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Answers to Text Questions
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Inquiry Questions

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Self Test

1). Which of the following experiments suggested that the nucleus is the repository for genetic information?
    a). Hammerling's experiment using Acetabularia
    b). Griffith's experiment using S. pneumoniae and mice
    c). the Hershey and Chase experiment using bacteriophages
    d). Franklin's X-ray diffraction
Answer: a

2). When Hershey and Chase differentially tagged the DNA and proteins of bacteriophages and allowed them to infect bacteria, what did the viruses transfer to the bacteria?
    a). radioactive phosphorus and sulfur
    b). radioactive sulfur
    c). DNA
    d). Both b and c are correct.
Answer: c

3). If one strand of a DNA molecule has the base sequence ATTGCAT, its complementary strand will have the sequence
    a). ATTGCAT
    b). TAACGTA
    c). GCCATGC
    d). CGGTACG
Answer: b

4). DNA is made up of building blocks called
    a). proteins.
    b). bases.
    c). nucleotides.
    d). deoxyribose.
Answer: c

5). X-ray diffraction experiments conducted by _____________ led to the determination of the structure of DNA.
    a). Francis Crick
    b). James Watson
    c). Erwin Chargaff
    d). Rosalind Franklin
Answer: d

6). Meselson and Stahl proved that
    a). DNA is the genetic material.
    b). DNA is made from nucleotides.
    c). DNA replicates in a semiconservative manner.
    d). DNA is a double helix held together with base-pairing.
Answer: c

7). DNA polymerase III can only add nucleotides to an existing chain, so _________________ is required.
    a). an RNA primer
    b). DNA polymerase I
    c). helicase
    d). a DNA primer
Answer: a

8). Okazaki fragments are
    a). synthesized in the 3' to 5' direction.
    b). found on the lagging strand.
    c). found on the leading strand.
    d). assembled as continuous replication.
Answer: b

9). Beadle and Tatum's experiment showed that each enzyme is specified by a single
    a). chromosome.
    b). gene.
    c). nucleotide.
    d). mutation.
Answer: b

10). What was significant about Ingram's work on sickle cell anemia?
    a). The gene for hemoglobin was missing.
    b). Proteins consisted of sequences of amino acids.
    c). A change in one amino acid can affect a protein's structure.
    d). One gene encodes one protein.
Answer: c

Test Your Visual Understanding

1). Based on the information in this figure, can you explain why pyrimidines don't base-pair with each other, and why purines don't base-pair with each other?
Answer: In order that the DNA maintain a diameter of about 2 nm across the double helix, it is necessary the smaller pyrimidines base pair with the larger purines. If two purines were to base-pair, the diameter of the double helix would be larger than 2 nm. Likewise, if two pyrimidines were to base-pair, the diameter of the double helix would be smaller than 2nm.

2). Can you also explain why adenine does not base-pair with cytosine. and why thymine does not base-pair with guanine?
Answer: Hydrogen bonding, which holds the two DNA strands together, determines which nucleotides will form complementary base-pairs. Adenine is able to form two hydrogen bonds and so it lines up with thymine, which is also able to form two hydrogen bonds. Cytosine, which is a smaller nucleotide like thymine, does not base-pair with adenine because their hydrogen-bonding atoms would not line up correctly. Likewise, guanine is able to form three hydrogen bonds and so it lines up with cytosine. which is also able to form three hydrogen bonds. Guanine, which is larger like adenine, does not-base pair with thymine because their hydrogen-bonding atoms would not line up correctly.

Apply Your Knowledge

1). The human genome contain approximately 3 billion (3 x 109) nucleotide base-pairs, and each nucleotide in a strand of DNA takes up about 0.34 nanometers (0.34 x 10-9 meters). How long would the human genome be if it were fully extended?
Answer: 3 x 109 base-pairs x 0.34 x 10-9 meters or:
(3 x 109) x (0.34 x 10-9) = 1.02 x 100 meters for 1.02 meters long

2). From an extract of human cells growing in tissue culture, you obtain a white, fibrous substance. How would you distinguish whether it was DNA, RNA, or protein?
Answer: First you could test the substance for the presence of amino acids; if present, the substance is a protein. If the substance does not contain amino acids, it is one of the two nucleic acids. To determine which nucleic acid, you can either test the substance for the presence of ribose or deoxyribose or you can test the substance for the presence of thymine or uracil. The presence of thymine or deoxyribose indicates the substance is DNA. The presence of uracil or ribose indicates the substance is RNA.

3). Cells were obtained from a patient with a viral infection. The DNA extracted from these cells consisted of two forms: double-stranded human DNA and single-stranded viral DNA. The base compositions of these two forms of DNA were as follows:

ACGT
Form 122.1%27.9%27.9%22.1%
Form 231.3%31.3%18.7%18.7%

Which form was the viral DNA, and which form was the human DNA? Explain your reasoning.
Answer: The single-stranded viral DNA does not have complementary base-pairing and so one would not expect the viral DNA to follow the Chargaff's rule of base-pairing. That being the case, Form 2 would be the viral DNA because the adenine and thymine are not present in equal proportions and the cytosine and guanine are not in equal proportions. In Form 1, the adenine and thymine are in equal proportions indicating that they are base-paired and the cytosine and guanine are in equal proportions indicating that they are base-paired. Therefore Form 1 is human DNA.








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