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【正文】 頁，共四十二頁。 第十七頁，共四十二頁。s chromosome. 第十五頁，共四十二頁。 Bacterial mutation ? Mutation rate ? Mutation and selectivity ? Backward mutation 第十四頁，共四十二頁。 What can cause mutation? ? Chemicals: nitrous acid。 Mechanism of bacterial variation ? Gene mutation ? Gene transfer and rebination ? Transformation ? Conjugation ? Transduction ? Lysogenic conversion ? Protoplast fusion 第十一頁，共四十二頁。 Transposons (Tn) Transposons are transposable geic elements that carry one or more other genes in addition to those which are essential for transposition. ? Nomenclature Transposons are given the designation Tn followed by a number. ? Structure The structure of a transposon is similar to that of an insertion sequence. The extra genes are located between the terminal repeated sequences. In some instances (posite transposons) the terminal repeated sequences are actually insertion sequences. ? Importance Many antibiotic resistance genes are located on transposons. Since transposons can jump from one DNA molecule to another, these antibiotic resistance transposons are a major factor in the development of plasmids which can confer multiple drug resistance on a bacterium harboring such a plasmid. These multiple drug resistance plasmids have bee a major medical problem because the indiscriminate use of antibiotics have provided a selective advantage for bacteria harboring these plasmids. 第九頁，共四十二頁。 attack. Phase variation is not unique to Salmonella flagellar antigens. It is also seen with other bacterial surface antigens. Also the mechanism of phase variation may differ in different species of bacteria (. Neisseria。 PLASMIDS Plasmids are extrachromosomal geic elements capable
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