🧬AP Biology
1. DNA is a double-stranded molecule that serves as the primary genetic material in most organisms. The stability of the DNA double helix structure is maintained by specific chemical bonds between the nitrogenous bases of the two strands.
Researchers investigated the thermal stability of DNA isolated from three different bacterial species: Species A, Species B, and Species C. They suspended the purified DNA from each species in a buffer solution with a pH of 7.0. To measure the stability of the DNA, the researchers slowly heated the samples from 50°C to 100°C while measuring the absorbance of UV light at 260 nm.
Single-stranded DNA absorbs more UV light than double-stranded DNA. Therefore, an increase in absorbance indicates that the hydrogen bonds holding the two strands together are breaking, a process known as denaturation. The temperature at which 50% of the DNA is denatured is referred to as the melting temperature (Tm). The results of the experiment are shown in Figure 1.
The researchers hypothesized that the differences in melting temperature were due to the nucleotide composition of the DNA from each species. They sequenced the DNA to determine the percentage of guanine (G) and cytosine (C) nucleotides. The data from this analysis are presented in Figure 2.
Describe the role of hydrogen bonds in the structure of the DNA double helix.
Figure 1. Effect of temperature on the relative absorbance of DNA from three bacterial species measured at 260 nm. Error bars show ±SE at each sampled temperature.
Identify the dependent variable in the experiment shown in Figure 1.
Justify the researchers' decision to use a buffer solution with a constant pH of 7.0 for all DNA samples in the experiment shown in Figure 1.
Based on Figure 1, describe the relationship between temperature and DNA structure for Species B.
Figure 2. Average GC base-pair percentage in DNA from three bacterial species. Error bars show ±SE.
Identify the independent variable in the experiment shown in Figure 2.
Based on Figure 2, identify the species with the lowest percentage of GC base pairs.
Based on the data in Figure 2, calculate the percentage of thymine (T) nucleotides in the DNA of Species C.
The researchers claim that Species C is likely an extremophile adapted to live in a high-temperature environment, such as a hot spring. Using data from Figure 1 and Figure 2, support the researchers' claim.
Justify the correlation between GC content and DNA melting temperature based on the chemical structure of DNA base pairs.
