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 (Tm). The scientists observed that the genomic DNA of T. aquaticus has a significantly higher Tm 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 (Tm) 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.
What the Scientific Investigation FRQ asks
This is FRQ 3 on the AP Biology exam. You get a research scenario and answer four parts about the experiment: the biology behind it, its procedures, a null hypothesis or prediction, and the reasoning for that prediction.
Identify or justify a part of the experimental design
1 pt
C
Part C: Hypothesis
State a null hypothesis or predict a result
1 pt
D
Part D: Justification
Justify the prediction with biological reasoning
1 pt
How Scientific Investigation FRQ practice works
Each question follows the exam’s format, from the full prompt to a score on every part.
1
Read the full question
The scenario and all four parts, A through D, laid out the way the exam shows them.
2
Write on a 10-minute timer
The time to plan for on exam day. Pause it or turn it off, and your response saves as you go.
3
Submit for a score out of 4
Your response is scored against the scoring guidelines written for that question. Your first score is included.
Feedback on every part
A summary at the top tells you what to work on next. Below it, each part shows whether you earned the point and what the scoring guidelines were looking for.
The response correctly describes that a decrease in primary producer biomass reduces the energy available at the base of the food web, which in turn reduces energy flow to secondary consumers. The mention of the 10% rule strengthens the response, though it was not required. Full credit earned.
Part B: 0/1 point
The response incorrectly identifies glucosinolate concentration as the indicator of reproductive effort. The correct answer is the average number of flowers produced per plant. Glucosinolate concentration is a measure of defensive response, not reproductive effort. No credit earned.
Part C: 1/1 point
The response provides a correct directional prediction: Group 2 (caterpillar-infested) plants will produce fewer flowers on average than Group 1 (herbivore-free) plants. The prediction is clear and specific. Full credit earned.
Detailed feedback on a practice Scientific Investigation FRQ
Questions about the Scientific Investigation FRQ
How many responses can I get scored?
You can read every question without a plan. Your first scored response is included. A plan unlocks unlimited scoring.
How is a null hypothesis different from a prediction?
A null hypothesis says there’s no difference or relationship between the variables. A prediction states the result you expect and its direction.
How long should each part be?
Two to four focused sentences usually does it. Each part is worth 1 point, so answer every part.
How is this different from the Data Analysis FRQ?
This question asks how an experiment is designed: its variables, controls, and predictions. The Data Analysis FRQ asks you to describe data and use it to evaluate a hypothesis.
More AP Biology practice
Experimental Results FRQ practice
Long FRQs with a research scenario, its figures, and all four parts.
