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📝 DNA as template for replication and repair (13 MCQs)

📖 From Principles of Biochemistry • 1. The Foundations of Biochemistry • 13 questions available

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What is DNA as template for replication and repair?

Definition:
DNA acts as a template for replication and repair by using the base sequence of one strand to specify the complementary sequence of a new strand, ensuring that the genetic information is accurately duplicated and maintained; during replication, the double helix unwinds, and each strand serves as a template for a new complementary strand, and during repair, the undamaged strand guides the repair of the damaged one, preserving the integrity of the genome.

Working:
DNA works as a template through the action of enzymes: DNA polymerase synthesizes a new strand by adding nucleotides complementary to the template strand, following the rules A-T and G-C, and the replication process is semiconservative, producing two double helices each with one old and one new strand; in DNA repair, enzymes like endonucleases remove damaged bases, and the intact strand is used as a template to fill the gap, ensuring that errors and damage are corrected, with the fidelity of replication being exceptionally high due to proofreading mechanisms.

Example:
A simple example is when a segment of DNA is damaged by UV radiation, causing a thymine dimer, and during repair, the undamaged complementary strand is used as a template to replace the damaged bases, restoring the correct sequence, and this process is essential for preventing mutations and maintaining genetic stability.

Reason:
DNA's role as a template is crucial for life, as it ensures that genetic information is accurately copied and transmitted during cell division and repaired when damaged, and this concept is fundamental to understanding genetics, inheritance, and the development of treatments for diseases like cancer.

Easy

Medium

Hard

📝 All DNA as template for replication and repair MCQs

Q1. A DNA strand has the sequence $5'-ACGTTAGC-3'$ . During copying, which newly synthesized strand would provide the most direct evidence that the original strand can act as a template?

5&#039;-TGCAATCG-3&#039;

5&#039;-CGATTGCA-3&#039;

✅

5&#039;-ACGTTAGC-3&#039;

5&#039;-GCTAACGT-3&#039;

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The complementary strand must pair antiparallel to the template. Because the given strand runs $5'$ to $3'$ , its complementary product is $3'-TGCAATCG-5'$ , which written $5'$ to $3'$ becomes $5'-CGATTGCA-3'$ .

Q2. A DNA molecule contains two complementary strands, but only one strand is initially exposed to a replication enzyme. Why can the exposed strand still determine the sequence of a newly synthesized strand?

A.The enzyme copies nucleotides randomly and later corrects them

B.The exposed strand provides pairing information that determines which nucleotide is incorporated at each position ✅

C.The exposed strand chemically converts directly into the new strand

D.The opposite strand is unnecessary because DNA contains identical bases on both strands

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: A template does not need to become part of the new strand to determine its sequence. Complementary base pairing allows each exposed template base to specify the appropriate incoming nucleotide, providing sequence information during synthesis.

Q3. A researcher replaces one nucleotide in a DNA template with a chemically similar but noncomplementary base. The replication machinery inserts a nucleotide opposite the altered position. What is the most likely immediate consequence?

A.The altered template position has no effect because DNA polymerases ignore template identity

B.The inserted nucleotide may be mismatched, increasing the probability of an error that requires correction ✅

C.Both daughter strands will automatically contain the original sequence

D.The DNA molecule will necessarily break at that position

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Template-directed synthesis depends on accurate molecular recognition and complementary pairing. A noncomplementary template base can promote incorrect nucleotide selection, creating a mismatch that may need proofreading or repair to preserve sequence fidelity.

Q4. A repair enzyme encounters a damaged base on one DNA strand while the opposite strand remains chemically intact. Why is the intact strand particularly valuable?

A.It provides a reference for determining which nucleotide should replace the damaged information ✅

B.It prevents all chemical reactions from occurring near the damaged site

C.It permanently becomes the repaired strand and is discarded afterward

D.It supplies energy directly to the repair enzyme

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: The undamaged complementary strand retains sequence information that can serve as a reference. Repair can therefore restore the damaged strand by using complementary pairing rather than guessing which nucleotide originally occupied the site.

Q5. Suppose a DNA segment contains $5'-GATTACA-3'$ . A repair system removes the middle nucleotide from one strand but leaves the complementary strand unchanged. Which strategy would best restore the missing information?

A.Insert a nucleotide selected solely from the surrounding bases

B.Use the intact complementary strand as a template and select the nucleotide required for complementary pairing ✅

C.Replace the entire DNA molecule with a newly synthesized molecule

D.Insert the most abundant nucleotide in the cell

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The complementary strand preserves the original sequence relationship even after one strand is damaged. Using that intact strand as a template allows the repair machinery to infer the missing nucleotide through complementary base pairing.

Q6. A mutant DNA molecule has normal base-pairing chemistry but its two strands cannot separate efficiently. During replication, what outcome is most directly expected?

A.Replication becomes difficult because template bases are less accessible for copying ✅

B.Replication becomes more accurate because the strands remain permanently paired

C.Repair becomes unnecessary because the sequence cannot change

D.Every nucleotide is copied twice onto the same strand

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Replication requires access to template information. If the two strands cannot separate sufficiently, the machinery cannot efficiently expose the bases that must guide complementary nucleotide incorporation, even if pairing chemistry itself remains normal.

Q7. Two DNA templates are compared. Template A produces 98 correct copies out of 100 attempts, while Template B produces 75 correct copies out of 100 attempts. Both have identical nucleotide composition but differ in how clearly their bases can be recognized. Which interpretation best fits the data?

A.Template B provides less reliable information to the copying machinery despite having the same base composition ✅

B.Template A must contain more nucleotides than Template B

C.Template B cannot participate in repair because it contains fewer bases

D.The difference proves that complementary pairing is irrelevant to replication fidelity

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: The results indicate that sequence information alone is not sufficient if recognition or accessibility is impaired. Template A supports more accurate copying, whereas Template B likely introduces more opportunities for incorrect nucleotide selection.

Q8. A replication experiment measures the percentage of correctly copied sites as the number of template bases examined increases. The observed values are: 10 bases, 99%; 100 bases, 98%; 1,000 bases, 90%; 10,000 bases, 65%. What is the strongest conclusion?

A.The copying mechanism becomes intrinsically less accurate for every individual nucleotide as DNA length increases

B.Errors become more noticeable in absolute number as the number of opportunities for errors increases ✅

C.Longer DNA molecules cannot contain complementary base pairs

D.Template strands stop functioning after 1,000 bases

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: As the number of copied positions increases, there are more opportunities for an error to occur somewhere in the molecule. The decreasing percentage therefore can reflect accumulated error opportunities rather than a change in individual base-pair recognition.

Q9. A student claims: 'Because the newly synthesized strand is complementary to the template, it should have exactly the same sequence as the template when both are written $5'$ to $3'$ .' What is the flaw?

A.Complementary strands contain unrelated nucleotides

B.The student ignores antiparallel orientation, so reversing the complementary sequence is required when both strands are written

5&#039;

3&#039;

✅

C.DNA strands are always parallel

D.The template sequence cannot be written in

5&#039;

3&#039;

form

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Complementarity and antiparallel orientation must be considered together. The complementary strand runs in the opposite direction, so converting it to the conventional $5'$ to $3'$ notation requires reversing the order of the complementary nucleotides.

Q10. A repair experiment compares two conditions. In Condition X, damaged DNA is repaired with an intact complementary strand available. In Condition Y, the complementary strand is also damaged. Repair accuracy is 97% in X and 61% in Y. What does the comparison most strongly support?

A.An intact complementary strand improves the reliability of sequence restoration ✅

B.DNA repair is independent of existing sequence information

C.Damage to both strands always makes repair impossible

D.The lower accuracy proves that damaged DNA contains no nucleotides

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: The major experimental difference is the availability of an intact complementary reference. The large accuracy difference supports the conclusion that preserved complementary information helps repair machinery reconstruct damaged or missing sequence information.

Q11. Two hypothetical repair strategies are tested. Method 1 uses the undamaged complementary strand as a sequence reference. Method 2 chooses replacement bases according to their frequency elsewhere in the genome. Which method should generally produce greater fidelity, and why?

A.Method 1, because local complementary information directly identifies the expected base ✅

B.Method 1, because nucleotide frequency always predicts every DNA sequence

C.Method 2, because common nucleotides are automatically correct

D.Method 2, because repair does not require a template

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Local complementary information is far more precise than genome-wide nucleotide frequency. A template provides position-specific instructions, whereas frequency-based selection can produce a common nucleotide that is nevertheless incorrect at the particular damaged position.

Q12. A theoretical DNA molecule contains complementary strands, but a mutation causes one strand to lose its ability to participate in specific base pairing while the other remains normal. Which prediction best combines replication and repair reasoning?

A.Both processes should remain completely unaffected because only one strand is altered

B.Replication and repair may both become less reliable because the altered strand provides poorer sequence-recognition information ✅

C.Only transcription can be affected because replication never uses templates

D.The intact strand will automatically repair the defective strand without molecular machinery

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Both replication and repair depend on reliable sequence information and complementary recognition. If one strand cannot participate normally in pairing, it may provide weaker guidance during copying or restoration, increasing the likelihood of sequence errors.

Q13. A scientist proposes that DNA fidelity depends only on having two strands, not on their complementarity. Which observation would most strongly refute this claim?

A.DNA molecules contain phosphate groups

B.Replacing a correctly paired nucleotide with a noncomplementary nucleotide increases copying errors ✅

C.DNA molecules can exist in different lengths

D.DNA contains four major nucleotide types

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The critical test is whether complementarity changes copying accuracy. If introducing a noncomplementary base increases errors, that directly demonstrates that the informational relationship between the two strands, rather than simply having two strands, contributes to replication fidelity.

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