AP Biology FRQ Practice

Practice FRQ 1 of 20

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.

A. 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.

A single-panel line graph showing three DNA melting (denaturation) curves.

Axes (must be exactly as specified):
- X-axis label: "Temperature (°C)".
- X-axis numeric range: from 50 to 100.
- X-axis tick marks and labels: 50, 60, 70, 80, 90, 100 (tick interval of 10 °C).
- Y-axis label: "Relative Absorbance at 260 nm".
- Y-axis numeric range: from 1.0 to 1.5.
- Y-axis tick marks and labels: 1.0, 1.1, 1.2, 1.3, 1.4, 1.5 (tick interval of 0.1).
- Origin requirement: the axes intersection is labeled "0" at the lower-left corner of the plotting area, even though the plotted x- and y-ranges begin at 50 and 1.0; the "0" label appears at the intersection as a standalone origin label.
- Arrows: arrows on the positive ends of both axes.
- No gridlines.

Data sampling locations (must be visibly implied by error bars):
- Error bars are present at exactly six x-axis positions aligned with the labeled ticks: 50, 60, 70, 80, 90, and 100 °C for each species.

General layout constraints:
- Three curves occupy the same axes and do not overlap at their steepest regions; the transitions are separated along the x-axis (Species A leftmost transition, Species B middle, Species C rightmost).
- All three curves share the same low-temperature baseline and the same high-temperature plateau.

Low-temperature baseline (all three species):
- From 50 °C through 60 °C, each curve is nearly horizontal and lies exactly on the y=1.0 baseline, indicating fully double-stranded DNA at low temperature.

High-temperature plateau (all three species):
- From 90 °C through 100 °C, each curve is nearly horizontal and lies exactly on the y=1.4 plateau, indicating predominantly single-stranded DNA at high temperature.

Species A curve (solid line):
- Line style: solid black line, medium thickness.
- Starts on the left edge at 50 °C at y=1.0, with a horizontal tangent (flat baseline).
- Remains flat at y=1.0 until the tick at 60 °C.
- Between 60 °C and 70 °C: a smooth S-shaped (sigmoidal) rise.
- The curve is concave up (U-shaped local curvature) in the early part of this interval, then passes through a single inflection point exactly at the midpoint of this 60–70 segment (halfway between the 60 and 70 tick marks), then becomes concave down (∩-shaped local curvature) in the latter part of the interval.
- At the inflection point, the curve crosses exactly halfway between the baseline and plateau (halfway between y=1.0 and y=1.4), i.e., it crosses y=1.2.
- After 70 °C: the curve levels to a horizontal plateau; by 80 °C it is exactly at y=1.4 and stays exactly at y=1.4 through 100 °C.

Species B curve (dashed line):
- Line style: black dashed line, same thickness as Species A.
- Starts on the left edge at 50 °C at y=1.0 with a horizontal tangent.
- Remains flat at y=1.0 through the tick at 70 °C.
- Between 70 °C and 80 °C: a smooth S-shaped (sigmoidal) rise.
- Concave up in the early part of the 70–80 interval, then a single inflection point exactly halfway between the 70 and 80 tick marks, then concave down in the late part of the interval.
- At that inflection point, the curve crosses exactly halfway between y=1.0 and y=1.4, i.e., y=1.2.
- After 80 °C: the curve levels off; by 90 °C it is exactly at y=1.4 and stays exactly at y=1.4 through 100 °C.

Species C curve (dotted line):
- Line style: black dotted line, same thickness as Species A.
- Starts on the left edge at 50 °C at y=1.0 with a horizontal tangent.
- Remains flat at y=1.0 through the tick at 80 °C.
- Between 80 °C and 90 °C: a smooth S-shaped (sigmoidal) rise.
- Concave up in the early part of the 80–90 interval, then a single inflection point exactly halfway between the 80 and 90 tick marks, then concave down in the late part of the interval.
- At that inflection point, the curve crosses exactly y=1.2 (the midpoint between baseline and plateau).
- After 90 °C: the curve is exactly at y=1.4 and remains perfectly horizontal through 100 °C.

Error bars (visual and numeric constraints):
- At each of the six temperature ticks for each species, a vertical error bar is centered on the curve value.
- Each error bar has a small horizontal cap at the top and bottom.
- All error bars for all species have identical total height.
- The error bar half-length equals exactly one-fifth of the y-axis tick interval (so each half-length is one-fifth of 0.1), making the bars visibly small relative to the 0.1 spacing, and consistent across all points.

Legend/labeling:
- Include a legend inside the plotting area in the upper-left corner.
- Legend entries match line styles exactly and are labeled: "Species A" (solid), "Species B" (dashed), "Species C" (dotted).
- No other text on the plot besides axes labels, tick labels, the origin "0" label, and the legend.

Curve behavior summary (must be visually enforced):
- Each species shows exactly one sigmoidal transition with exactly one inflection point located at the midpoint of its designated 10 °C transition window.
- Baselines and plateaus are strictly horizontal segments aligned exactly with y=1.0 and y=1.4, respectively.

i. Identify the dependent variable in the experiment shown in Figure 1.

ii. 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.

iii. 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.

A single-panel vertical bar graph comparing GC content among three species.

Axes (must be exactly as specified):
- X-axis label centered below categories: "Species" (no units).
- X-axis categories (left to right, evenly spaced): "Species A", "Species B", "Species C".
- Y-axis label: "GC Content (%)".
- Y-axis numeric range: from 0 to 100.
- Y-axis tick marks and labels: 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 (tick interval of 10%).
- Origin requirement: the intersection at the bottom-left is labeled "0".
- Arrows: arrows on the positive ends of both axes.
- No gridlines.

Bars (exact heights and styling):
- Three solid-fill rectangular bars with identical width.
- All bars start exactly at the y=0 baseline.
- Species A bar height reaches exactly the y=30 tick mark.
- Species B bar height reaches exactly the y=50 tick mark.
- Species C bar height reaches exactly the y=70 tick mark.
- Bar fills are grayscale: Species A light gray, Species B medium gray, Species C dark gray (or all the same gray if color is not available), with thin black outlines.

Error bars (exact constraints):
- Each bar has a single vertical error bar centered at the bar’s top.
- Each error bar has a horizontal cap at the top and bottom.
- The error bar half-length is exactly 2 percentage points for all three bars.
- Therefore, the Species A error bar extends from exactly 2 units below the 30% bar top to exactly 2 units above it; likewise for Species B centered on 50% and Species C centered on 70%.
- Error bars are thin black lines and identical in length across all bars.

Text/labels:
- Category labels "Species A", "Species B", "Species C" appear directly under each bar.
- No numeric value labels printed on bars (students read values from axis ticks).
- No legend (categories are already labeled on the x-axis).

Geometric layout constraints:
- Spacing between bars is uniform.
- Bars occupy the central region of the plotting area, leaving clear margins on left and right so that no bar touches the y-axis or the right boundary.

i. Identify the independent variable in the experiment shown in Figure 2.

ii. Based on Figure 2, identify the species with the lowest percentage of GC base pairs.

iii. Based on the data in Figure 2, calculate the percentage of thymine (T) nucleotides in the DNA of Species C.

i. 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.

ii. Justify the correlation between GC content and DNA melting temperature based on the chemical structure of DNA base pairs.