Bacteria in Soil I

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     The diversity of species in bacterial communities is often studied by phenotypic characterization. A problem with this method is that phenotypic methods can be used only on bacteria which can be isolated and cultured, and most soil bacteria that have been observed by fluorescence microscope cannot be isolated and cultured.
     DNA can be isolated from bacteria in soil to obtain genetic information about the nonculturable bacteria therein. The heterogeneity of this DNA is a measure of the total number of genetically different bacteria, or the number of species. DNA heterogeneity can be determined by thermal denaturation and reassociation. In general, renaturation of homologous single-stranded DNA follows second-order reaction kinetics. In other words, the fraction of DNA that has renatured within a given time period is proportional to the genome size or the complexity of DNA, defined as the number of nucleotides in the DNA of a haploid cell, without repetitive DNA. The genetic diversity of a bacterial community can be inferred in a similar manner.
     Vigdis Torsvik, Jostein Goksøyr, and Frida Lise Daae used this process to analyze soil samples taken from the soil from a beech forest north of Bergen, Norway. The reassociation curves for the main DNA fraction did not follow ideal second-order reaction kinetics, so the half-life values gave only approximate, underestimated values for the number of genomes present. Nevertheless, the soil bacterium DNA was very heterogeneous; the diversity corresponded to about 4,000 distinct genomes of a size typical of standard soil bacteria. This diversity was about 200 times as many species as could have been isolated and cultured.
     Various procedures for isolating DNA from river sediments and seawater are known. This opens up the possibility of applying the thermal denaturation method to systems other than soil. The results of the Norway study indicated that the genetic diversity of the total bacterial community in a deciduous-forest soil is so high that heterogeneity can be determined only approximately. In environments with pollution or extreme conditions, the genetic diversity might be easier to determine precisely.                

According to the passage, the primary reason that thermal denaturation and reassociation gives information about the genetic diversity in a soil sample is that

Review: Bacteria in Soil I


Explanation

This question asks about the denaturation and reassociation method, since we know that the evidence supporting our answer will reside in the second paragraph, where the method is described. As we have discussed, the short answer is "time," which is roughly equal in the passage to the "second-order kinetics" (lines 15-19). Choices (A) and (B) speak to time/second-order kinetics, but we know from the third paragraph that the process did not perfectly demonstrate second-order kinetics. So our answer is almost certainly (B). Choice (C) contradicts the fact that this process works for a single cell, as described in the second paragraph. Choice (D) is not assumed or required by the passage; nor would it appear to be possible, since the sample is taken from nature. The "extreme conditions" of (E) are mentioned only at the end of the passage, but in that line the author states that it will be easier to apply the method when there are extreme conditions.

The correct answer is (B).


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