The Diversity of Species in Bacterial Communities is Often Studied by Phenotypic Characterization GMAT Reading Comprehension

Reading Passage Question

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 diversity of species in bacterial communities is often studied by phenotypic characterization is a GMAT reading comprehension passage with answers. Candidates need a strong knowledge of English GMAT reading comprehension.

This GMAT Reading Comprehension consists of 6 comprehension questions. The GMAT Reading Comprehension questions are designed for the purpose of testing candidates’ abilities in understanding, analysing, and applying information or concepts. Candidates can actively prepare with the help of GMAT Reading Comprehension Practice Questions.

Solution and Explanation

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

1. the process has been demonstrated to manifest second-order reaction kinetics exactly
2. the time it takes the genetic material to reassociate reflects the diversity in the sample
3. the level of genetic diversity in a typical soil sample is sufficiently high for the process to work ideally
4. the genetic material in such a soil sample does not contain repetitive DNA
5. the ecosystem under study is not subject to extreme climatic conditions

2. The passage suggests that, in the context of thermal denaturation and reassociation, a bacterial community is comparable to

1. A snapshot taken by fluorescence microscope
2. A multiple of 200 of the number of species that can be cultured
3. A sample of repetitive DNA from non-bacterial organisms
4. A single haploid cell
5. A single species

3. With which of the following statements would the author of the passage most likely agree?

1. Isolating and culturing specific bacteria is an inferior method of determining the number of species in a soil sample, but it may be useful to study certain culturable species more directly than is possible by thermal denaturation.
2. Isolating and culturing specific bacteria no longer has any useful application, especially in environments with pollution or extreme conditions.
3. Isolating and culturing specific bacteria is a superior but unfeasible method of studying bacteria communities.
4. Isolating and culturing specific bacteria might be useful to obtain a basic estimate of the number of species in a sample that can then be confirmed through the thermal denaturation method.
5. Isolating and culturing specific bacteria, while not always possible, might be useful to give estimates of the number of species in bacteria communities that fail to exhibit second-order reaction kinetics.

4. The author mentions that renaturation of the sample taken in Bergen, Norway did not follow ideal second-order kinetics in order to

1. highlight that aspects of the renaturation method remain unproven
2. point out a difference between the expected and the actual characteristics of the genetic material in the sample
3. emphasize the superiority of the renaturation method over isolating and culturing bacteria
4. imply that the genetic diversity in the sample might be even higher than the reported value
5. identify a limitation of the usefulness of the renaturation method

5. The passage suggests that the whether the thermal denaturation method can be applied in a specific environment depends primarily on which of the following considerations?

1. Whether the environment is free of pollution and extreme conditions
2. Whether the average DNA complexity of the species present is sufficiently low
3. Whether a majority of the bacteria in that environment can be cultured
4. Whether the bacteria in that environment follow second-order reaction kinetics
5. Whether the bacterial DNA in a terrestrial or water sample from that environment can be isolated

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

1. the average frequency, by mass, of pollutants present in a sample
2. the maximum half-life of reassociation for the bacterial community
3. the average size of the genome of a species in the community
4. the number of nonrepeating nucleotides in the DNA of a haploid cell
5. the percentage of species in the community that are culturable

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