AP Biology Scoring Guidelines from the 2018 Exam ...

2018

AP Biology

Scoring Guidelines

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AP? BIOLOGY 2018 SCORING GUIDELINES

Question 1

Figure 1. Phylogenetic tree representing the evolutionary relatedness among bear populations based on mitochondrial DNA sequence comparisons

Polar bears are highly adapted for life in cold climates around the North Pole. Brown bears, black bears, and pandas are found in warmer environments. Researchers collected complete mitochondrial DNA sequences from several populations of bears and constructed a phylogenetic tree to represent their evolutionary relatedness (Figure 1).

A researcher studying adaptation in bears sequenced the nuclear gene encoding a lysosomal trafficking protein (LYST) in polar bears, brown bears, black bears, and panda bears. There are seven inferred amino acid substitutions that are found only in polar bears. Mutations that cause similar substitutions in the human LYST protein are associated with Chediak-Higashi syndrome, an autosomal recessive condition in which pigment is absent from the hair and eyes. The researcher used the inferred amino acid sequences to build the distance matrix shown in Table 1.

TABLE 1. AMINO ACID DIFFERENCES IN THE LYST PROTEIN AMONG BEAR SPECIES

Panda Black Brown Polar

Panda

-

Black

33

-

Brown

34

1

-

Polar

40

7

8

-

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AP? BIOLOGY 2018 SCORING GUIDELINES

Question 1 (continued)

(a) Use the phylogenetic tree in Figure 1 to estimate the age in hundreds of thousands of years of the most recent common ancestor of all brown bears. Identify the population of brown bears to which polar bears are most closely related based on the mitochondrial DNA sequence comparison. Identify two populations whose positions could be switched without affecting the relationships illustrated in the phylogenetic tree.

Estimate (1 point) ? First two digits of the answer must be between 30 and 35.

Identification (1 point) ? European

Identification (1 point) ? European/Polar OR Asian/Western (North American)

(b) Construct a cladogram on the template to represent a model of the evolutionary relatedness among the bear species based on the differences in LYST protein sequences (Table 1). Circle the position on the cladogram that represents the out-group.

Construction (1 point) ? Correctly illustrated evolutionary relationship among the four species

Circling (1 point) ? Correctly circled out-group based on orientation of cladogram

(c) A student claims that mitochondrial DNA sequence comparisons provide a more accurate phylogeny of bear species than do LYST protein sequence comparisons. Provide ONE piece of reasoning to support the student's claim.

Reasoning (1 point) ? Genes show more variability (in nucleotide sequence) than proteins do (in amino acid sequences). ? mtDNA genome contains multiple genes vs. one lyst gene. ? The phenotype associated with the lyst gene is under strong selection.

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AP? BIOLOGY 2018 SCORING GUIDELINES

Question 1 (continued)

(d) A researcher genetically engineers a mouse strain by deleting the mouse lyst gene and replacing it with the polar bear lyst gene. Predict the most likely difference in phenotype of the transgenic mouse strain compared to the wild-type mouse strain. Justify your prediction.

Prediction (1 point) ? Mouse fur and/or eyes will not have pigment/will have reduced pigment. ? Mouse (fur) will be white/lighter.

Justification (1 point) ? Polar bear lyst gene/LYST protein is associated with a lack of pigment/white hair. ? Mutated human lyst gene/ LYST protein is associated with a lack of pigment in hair and eyes.

(e) Describe how the mutation in the lyst gene became common in the polar bear population. If the lyst gene were the only determinant of fur color, predict the percent of white offspring produced by a mating between a polar bear and a brown bear.

Description (1 point) ? Natural selection for the white fur phenotype

Prediction (1 point) ? 0%

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AP? BIOLOGY 2018 SCORING GUIDELINES

Question 2

Figure 1. Cellular response to infection by pathogenic bacteria

Some pathogenic bacteria enter cells, replicate, and spread to other cells, causing illness in the host organism. Host cells respond to these infections in a number of ways, one of which involves activating particular enzymatic pathways (Figure 1). Cells normally produce a steady supply of inactive caspase-1 protein. In response to intracellular pathogens, the inactive caspase-1 is cleaved and forms an active caspase-1 (step 1). Active caspase-1 can cleave two other proteins. When caspase-1 cleaves an inactive interleukin (step 2), the active portion of the interleukin is released from the cell. An interleukin is a signaling molecule that can activate the immune response. When caspase-1 cleaves gasdermin (step 3), the N-terminal portions of several gasdermin proteins associate in the cell membrane to form large, nonspecific pores.

Researchers created the model in Figure 1 using data from cell fractionation studies. In the experiments, various parts of the cell were separated into fractions by mechanical and chemical methods. Specific proteins known to be located in different parts of the cell were used as markers to determine the location of other proteins. The table below shows the presence of known proteins in specific cellular fractions.

CELL FRACTIONS CONTAINING DIFFERENT CELLULAR PROTEINS

Whole cell sample

Aconitase

(Krebs cycle protein)

DNA polymerase

+

+

GAPDH (glycolytic

protein)

+

Sodiumpotassium

pump

+

NF-kB (Immune response protein)

+

Fraction 1

+

Fraction 2

+

+

Fraction 3

+

+

Fraction 4

+

+ = presence of protein

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