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🧬AP Biology
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🧬AP Biology

FRQ 3 – Scientific Investigation (Short)
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Unit 1: Chemistry of Life
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Practice FRQ 1 of 131/13

3. Certain bacteria, such as Thermus aquaticus, thrive in high-temperature environments like hot springs, while others, such as Escherichia coli, inhabit moderate environments like the mammalian gut. The stability of biological macromolecules, including DNA, is critical for survival in these diverse habitats.

Scientists conducted an experiment to investigate the thermal stability of DNA from these two species. They extracted purified DNA from T. aquaticus and E. coli and suspended samples of each in a buffer solution. The scientists slowly heated the DNA samples from 25°C to 95°C while monitoring the absorbance of UV light. As the two strands of the DNA double helix separate (denature), UV absorbance increases. The temperature at which 50% of the DNA strands have separated is defined as the melting temperature (TmT_mTm​). The scientists observed that the genomic DNA of T. aquaticus has a significantly higher TmT_mTm​ than the genomic DNA of E. coli.

A.

Describe the specific chemical interaction that connects the nitrogenous bases of complementary DNA strands and holds the double helix together.

B.

Identify one variable, other than the rate of heating, that the scientists must keep constant between the two DNA samples to ensure the validity of the results.

C.

State the null hypothesis for the experiment comparing the melting temperatures (TmT_mTm​) of the DNA from the two bacterial species.

D.

The scientists claim that the DNA of T. aquaticus contains a higher percentage of guanine-cytosine (G-C) base pairs than the DNA of E. coli. Based on the chemical structure of DNA, justify the scientists' claim.

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Free Response Question Practice

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