Bacteria in Soil VI

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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.                

The passage suggests that employing the thermal denaturation and reassociation method entails assuming a value for which of the following?

Review: Bacteria in Soil VI


Explanation

This question asks for something which, on first blush, may appear to have no basis in the passage. We can begin by approaching the answer choices armed with our current understanding of the passage and its main points; we will then refine by ruling in and out answer choices based on specific lines from the passage. In choice (A), pollution is wrong, as it has been in every question so far; the method works at low and high levels of pollution. So (A) is out. Choice (E) is inaccurate; the point of the thermal method is that it doesn't require species to be culturable (and it doesn't require them not to be culturable). So (E) is out. We are left with (B), (C) and (D). One way to evaluate them is to consider the most unique word in each answer choice and consider whether and where they are discussed in the passage. Being overly literal would be risky, but the point is to look for objective grounding for a statement. Does the passage discuss "half-lives," "genome sizes," or "nucleotides"? In lines 19 and 20, the genome size is defined as the number of nucleotides. Genome size appears again in lines 32-34: "the diversity corresponded to about 4,000 distinct genomes of a size typical of standard soil bacteria." This detail is important, because the passage is indicating that the number of distinct genomes, or different species, has been inferred based on the "size typical of standard soil bacteria." That typical size is the "value" referred to in the question.

Therefore, the correct answer is (C).


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