LS4 Lect_10

LS4 Lect_10 - 2/3/10 Lwoff concludes that; 1. lysogenic...

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2/3/10 1 Lwoff concludes that; 1. lysogenic bacteria contain a noninfective structure, the prophage . 2. the prophage can turn into an infective phage and that no new phage particle is required for this. 3. In a small percentage of lysogenic bacteria, the prophage becomes induced , producing a burst of new phage. 1 What happens if an Hfr strain is a lysogen? The Hfr ( λ ) strain is lysogenic for λ . The prophage λ makes a repressor that prevents the prophage from entering the lytic cycle. The F - cell does not contain a prophage and therefore does not contain the repressor protein. When the prophage is transferred into the F - cell it ±nds itself in an environment with no repressor. This induces the prophage and results in phage growth and the lysis of the F - bacteria. This process is called Zygotic Induction . Using an Hfr cross the location of the prophage insertion can be mapped. For λ the location of the prophage is always the same site, called att λ . 2
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2/3/10 2 TRANSDUCTION Sometimes phage carry bacterial instead of their own genes. This phenomena is called transduction. There are two kinds: 1. Generalized Transduction (Phage P1 and P22) 2. Specialized Transduction (Phage l) Generalized Transduction. Lytic Life Cycle The phage can make a mistake and package the bacterial DNA instead of its own.The bacterial DNA could then injected into a second bacteria where the DNA can recombine with the chromosomal DNA. Transduction as a genetic tool. The E. coli chromosome is 4,700 kb (kilobases) long. P1 and P22 can package about 40 kb of DNA. About 1 in a thousand P1 or P22 phage will make a mistake and package bacterial DNA. Therefore an individual gene can be rescued by transduction at a frequency of about 10-5.
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2/3/10 3 Co-transduction Three genes; X, Y and Z. Select for X, assay Y; 0% cotransduction Select for X, assay Z; 20% cotransduction Select for Y, assay Z; 35% cotransduction Therefore the map is Specialized Transduction - the prophage sometimes makes a mistake during excision and removes some of the adjacent bacterial DNA. This
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LS4 Lect_10 - 2/3/10 Lwoff concludes that; 1. lysogenic...

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