SAT - Biology Subject DNA Structure and Replication Questions and Answers — Questions and Answers
Question 1: In a DNA double helix, the two polynucleotide chains are held together by which type of bond between the complementary nitrogenous bases?
- Covalent bonds
- Hydrogen bonds (Correct answer)
- Ionic bonds
- Peptide bonds
Correct answer: Hydrogen bonds
The two strands of the DNA double helix are held together by hydrogen bonds that form between complementary base pairs: two bonds between adenine and thymine, and three bonds between guanine and cytosine. Covalent bonds form the strong sugar-phosphate backbone of each strand, while peptide bonds link amino acids in proteins.
Question 2: A scientist analyzes a sample of DNA and finds that it contains 28% adenine. Based on the rules of base pairing, what percentage of guanine should be present in this sample?
- 28%
- 72%
- 22% (Correct answer)
- 44%
Correct answer: 22%
According to Chargaff's rules, adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C). Therefore, the percentage of A equals the percentage of T, and the percentage of G equals the percentage of C. If A = 28%, then T must also be 28%. Together, A and T make up 56% of the sample (28% + 28%). The remaining percentage (100% - 56% = 44%) must be composed of G and C. Since G and C are equal, the percentage of guanine is 44% / 2 = 22%.
Question 3: Which of the following enzymes is responsible for unzipping the DNA double helix by breaking the bonds between complementary bases to initiate replication?
- DNA polymerase III
- DNA ligase
- Primase
- DNA helicase (Correct answer)
Correct answer: DNA helicase
DNA helicase is the enzyme that unwinds the DNA double helix at the replication fork, separating the two parental strands so they can be used as templates. DNA polymerase III synthesizes the new DNA strand, DNA ligase joins Okazaki fragments, and primase synthesizes RNA primers.
Question 4: A researcher uses a chemical that specifically inhibits the function of DNA ligase. Which of the following would be an expected result of this inhibition during DNA replication?
- The parental DNA strands would fail to separate.
- Synthesis of the leading strand would be unable to start.
- The newly synthesized lagging strand would consist of unjoined Okazaki fragments. (Correct answer)
- RNA primers would not be replaced with DNA nucleotides.
Correct answer: The newly synthesized lagging strand would consist of unjoined Okazaki fragments.
DNA ligase is responsible for joining the sugar-phosphate backbones of the Okazaki fragments on the lagging strand to create a continuous DNA strand. If DNA ligase is inhibited, the lagging strand would be synthesized as a series of disconnected fragments.
Question 5: The semiconservative model of DNA replication was confirmed by the Meselson-Stahl experiment. This model proposes that after one round of replication, each new DNA molecule consists of:
- Two newly synthesized strands.
- One parental strand and one newly synthesized strand. (Correct answer)
- Two parental strands.
- A mix of parental and new DNA segments on each strand.
Correct answer: One parental strand and one newly synthesized strand.
The semiconservative model states that each of the two parental DNA strands acts as a template for a new strand to be synthesized. The result is two new DNA molecules, each composed of one original (parental) strand and one newly synthesized strand. 'Semiconservative' means half of the original molecule is conserved in each new molecule.
Question 6: Which of the following statements correctly describes a key difference between the leading and lagging strands during DNA synthesis?
- The leading strand is synthesized in the 3' to 5' direction, while the lagging strand is synthesized in the 5' to 3' direction.
- The leading strand requires multiple RNA primers, while the lagging strand requires only one.
- The lagging strand is synthesized as a series of Okazaki fragments, while the leading strand is synthesized continuously. (Correct answer)
- The lagging strand serves as the template, while the leading strand is the newly synthesized DNA.
Correct answer: The lagging strand is synthesized as a series of Okazaki fragments, while the leading strand is synthesized continuously.
Because DNA polymerase can only add nucleotides to the 3' end of a growing strand, synthesis must always proceed in the 5' to 3' direction. The leading strand is synthesized continuously towards the replication fork. The lagging strand, however, is synthesized discontinuously away from the replication fork in short pieces called Okazaki fragments, which are later joined together.
SAT - Biology Subject DNA Structure and Replication Questions and Answers
In a DNA double helix, the two polynucleotide chains are held together by which type of bond between the complementary nitrogenous bases?