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Ch. 9_Microbial Genetics

Ch. 9_Microbial Genetics - Ch 9 Genetic Mapping in Bacteria...

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Ch. 9. Genetic Mapping in Bacteria & Bacteriophages Gene transfer in bacteria can occur by transformation, transduction and conjugation (described in detail later). In all 3 cases, DNA transfer is unidirectional. The size of DNA transferred by the 3 methods is different. Therefore, they have different resolving powers (see later). E. coli has been widely used for genetic experiments. It is a rod, ~1-3 μ M long and 0.5 μ M in diameter ( Figs. 9.1, 9.2) . There is no nuclear membrane and no formal nuclear division like in eukaryotes. There is no metaphase and no highly condensed chromosomes like in eukaryotes. In addition to the chromosome, bacteria often have extrachromosomal DNA called plasmids. These are small circular double-stranded molecules ( Fig. 9.1 ) with an origin of DNA replication and capable of self-replication. Often they carry one or more genes for resistance to antibiotics. Plasmids range in size from a few thousand to several hundred thousand bp in size. A large plasmid found in E. coli cells is called F-factor or Sex-Factor . This plasmid carries genes that enable it to make tube like structures ( pilus ) on the surface of the E. coli cell ( Fig. 9.2) . A cell containing the F-factor and capable of making pili is called a F + or male cell. These cells make contact with F - or female cells and establish a connection using a pilus. Plasmids, like the F-factor, capable of mediating conjugation are called conjugative plasmids . Not all plasmids can do this. During mating, a copy of the sex-factor plasmid is transferred to the F - (female) cell using the conjugation tube . The F-factor replicates by a rolling circle mechanism and the resulting single-stranded tail of DNA is transferred to the F - cell ( Fig. 9.3) . The single strand of DNA is converted to a DS DNA in the recipient cell. This results in the conversion of the F - recipient cell into a F + cell. Insertion sequences, transposons and antibiotic-resistance gene cassettes. As agents of mutation and large-scale genetic changes, transposons were discussed in the Chapter 14 lecture earlier (Mutations and DNA Repair). In this chapter, the role of transposons as carriers of antibiotic-resistance genes will be described. The simplest bacterial transposons are called Insertion Sequences (IS elements; Fig. 9.4) . These are typically about 1-3 Kb in size and have 3 characteristic features. (1) they have short inverted repeat sequences at their ends. (2) They create very short (5-9 bases) direct repeats at the site of insertion. (3) They have agene in the middle (flanked by the inverted repeats) for an enzyme called transposase, that catalyzes their movement from one location to another. More complicated transposons often have antibiotic- resistance genes in addition to the transposase gene ( Fig. 9.4B ). Transposons can insert themselves into any DNA in the bacterial host cell, the chromosome and any plasmids that may be present. When the same transposons are present in a plasmid and the chromosome, the 2 transposon copies may recombine with each other. Due to the
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