Bianci and Kowalczykowski reported their studies of the Chi sequence in a 1997 p
ID: 213802 • Letter: B
Question
Bianci and Kowalczykowski reported their studies of the Chi sequence in a 1997 paper in the USA Proceedings of the National Academy of Sciences (PNAS: June 24, 1997.94 (13) 6706-6711). To recognize Chi, the RecBCD enzyme must identify this 8-nt element uniquely, while translocating at 1,000 bp/sec. group used a DNA containing two adjacent Chi sites. To study RecBCD interaction with Chi, in one experiment, they created a recombinant plasmid containing two copies of the Chi sequence located adjacent to each other. Which of the following explanations is the most likely reason for using a construct with two copies of Chi? A) The Chi sequence serves as a signal to the RecBCD helicase-exonuclease that triggers a major change in the activities of this enzyme. Upon encountering the Chi sequence as it unwinds DNA, RecBCD cuts the DNA a few nucleotides to the 3' side of Chi; depending on the reaction conditions, this cut is either a simple nick on one strand or the change of nuclease activity from cutting the 3'-ended strand to cutting the 5'ended strand. By having two Chi sites, the DNA is cut on both strands B) Because recombination can occur with small probability at any location along chromosome, the between two locations depends on their distance. T hera ioreugh to frequency of recombination between two locations depends on their distance. Therefore, for genes sufficiently distant on the same chromosome the amount of crossover is high enough to destroy the correlation between alleles C Two chi sites are better than one -double the pleasure, double the fun D) RecA, the chief recombinase found in Escherichia coli, is responsible for the repair of DNA double strand breaks (DSBs). In yeast and other eukaryotic organisms there are two recombinases required for repairing DSBs. The RAD51 protein is required for mitotic and meiotic recombination, whereas the DMC1 protein is specific to meiotic recombination. The two chi sites is more like a meiotic organization E) Not all RecBCD molecules are altered when they encounter a Chi site. Having two such sites in the substrate DNA increased the probability that all RecBCDs in the experiment would be affected, creating a more homogenous population of RecBCD, and thus making it easier to measure changes in RecBCD at ChiExplanation / Answer
The answer is letter B, it was the way of the researchers to prove that the system led by the RecBCD enzyme works as soon it recognizes the Chi site, which generates a genetic recombination, creating this close sites, although really they are more separated each other.
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