← Back to 1. The Foundations of Biochemistry

📝 DNA double helix structure (9 MCQs)

📖 From Principles of Biochemistry • 1. The Foundations of Biochemistry • 9 questions available

←Previous Topic:Genetic continuity in DNA📝 DNA double helix structureComplementary base pairing A-T G-C:Next Topic→

What is DNA double helix structure?

Definition:
The DNA double helix is the three-dimensional structure of DNA, discovered by Watson and Crick in 1953, consisting of two antiparallel polynucleotide strands wound around a common axis, with the sugar-phosphate backbones on the outside and the nitrogenous bases (adenine, thymine, guanine, cytosine) on the inside, paired by specific hydrogen bonds (A-T and G-C), and this structure is essential for storing genetic information and for replication and transcription.

Working:
The double helix works by providing a stable yet accessible structure for genetic information; the two strands are held together by hydrogen bonds (two for A-T, three for G-C), and the antiparallel orientation allows for complementary base pairing, and the helical twist (about 10 bases per turn) creates major and minor grooves that are important for protein binding; the structure is dynamic, allowing for unwinding during replication and transcription, and the base sequence encodes the genetic information, with the stability described by the equation $\Delta G = \Delta H - T\Delta S$ .

Example:
A simple example is the structure of a short DNA segment: 5'-ATGC-3' paired with 3'-TACG-5', where A pairs with T (two H-bonds) and G pairs with C (three H-bonds), forming a stable double helix, and this structure is found in all living organisms, from bacteria to humans, illustrating the universality of DNA.

Reason:
The double helix is fundamental to molecular biology because it explains how genetic information is stored, replicated, and expressed, and it is the basis for understanding genetics, evolution, and biotechnology, as well as for developing therapies for genetic disorders.

Easy

Medium

Hard

📝 All DNA double helix structure MCQs

Q1. Which structural feature most directly explains why each DNA strand can serve as a template for producing a complementary strand?

A.The sugar-phosphate backbone contains many negatively charged groups

B.The two strands contain complementary bases held together by specific hydrogen-bonding patterns ✅

C.DNA contains a large number of phosphate groups between bases

D.The helix is stabilized mainly by covalent bonds between opposite strands

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Complementary base pairing is the key feature. Because each strand contains a sequence that determines the matching bases on the opposite strand, separation of the double helix exposes an accurate template for synthesis of a new complementary strand.

Q2. A researcher replaces several normal DNA base pairs with pairs that cannot form their usual specific hydrogen bonds, while leaving the sugar-phosphate backbone intact. What is the most likely consequence during copying?

A.Replication becomes more accurate because the backbone is unchanged

B.Template recognition and accurate base selection become less reliable ✅

C.DNA replication becomes completely independent of sequence

D.The replacement necessarily converts DNA into RNA

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The backbone provides structural support, but accurate information transfer depends strongly on complementary base recognition. Altering the normal hydrogen-bonding patterns can reduce the ability of replication machinery to distinguish correct from incorrect partners.

Q3. Two DNA molecules have identical base sequences, but Molecule X is a stable double helix whereas Molecule Y has frequent disruptions between paired bases. Which prediction is most reasonable?

A.X should generally provide a more reliable template for faithful copying ✅

B.Y should always replicate faster because disrupted bases are exposed

C.X cannot be repaired because both strands are paired

D.Y must contain RNA instead of DNA

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: A stable double-stranded structure preserves complementary information while allowing controlled strand separation when needed. Frequent disruptions in pairing could interfere with template integrity, recognition, and accurate restoration of the original sequence.

Q4. A DNA segment contains the sequence $5'-A\,G\,C\,T-3'$ . If one strand is separated and copied using normal complementary pairing, which newly synthesized sequence is expected when written $5'$ to $3'$ ?

5&#039;-A\,G\,C\,T-3&#039;

5&#039;-T\,C\,G\,A-3&#039;

✅

5&#039;-A\,C\,G\,T-3&#039;

5&#039;-T\,G\,C\,A-3&#039;

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: Each template base specifies a complementary partner: A pairs with T and G pairs with C. Because the newly synthesized strand is antiparallel to the template, its sequence written from $5'$ to $3'$ is $5'-T\,C\,G\,A-3'$ .

Q5. A mutation changes one DNA base so that it now preferentially pairs with an incorrect partner during copying. Which sequence of events best explains how this could reduce fidelity?

A.The altered base changes the backbone into protein, preventing all replication

B.The altered pairing can cause an incorrect nucleotide to be incorporated, potentially propagating the sequence change ✅

C.The mutation automatically removes the entire chromosome

D.The mutation guarantees that repair enzymes will insert the original base

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: A changed base can alter pairing preferences and therefore increase the probability of incorrect nucleotide incorporation. If the mismatch escapes correction before another round of copying, the altered information can become permanently established.

Q6. A student argues: 'Because the two DNA strands are held together by relatively weak interactions, DNA cannot be a stable information-storage molecule.' What is the strongest correction?

A.Weak strand separation interactions can coexist with strong overall stability and facilitate controlled replication and repair ✅

B.DNA is stable only because its strands are connected by peptide bonds

C.DNA stability requires the absence of any strand separation

D.The bases do not contribute to DNA stability

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: DNA must balance stability with accessibility. The interactions between complementary strands can be disrupted when necessary, while base stacking and the overall molecular architecture contribute substantial stability. This balance supports both information storage and copying.

Q7. A DNA molecule contains a localized mismatch immediately after replication. The repair system can compare the two strands before the error becomes permanent. Why is the double-stranded arrangement especially useful in this situation?

A.One strand can provide an undamaged sequence reference for correcting the mismatched region ✅

B.Both strands must be destroyed so that a new DNA molecule can form

C.The mismatch is automatically converted into a protein

D.The phosphate backbone directly identifies which base is biologically correct

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: A paired DNA molecule contains redundant sequence information. When one strand contains an error, the complementary strand can serve as a reference for determining the expected base, allowing repair mechanisms to restore the correct sequence.

Q8. Two hypothetical DNA designs are compared. Design P uses complementary base pairs and antiparallel strands; Design Q uses identical bases at corresponding positions on both strands. During damage repair, which design has the greater informational advantage, and why?

A.P, because the complementary strand can provide sequence information for restoring damaged bases ✅

B.Q, because identical strands prevent all mutations

C.P, because antiparallel strands eliminate the need for replication

D.Q, because identical bases always form stronger covalent bonds

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Design P contains complementary sequence information, so an intact strand can guide restoration of damage on its partner. Design Q lacks the same complementary coding relationship and therefore provides less direct information for identifying the correct replacement.

Q9. A theoretical DNA molecule has extremely strong interactions between its two strands, making strand separation energetically very difficult. Another molecule has much weaker and less specific base pairing. Which molecule would most plausibly provide the best balance for accurate replication?

A.The first, because maximum strand strength always maximizes replication accuracy

B.The second, because weaker interactions always produce better DNA

C.Neither; accurate replication requires both stable information storage and sufficiently controllable strand separation ✅

D.Both are equally suitable because strand interactions do not affect replication

💡 Difficulty: easy | ✅ Correct: C

📖 Explanation: Accurate replication requires a balance rather than an extreme. DNA must remain sufficiently stable to preserve information, yet its strands must be separable in a controlled manner so each can act as a template. Specific pairing also supports fidelity.

🔗 Related Topics (MCQs)

📝 Cellular Foundations MCQs 📝 Cell structure and functional units MCQs 📝 Cell dimensions and oxygen diffusion MCQs 📝 Archaea (archaebacteria) MCQs 📝 Bacteria (eubacteria) MCQs

←Previous Topic:Genetic continuity in DNA📝 DNA double helix structureComplementary base pairing A-T G-C:Next Topic→