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BIOL 1010
Igor Kovalchuk

Genetics of Bacteria Genetics of Bacteria - The major component of the bacterial genome is one double-stranded, circular DNA molecule. - E.coli genome consists of 4.6 million nt pairs, representing 4300 genes (size of genome is 100:1 vs. virus 1:1000) - Densely packed, it forms the region called nucleoid. - Bacterial cells divide by fission (20 min per division in optimal condition) proceeded by replication of the chromosome. - Single cell- 12 hr- 10 -10 cells - Fission is asexual process-most of the bacteria in a colony are genetically identical to the parent cell. -7 - Spontaneous mutation rate of 10 per cell division result in 2000 E.coli mutants in a human colon where 2x10 cells are produced per day. - Thus new mutations, although rare, can have a significant impact on genetic diversity when reproductive rates are very high - The prokaryotic chromosome is a single DNA molecule that first replicates, then attaches each copy to a different part of the cell membrane. - When the cell beings to pull apart, the replicate and original chromosomes are separated - Following cell splitting (cytokinesis), there are then two cells of identical genetic composition (except for the rare chance of spontaneous mutation) Replication of the bacterial chromosome - From one origin, DNA replication progresses in both directions around the circular chromosome until the entire chromosome has been reproduced. - Enzymes that cut, twirl (magenta arrow), and reseal the double helix prevent the DNA from knotting. - Though mutations are rare events, they can impact genetic diversity in bacteria because of their reproductive rate. - Though mutation can be a major source of genetic variation in bacteria, it is not a major source of genetic variation in bacteria, it is not a major source in more slowly reproducing organisms (humans) - In higher organisms, genetic recombination from sexual reproduction is responsible for most of the genetic diversity between populations. Genetic recombination produces several new bacterial strains - Thee natural process of genetic recombination in bacteria: o Transformation o Transduction o Conjugation - These mechanisms of gene transfer occur separately from bacterial reproduction, and in addition to mutation, are another major source of genetic variation in bacteria. Transformation - Process of gene transfer during which bacterial cell assimilates foreign DNA from surroundings - Many bacteria have surface proteins that recognize and import naked DNA from closely related bacterial species - Lacking such proteins, E.coli can be artificially induced to take up foreign DNA by incubating them in a culture medium that has a high concentration of calcium ions. - This technique is used by biotechnology industry to introduce foreign genes into bacterial genomes so that bacterial cells can produce proteins characteristic of other species (human insulin and growth hormone) - Certain cell types of bacteria can “donate” a piece of their DNA to a recipient cell. - The recombination is the bacterial equivalent of sexual reproduction in eukaryotes. - Note that the entire DNA is not usually transferred, only a small piece. - By this mechanism, harmless Streptococcus pneumonia can be transformed by pneumonia- causing cells and become pathogenic. Transduction - Gene transfer from one bacteria to another by a bacteriophage - Generalized transduction- occurs when random pieces of host cell DNA are packaged within a phage capsid during the lytic cycle of a phage. o This process can transfer almost any host gene and a little or no phage genes  Phage infects bacterial cell  Host DNA is hydrolyzed into pieces, phage DNA and proteins are made  Occasionally a bacterial DNA fragment is packaged in a phage capsid  Transducing phages infect new host cells, where recombination can occur + - + +  The recombinants have genotypes (A B) different from either the donor (A B ) or recipient (AB) - - Specialized transduction- occurs when macrophage excises from the bacterial chromosome and carries with it only certain host genes adjacent to the excision site (restricted transduction) o Carried out by only temperate phages  Bacterial cells had prophage integrated between genes A and B  Occasionally, prophage DNA
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