sometimes a membranous envelope. The genome may be single- or doublestranded DNA, or single- or -doublestranded RNA. Viruses can reproduce only within a host cell • Viruses use enzymes, ribosomes, and small molecules of host cells to synthesize progeny viruses. Each type of virus has a characteristic host range, determined by specific receptors on host cells. Simplified Viral Reproductive Cycle • video Phages reproduce using lytic or lysogenic cycles • In the lytic cycle, injection of a phage genome into a bacterium programs destruction of host DNA, production of new phages, and digestion of the host’s cell wall, releasing the progeny phages. • In a lysogenic cycle, a temperate phage inserts its genome into the bacterial chromosome as a prophage, which is passed on to host daughter cells until it is stimulated to leave the chromosome and initiate a lytic cycle. Phage Lytic Cycle • video Phage Lysogenic and Lytic Cycles • video Animal viruses are diverse in their modes of infection and replication • Animal viruses often have an envelope acquired from host cell membrane, which allows them to enter and exit the host cell. • Retroviruses (such as HIV) are RNA viruses that use the enzyme reverse transcriptase to synthesize DNA from their RNA template. • The DNA can then integrate into the host genome as a provirus. • Vaccines against specific viruses stimulate the immune system to defend the host against an infection. • Emerging viruses that cause new outbreaks of disease are usually existing viruses that manage to expand their host territory. • Tumor viruses insert viral DNA into host cell DNA, triggering cancerous changes through their own or host cell oncogenes. Retrovirus (HIV) Reproductive Cycle • video Plant viruses are serious agricultural pests • Most are single-stranded RNA viruses. They enter plant cells through damaged cell walls or are inherited from a parent. Viroids and prions are infectious agents even simpler than viruses • Plant diseases can be caused by viroids, naked RNA molecules that disrupt plant growth. • Prions are infectious proteins that cause brain diseases in mammals. Viruses may have evolved from other mobile genetic elements • Evidence points to their origin as packaged fragments of cellular nucleic acid. THE GENETICS OF BACTERIA The short generation span of bacteria helps them adapt to changing environments • The bacterial chromosome is a circular DNA molecule with few associated proteins. • Plasmids are smaller rings of DNA with accessory genes. • Because bacteria proliferate rapidly and have a short generation span, new mutations can affect a population’s genetic variation quickly. Genetic recombination produces new bacterial strains • The mechanisms of gene transfer between bacteria are transformation, transduction, and conjugation. In transformation, naked DNA enters the cell from the surroundings. • In transduction, bacterial DNA is carried from one cell to another by phages. • In conjugation, an F factor-containing "male" cell transfers DNA to an F- cell. • F+ cells transfer only the F plasmid. • The F factor of an Hfr cell, which is integrated into the bacterial chromosome, brings some chromosomal DNA along with it when it transfers to an F- cell. • R plasmids confer resistance to various antibiotics. Their transfer between bacterial cells poses medical problems. • Transposons, DNA segments that can insert at multiple sites in a cell’s DNA, also contribute to genetic shuffling in bacteria (and eukaryotes).. • Insertion sequences, the simplest bacterial transposons, can affect gene function as they move about. • They consist of inverted repeats of DNA flanking a gene for transposase. • Composite transposons have additional genes, such as those for antibiotic resistance. The control of gene expression enables individual bacteria to adjust their metabolism to environmental change • Cells control metabolism by regulating enzyme activity or by regulating enzyme synthesis through activating or inactivating genes. • In bacteria, coordinately regulated genes are often clustered into operons, with one promoter serving several adjacent genes. • An operator site on the DNA switches the operon on or off. In a repressible operon, binding of a specific repressor protein to the operator shuts off transcription by blocking the attachment of RNA polymerase. • The repressor is activated by binding a small corepressor molecule, usually the end product of an anabolic pathway. • In an inducible operon, an innately active repressor is inactivated by binding an inducer, thereby turning on the genes of the operon only when necessary. • Inducible enzymes usually function in catabolic pathways. • Operons can also be subject to positive control via a stimulatory activator protein. – For example, the active form of cAMP receptor protein (CRP) stimulates transcription by binding to a site next to the promoter and enhancing its ability to bind RNA polymerase. The lac Operon in E. coli • video