Lecture10S10

Lecture10S10 - BIS101/Engebrecht 4/20/10 Lecture10...

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BIS101/Engebrecht 4/20/10 Lecture10 Announcements: Midterm I room assignments: Last Name A-N = Haring 2205; O-Z = Storer 1322. Bring blue xcantrons and number 2 pencils! I reviewed conjugation mapping. Since the probability of a gene being transferred is a function of distance from the origin, it follows that genes closer to the origin are transferred first, and those farther away from the origin are transferred later. Thus, it is possible to do interrupted matings over time to establish a rough map of the bacterial genome. Please see Handout09 posted on the web, (Please note that the nomenclature for this is: if leu+, then prototrophic for leucine biosynthesis and if leu-, then auxotrophic). We saw that although interrupted matings give a rough gene order, two closely linked genes will be transferred at approximately the same time. How can we determine the order of the genes in this situation? By examining the recombinant phenotypes after a defined mating experiment (please refer to the Handout09 ) . To do these experiments, you need to first select for the last marker you are interested in - in our example, this was the leu+ gene. You then examine the phenotypes among the leu+ colonies to determine the genotype of the other markers. Remember that in order to maintain the circular chromosome, you need to have an even number of recombination events. Also remember that you must understand the media you are growing the cells on to determine what genes are recombining. If you grow on medium with added leucine and arginine but no methionine, you are looking at the cells that have become met+. Just like we saw with the 3point crosses in eukaryotic mapping, it is the events where there are multiple crossovers that delineate gene order. In this case, you need to examine the quadruple crossover class to distinguish which gene is in the middle – this will be the least frequent class. The frequency of the recombinants also define the map distance. This is expressed as mu and not cM because cM is only applicable to meiotic mapping. In our example, the % recombinants were 5 and therefore, the genes were 5 map units apart. CQ1: Since his+ enters last, you must plate the conjugates on medium lacking histidine and containing streptomycin (to select against the donor) = Answer C . CQ2: To find the correct gene order, look at the least frequent recombinants, this will be the quadruple cross over class. Since gly has changed marker configurations but the others haven’t, gly must be in the middle so the Answer is B (but remember that either leu gly his or his gly leu would be correct). CQ3: To find the map distances you need to distinguish the recombinants between leu and gly
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This note was uploaded on 07/02/2010 for the course BIS 101 taught by Professor Simonchan during the Spring '08 term at UC Davis.

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Lecture10S10 - BIS101/Engebrecht 4/20/10 Lecture10...

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