AP Biology Student Samples from the 2023 Exam Administration

2023

Biology

Sample Student Responses

and Scoring Commentary

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Free-Response Question 6

 Scoring Guidelines

 Student Samples

 Scoring Commentary

AP® Biology 2023 Scoring Guidelines

Question 6: Analyze Data 4 points

Housekeeping genes encode proteins involved in universally important processes such as transcription,

translation, and glycolysis. Because these genes appear to be expressed in all cells at constant levels, the

expression of housekeeping genes is often used as a control when comparing how the expression of other

genes varies under different conditions.

Researchers studying the effect of pesticides on declining bee populations wanted to determine whether the

expression of four housekeeping genes (GAPDH, RPL32, RPS5, and TBP-AF) was in fact constant in bees across

different variables. The researchers collected samples of mRNA for each of the four genes and compared

how their expression varied across the developmental stage of the bee, the sex of the bee, and the cell type

from which the sample was taken. The mRNA from the samples was reverse transcribed to produce DNA

copies of each gene. PCR was then used to amplify the DNA, and the Cq value was determined. The Cq value

is the number of PCR cycles needed to produce a specified number of DNA copies (Figure 1). A high Cq value

for a sample indicates the gene was expressed at a low level.

To analyze whether any of the examined variables affected expression of the housekeeping genes,

researchers examined the range of Cq values for each gene in response to each variable. Genes with a wide

range of Cq values were determined to be affected by the variable, while genes with a narrow range of Cq

values were determined to be unaffected by the variable.

Figure 1. The effect of developmental stage, sex, and cell type on the Cq value of four housekeeping genes

AP® Biology 2023 Scoring Guidelines

(a)

Based on the data in Figure 1, identify the gene that had the lowest median Cq value

when bees of different developmental stages were compared.

• RPS5

1 point

(b)

The Cq value is inversely proportional to the amount of mRNA from that gene in the

starting sample. Based on the data in Figure 1, identify the gene that has the lowest level

of gene expression regardless of variable.

• TBP-AF

1 point

(c)

The scientists investigated the effect of pesticides on the expression of other genes in one

cell type of a group of bees containing males and females of the same developmental

stage. They hypothesized that TBP-AF would serve as the best control gene for this

experiment. Use the data to evaluate their hypothesis.

• Their hypothesis is supported because TBP-AF has the smallest Cq range/most

constant expression (when comparing sexes).

1 point

(d)

Explain how expression of a gene such as GAPDH can vary from one cell type to another

within the same bee.

• Different cell types contain different levels of/different transcription factors, and

therefore regulate the expression of genes in different ways.

1 point

Total for question 6

4 points

Q6 Sample A 1 of 1

Q6 Sample B 1 of 1

Q6 Sample C 1 of 1

Biology 2023 Scoring Commentary

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Question 6

Note: Student samples are quoted verbatim and may contain spelling and grammatical errors.

Overview

Question 6 described housekeeping genes that tend to be expressed at constant levels in all cells. The stimulus

also described an experiment designed to determine whether the expression of four specific housekeeping genes

was indeed constant across different developmental stages, sexes, and cell types of bees. Gene expression was

measured by collecting mRNA, performing reverse transcription, and then using PCR to amplify the DNA. This

process results in a

value, defined as “the number of PCR cycles needed to produce a specified number of

DNA copies.” Data from the experiment were presented in a box-and-whisker plot.

In part (a) students were asked to identify the gene that had the lowest median

value when comparing bees of

different developmental stages (Skill 4.B).

Part (b) explained that the

value is inversely proportional to the amount of mRNA collected. Students were

asked to identify the gene with the lowest level of gene expression (Skill 4.B; Learning Objectives [LOs]

IST-1.N, IST-2.A from the AP Biology Course and Exam Description [CED]).

Part (c) described an investigation into the effect of pesticides on gene expression in male and female bees of the

same developmental stage. Students were asked to use the data from the box-and-whisker plot to evaluate the

scientists’ hypothesis about the best control gene for the investigation (Skills 4.B and 5.D).

In part (d) students were asked to explain how expression of a specific gene can vary in different cell types of the

same bee (Skill 6D; LO IST-2.D).

Sample: 6A

Score: 4

The response earned 1 point in part (a) for identifying RPS5 as the gene with the lowest median

value. The

response earned 1 point in part (b) for identifying TBP-AF as the gene with the lowest level of gene expression. The

response earned 1 point in part (c) for correctly evaluating the hypothesis as being accurate because “the TBP-AF

gene has the smallest range of

values.” The response earned 1 point in part (d) for explaining how different

levels of gene expression in different cells are due to transcription factors, and “[c]ells that have the correct

transcription factors will ... express the GAPDH gene.”

Sample: 6B

Score: 3

The response earned 1 point in part (a) for identifying RPS5 as the gene with the lowest median

value. The

response earned 1 point in part (b) for identifying TBP-AF as the gene with the lowest level of gene expression. The

response earned 1 point in part (c) for correctly evaluating the hypothesis as being supported because TBP-AF has

the narrowest range of

values. The response did not earn a point in part (d) because it does not explain how

different levels of/different transcription factors in different cell types lead to different levels of gene expression.

Sample: 6C

Score: 2

The response earned 1 point in part (a) for identifying RPS5 as the gene with the lowest median

value. The

response earned 1 point in part (b) for identifying TBP-AF as the gene with the lowest level of gene expression. The

response did not earn a point in part (c) because, although it does state the hypothesis is supported, it does not

Biology 2023 Scoring Commentary

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correctly evaluate the hypothesis as being supported by TBP-AF having the smallest

range. The response did

not earn a point in part (d) because it does not explain how different levels of/different transcription factors in

different cell types lead to different levels of gene expression.