🧬AP Biology
🧬AP Biology
FRQ 1 – Interpreting and Evaluating Experimental Results (Long)
Unit 1: Chemistry of Life
Practice FRQ 1 of 201/20
1. DNA is the primary genetic material in most living organisms and consists of two strands of nucleotides wound into a double helix.
The stability of the DNA double helix is maintained by interactions between the nitrogenous bases of the two strands. Researchers investigated the factors that influence the stability of the DNA molecule by measuring the melting temperature (), which is the temperature at which 50% of the double-stranded DNA molecules separate into single strands.
In their first experiment, the researchers synthesized DNA fragments with varying percentages of guanine and cytosine (GC) content. They dissolved the DNA in a buffer solution and slowly heated the samples while monitoring the separation of the strands. The melting temperature () for each sample was recorded (Figure 1).
The researchers then isolated genomic DNA from two different bacterial species: Escherichia coli, which is typically found in the intestines of mammals, and Thermus aquaticus, which is found in hot springs. They analyzed the nucleotide composition of the DNA from each species (Figure 2).
A. Describe the structural feature of the DNA molecule that allows the two strands of the helix to separate during the process of replication.
Figure 1. Effect of GC content (%) on the melting temperature (Tm, °C) of synthetic DNA fragments. Each marker is the mean Tm; vertical error bars show ±2SE x . Values increase by exactly 7 0C for each 20% increase in GC content.
B.
i. Identify the dependent variable in the experiment shown in Figure 1.
ii. Justify why the researchers included a synthetic DNA fragment with 0% GC content in their experiment.
iii. Based on Figure 1, describe the relationship between the percentage of GC content and the melting temperature of the DNA.
Figure 2. Average percentage of each nucleotide base (A, C, G, T) in genomic DNA of Escherichia coli and Thermus aquaticus. Each bar is the mean percent; vertical error bars show 11%. Base totals per species sum to exactly 100%.
C.
i. Identify the independent variable in the researchers' second experiment (data shown in Figure 2).
ii. Based on Figure 2, identify the bacterial species that has the higher percentage of Cytosine nucleotides in its genome.
iii. The genome of Thermus aquaticus is approximately nucleotides in length. Based on the data in Figure 2, calculate the number of Guanine nucleotides in the genome of Thermus aquaticus.
D.
i. Researchers claim that Thermus aquaticus is better adapted to live in high-temperature environments than Escherichia coli. Using data from Figure 1 and Figure 2, support the researchers' claim.
ii. Justify the researchers' claim based on the chemical structure of the DNA bases and the types of bonds that hold them together.
