Exam 2 Material - Lecture 14 Most prokaryotic genomes...

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Lecture 14 Most prokaryotic genomes are comprised of a single, circular, supercoiled DNA chromosome. Some prokaryotic genomes are comprised of multiple, haploid chromosomes. Some prokaryotic genomes are comprised of linear and circular chromosomes. All prokaryotic DNA chromosomes are supercoiled. Supercoiling is essential to pack the chromosome into the small volume of the prokaryotic cell. DNA supercoiling is a consequence of DNA gyrase and DNA topoisomerase enzymes. Prokaryotic genomes do not all supercoil in the same direction. Bacteria use DNA gyrase to underwind the relaxed, closed-circular, double-stranded DNA molecule into a negative supercoil. By contrast, Archaea use topoisomerase to overwind their genomes in to positive supercoils. The E.coli chromosome is organized into 40-50 domains. Domains are relaxed, cytoplasmic loops of chromosomal DNA – their supercoiling eliminated by DNA gyrase nicking. Domains are chromosomal DNA sequences spaced between sequences of high protein binding. Domain formation is crucial for prokaryotic gene expression and chromosome replication. Quinolones are a group of antibacterial compounds - like Cipro - that interfere with DNA gyrase and topoisomerase IV enzyme activity. Quinolones inhibit supercoiling, strand nicking, and supercoil relaxation. As a result, prokaryotic DNA replication is prevented, making quinolones extremely effective antibacterial antibiotics. Many prokaryotes contain plasmids encoding important, dispensable functions. Plasmids are typically circular (some are linear) molecules of supercoiled DNA. Plasmids differ from chromosomes in the following way; they are usually much smaller (typically 1/10th to 1/100th the size of a chromosome). Plasmids are not present in all archaeal or bacterial species. Plasmids may be present in some members of a species, but not all. Plasmids encode no functions essential to cell growth. Plasmids are often present in multiple copies in a cell– the copy number of each plasmid varies. Plasmids are often a unique and important identifier of a prokaryotic cell – in particular with disease-causing/pathogenic bacteria. The gene functions encoded by plasmids include; Antibiotic resistance, Metabolism of exotic organic compounds, Plasmid incompatibility – which are the genes for plasmid DNA replication, Segregation – which determines which cytoplasm a plasmid occupies during cell division, Conjugation - cell-to-cell plasmid transfer, and Host chromosome DNA transfer – performed by a subset of conjugative plasmids. Most of the typical prokaryotic genome is coding sequence - that is DNA sequences encoding proteins & RNA. There is little “junk” DNA in prokaryotic genomes. Prokaryote genomes have comparatively fewer nucleotides between their genes.
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