Passage
DNA polymerization is one of the most conserved mechanisms of genome replication. Synthesis of a complete DNA strand requires a template, primers, a polymerase enzyme, and sufficient deoxyribonucleotide triphosphates (dNTPs) . The DNA polymerase enzyme binds consecutive base pairs on the template strand and extends the double helix by adding dNTPs to the primer. The amino acid residues in the active site of DNA polymerase form hydrogen bonds with Watson-Crick donors and acceptors on incoming DNA nucleotides to facilitate base pairing.The formation of the DNA double helix creates opposing changes in entropy and enthalpy. Favorable bonding interactions via hydrogen bonds during Watson-Crick base pairing results in negative enthalpy, and restricted rotation and flexibility of the DNA backbone generates negative entropy. Scientists hypothesize that hydrogen bonding between bases not only stabilizes the double helix but is also crucial for selective and efficient replication.Analogs that are similar in size and shape to naturally occurring bases can be used to determine the influence of hydrogen bonding on base pair selectivity. To mimic the structure of deoxythymidine triphosphate (dTTP) , researchers synthesized dNTP derivatives of difluorotoluene (dFTP) , a nonpolar analog that lacks Watson-Crick hydrogen bonding. Klenow fragment (KF) polymerase, which has 3′-5′ but not 5′-3′ exonuclease activity, was incubated with a mixture of DNA template, primers, and dNTPs, including dFTP derivatives. The efficiency of dFTP and natural dTTP nucleotide incorporation into a growing primer strand by KF is shown in Figure 1.
Figure 1 Template-specific selection of dFTP and dTTP by the KF enzyme
Adapted from Moran S, Ren RX, Kool ET. A thymidine triphosphate shape analog lacking Watson-Crick pairing ability is replicated with high sequence selectivity. Proc Natl Acad Sci USA. 1997;94(20) :10506-11.
-The experimental results described in the passage are inconsistent with which hypothesis?

Question

Passage
DNA polymerization is one of the most conserved mechanisms of genome replication. Synthesis of a complete DNA strand requires a template, primers, a polymerase enzyme, and sufficient deoxyribonucleotide triphosphates (dNTPs) . The DNA polymerase enzyme binds consecutive base pairs on the template strand and extends the double helix by adding dNTPs to the primer. The amino acid residues in the active site of DNA polymerase form hydrogen bonds with Watson-Crick donors and acceptors on incoming DNA nucleotides to facilitate base pairing.The formation of the DNA double helix creates opposing changes in entropy and enthalpy. Favorable bonding interactions via hydrogen bonds during Watson-Crick base pairing results in negative enthalpy, and restricted rotation and flexibility of the DNA backbone generates negative entropy. Scientists hypothesize that hydrogen bonding between bases not only stabilizes the double helix but is also crucial for selective and efficient replication.Analogs that are similar in size and shape to naturally occurring bases can be used to determine the influence of hydrogen bonding on base pair selectivity. To mimic the structure of deoxythymidine triphosphate (dTTP) , researchers synthesized dNTP derivatives of difluorotoluene (dFTP) , a nonpolar analog that lacks Watson-Crick hydrogen bonding. Klenow fragment (KF) polymerase, which has 3′-5′ but not 5′-3′ exonuclease activity, was incubated with a mixture of DNA template, primers, and dNTPs, including dFTP derivatives. The efficiency of dFTP and natural dTTP nucleotide incorporation into a growing primer strand by KF is shown in Figure 1.

Figure 1 Template-specific selection of dFTP and dTTP by the KF enzyme
Adapted from Moran S, Ren RX, Kool ET. A thymidine triphosphate shape analog lacking Watson-Crick pairing ability is replicated with high sequence selectivity. Proc Natl Acad Sci USA. 1997;94(20) :10506-11.
-The experimental results described in the passage are inconsistent with which hypothesis?

Quizplus · Accepted Answer

11ecba2d_0e9e_90c8_a3bf_a19276ca8fb1_MD0008_00 In DNA replication, dNTPs containing thymine (dTTP) are selectively inserted opposite adenine nucleotides rather than guanine, cytosine, or other thymine nucleotides in the template strand. Scientists generally attribute this selectivity to the hydrogen bonding between thymine and adenine in the Watson-Crick model of DNA. The study described in the passage tested this hypothesis by synthesizing dNTP derivatives of dFTP, an analog that lacks Watson-Crick hydrogen bonding capacity but is similar in size and shape to thymine. The effect of hydrogen bonding on base selectivity was measured as the efficiency with which dTTP and dFTP were inserted into a growing primer strand by KF. Scientists predicted that only dTTP nucleotides would be selectively paired with adenine and dFTP would not be added to the growing strand. However, the results depicted in Figure 1 show no significant difference in the base selectivity between dTTP and its structural analog dFTP. When KF detected adenine in the template strand, dFTP was inserted with the same efficiency as dTTP. Similarly, when the template base was guanine, cytosine, or thymine, the efficiency of dFTP and dTTP insertion was greatly reduced. Therefore, dFTP exhibited the same base pair selectivity as dTTP despite its inability to form Watson-Crick hydrogen bonds. (Choice A) dFTP was efficiently inserted into DNA strands opposite adenine; therefore, it serves as a good substrate for DNA polymerase. (Choice C) The template strand in the experiment did not contain dFTP nucleotides, so the results are neither consistent nor inconsistent with the hypothesis that dFTP is an effective template nucleotide. (Choice D) dFTP is similar in shape to dTTP. Its selective incorporation into DNA suggests that shape is involved in selectivity. Educational objective: DNA polymerase selectively inserts complementary nucleotides that form hydrogen bonds with bases on the template strand. The Watson-Crick model shows that adenine is paired with thymine whereas guanine is paired with cytosine.

Passage DNA Polymerization Is One of the Most Conserved Mechanisms of of Genome