3.4 B: DNA is the Heritable Material Quiz PROCTOR VERSION

1. Scientist Frederick Griffith performed an experiment in 1928 in which harmless pneumococcal bacteria were changed into a deadly strain by what Griffith identified as a “transforming principle.” The results of the Griffith experiment are shown in the diagram below.

In 1944, scientists , Colin MacLeod, and Maclyn McCarty were able to isolate the “transforming principle” and test it. Chemical analysis showed that the compound contained carbon, hydrogen, nitrogen, and phosphorus. The scientists then exposed the compound to several hydrolytic enzymes. The results of the scientists’ experiment are shown in the table below.

Page 1 of 8 3.4 B: DNA is the Heritable Material Quiz PROCTOR VERSION

Which statement best describes how the result of the Avery-MacLeod-McCarty experiment provided insight into the observation made in the Griffith experiment?

(A) The results of the Avery-MacLeod-McCarty experiment show that DNA is the “transforming principle” and that the DNA codes for RNA or proteins. Distractor Rationale: This answer suggests the student may understand that the experiment shows that DNA is the genetic material in bacteria that corresponds to the “transforming principle,” but does not understand that the results show no evidence that DNA codes for RNA or proteins, but instead show that RNA and proteins are not the genetic material in cells because the addition of enzymes that break down these components still resulted in a fully functional compound.

(B) The results of the Avery-MacLeod-McCarty experiment show that proteins cannot be the “transforming principle” since the proteins are easily broken down by many substances. Distractor Rationale: This answer suggests the student may understand that trypsin and chymotrypsin break down proteins, but does not understand that this does not indicate that protein is the source of the genetic material, because all of the substances in the experiment are easily broken down by one enzyme or another. When the proteins were changed/broken down, the compound remained functional.

(C) The results of the Avery-MacLeod-McCarty experiment show that DNA is the “transforming principle” because enzymes that disable DNA make the “transforming” compound non- functional. Rationale: This answer suggests the student understands that the experiment shows that only the enzyme that breaks down DNA can render the transforming compound non-functional; thus, DNA must be the heritable material that is passed from one cell to another, corresponding to the “transforming principle.”

(D) The results of the Avery-MacLeod-McCarty experiment show that the “transforming principle” has a double-helix structure because only the double helix can resist trypsin, chymotrypsin, and ribonuclease.

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Distractor Rationale: This answer suggests the student may understand that DNA is not broken down by trypsin, chymotrypsin, or ribonuclease, but does not understand that the results do not indicate that DNA is the genetic material, but only indicate that DNA is not affected by these substances.

Aligned to: LO 3.4 CA 3.4: Represent Genetic Information

2. In the 1930s, most scientists agreed that proteins are the genetic material in cells. Which statement would scientists from the 1930s most likely cite as the strongest evidence that proteins, and not DNA, store genetic material in cells?

(A) DNA has a simple, linear structure and is found primarily in the nucleus, whereas proteins can take on a diversity of shapes and are found throughout the cell. Distractor Rationale: This answer suggests the student may understand that proteins have greater structural and functional diversity than DNA, but does not understand that this was not cited as evidence of their ability to carry the genetic code because greater structural and functional diversity does not necessarily make the molecules better at storing information.

(B) DNA has only two levels of organization, whereas proteins have four levels of organization, allowing proteins to store a greater amount of information than DNA can. Distractor Rationale: This answer suggests the student may understand that proteins have four levels of organization, but does not understand that this was not cited as evidence of their ability to carry the genetic code because having more levels of organization does not necessarily make a better “language” for a genetic code.

(C) DNA cannot initiate changes within the cell without the help of proteins, whereas proteins can serve as enzymes and hormones, enabling proteins to interact with and cause greater change within cells than DNA can. Distractor Rationale: This answer suggests the student may understand that proteins can have multiple functions, from catalyzing reactions to serving as structural components of a cell, but does not understand that this was not cited as evidence of the ability of proteins to carry the genetic code because the code must store information, not perform a diversity of tasks.

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(D) DNA is made up of four different types of nucleotides, whereas proteins are made up of 20 different types of amino acids, enabling proteins to create many more chemical messengers than DNA can. Rationale: This answer suggests the student understands that before DNA was identified as the source of heritable information in the cell, scientists posited that since proteins have 20 different monomers, compared to four in DNA, proteins could make a better molecule for storing genetic information because 20 “letters” would allow for many more “words” in the genetic “language,” compared to only four in DNA.

Aligned to: LO 3.4 CA 3.4: Represent Genetic Information

3. In 1952, scientists Alfred Hershey and Martha Chase performed an experiment using bacteriophages (viruses that infect bacteria) to determine the source of genetic material found in organisms. Bacteriophages are composed of a protein shell and DNA. One group of bacteriophages was exposed to the radioactive isotope sulfur-35 (35S) and another group of bacteriophages was exposed to the radioactive isotope phosphorus-32 (32P). The radioactive sulfur-35 effectively labeled the protein in the bacteriophages because only the protein, not DNA, contains the element sulfur, and the radioactive phosphorus-32 effectively labeled the DNA in the bacteriophages because only DNA, not the protein, contains the element phosphorus. The bacteriophages were then allowed to infect bacterial cultures, as shown in the diagram below.

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After infection, the bacterial cultures were placed in a blender and then centrifuged to separate the phage shells and media from the bacteria. Upon analysis of the centrifuged material, the following observations shown in the table below were noted.

Which conclusion is best supported by the results of the Hershey-Chase experiment?

(A) Nucleic acids are the genetic material in cells, because phosphorus is a component of nucleic acids and is transferred from the phages to the bacterial cells. Rationale: This answer suggests the student understands that evidence of phosphorus, not sulfur, being transferred from a phage to bacteria supports the conclusion that nucleic acids, not proteins, are the genetic material in organisms.

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(B) Sulfur is an important component of genetic material in phages but not in bacterial cells, because sulfur remains in the viruses and is not observed in the bacteria. Distractor Rationale: This answer suggests the student may understand that sulfur is a component in viral proteins, but does not understand that this is not evidence of its ability to carry a genetic code because it is not the compound inserted into the bacteria by the virus.

(C) Phosphates are a key genetic material in cells, because phosphorus is transferred from the phages to bacterial cells, and phosphorus is a component of the phosphate-sugar chains that carry the genetic code. Distractor Rationale: This answer suggests the student may understand that phosphorus is found in phosphate, and that phosphates and sugar form the “backbone” of the genetic code, but does not understand that it is the nucleotide bases that make up the genetic code in nucleic acids.

(D) Phosphorus is the genetic material found in cells, because phosphorus is transferred from the phage to the bacterial cells and can therefore be used to alter cell activities. Distractor Rationale: This answer suggests the student may understand that the experiment shows the transfer of phosphorus from the phage to bacterial cells, but does not understand that this does not support the conclusion that phosphorus is the sole genetic material, but instead supports the conclusion that phosphorus is an element found in nucleic acids and provides evidence that the nucleic acids are inserted into the bacteria by the phages.

Aligned to: LO 3.4 CA 3.4: Represent Genetic Information

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4. The table below summarizes some of the most influential experiments in the study of DNA.

In 1953, scientists Watson and Crick discovered the structure of DNA. Which statement identifies and explains which of the experiments listed in the table was the most useful to Watson and Crick in determining DNA’s structure?

(A) The Hershey-Chase experiment was the most useful, because the observation that bacteriophages insert isotopes of phosphorus, but not isotopes of sulfur, into infected bacteria cells helped Watson and Crick determine the structure of the backbone in DNA’s double helix. Distractor Rationale: This answer suggests the student may understand that the Hershey-Chase experiment showed that DNA, instead of proteins, could be the genetic material, but does not understand that this information would not be useful in determining the structure of DNA because it deals only with DNA’s function as a carrier of genetic information.

(B) The Avery-MacLeod-McCarty experiment was the most useful, because the discovery that proteases and ribonucleases do not alter the molecule identified as Griffith’s “transforming principle” showed that DNA is the source of heritable information in organisms and thus helped Watson and Crick determine the significance of DNA’s double-helix shape.

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Distractor Rationale: This answer suggests the student may understand that the Avery-MacLeod-McCarty experiment shows that DNA was most likely the genetic material in organisms, but does not understand that this evidence would not be useful in determining the structure of DNA because it deals only with DNA’s function.

(C) Chargaff’s Rule 1 experiment was the most useful, because evidence that the percentages of adenine and thymine are the same and the percentages of guanine and cytosine are the same in all species helped Watson and Crick determine the structure of the DNA double helix. Rationale: This answer suggests the student understands that Chargaff’s evidence of similar amounts of AT bases, as well as GC bases, in all species would be useful data to support the conclusion by Watson and Crick that the structure of DNA is a double strand of complementary base pairs, because their discovery of complementary base pairing was made possible by Chargaff’s discovery, and complementary base pairing was the key to DNA’s structure.

(D) Chargaff’s Rule 2 experiment was the most useful, because the discovery that different species have different relative percentages of AT and CG helped Watson and Crick determine the structure of the DNA double helix in all species. Distractor Rationale: This answer suggests the student may understand that Chargaff’s discovery of variations in the relative percentages of the four bases among species shows that DNA could be a component of the genetic material in cells, but does not understand that the knowledge that percentages vary from species to species would not be useful in determining the structure of DNA.

Aligned to: LO 3.4 CA 3.4: Represent Genetic Information

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