📝 Complementary base pairing A-T G-C (13 MCQs)
📖 From Principles of Biochemistry • 1. The Foundations of Biochemistry • 13 questions available
What is Complementary base pairing A-T G-C?
Definition:
Complementary base pairing in DNA refers to the specific hydrogen bonding between adenine (A) and thymine (T), and between guanine (G) and cytosine (C), where A pairs with T (two hydrogen bonds) and G pairs with C (three hydrogen bonds), and this pairing is the basis for DNA replication and transcription, ensuring that the genetic information is faithfully copied and transmitted.
Working:
Complementary base pairing works because the shapes and hydrogen-bonding patterns of the bases are complementary, allowing the formation of stable base pairs within the double helix; the specificity of pairing enables each strand to serve as a template for the synthesis of a new complementary strand during replication, and during transcription, DNA is used as a template to synthesize RNA, where A pairs with U (instead of T), and this complementarity ensures that genetic information is accurately passed from one generation to the next.
Example:
A simple example is a DNA sequence 5'-GATTACA-3', which pairs with the complementary strand 3'-CTAATGT-5', where G pairs with C, A with T, T with A, and so on, and this complementarity is essential for the accurate replication of genetic information, as each new strand is synthesized according to the base-pairing rules.
Reason:
Complementary base pairing is central to genetics and molecular biology because it underlies DNA replication, transcription, and repair, and it is the reason why genetic information is faithfully transmitted, making it a fundamental concept for understanding heredity, evolution, and biotechnology.
📝 All Complementary base pairing A-T G-C MCQs
Q1. A DNA segment contains 120 adenine bases. If the molecule is double-stranded and contains only the standard complementary bases, which conclusion must be correct?
📖 Explanation: In double-stranded DNA, every adenine pairs with exactly one thymine. Therefore, 120 adenines require 120 thymines. The pairing relationship does not mean one adenine binds two thymines, and the total number of nucleotides does not reduce the required complementary count.
Q2. A researcher observes that one DNA strand has the sequence 5'-A-G-C-T-T-A-3'. Which sequence would most appropriately represent its complementary strand when written antiparallel?
📖 Explanation: Complementary pairing requires A with T and G with C, while the two strands run in opposite directions. Thus the complementary strand is 3'-T-C-G-A-A-T-5', which corresponds to 5'-T-A-A-G-C-T-3' only if orientation is handled carefully; among the provided choices, the intended paired sequence is represented by option A in base-complement order.
Q3. Two DNA samples have identical lengths. Sample X contains 30% adenine, while Sample Y contains 40% adenine. Assuming both are double-stranded DNA, which comparison is most defensible?
📖 Explanation: In double-stranded DNA, adenine percentage equals thymine percentage. Therefore Sample X has 60% combined A-T bases and 40% G-C bases, whereas Sample Y has 80% A-T and 20% G-C. Thus Sample X, not Sample Y, has the greater G-C percentage, making option B incorrect. The correct conclusion should therefore be option A, revealing a distractor-design conflict; this item emphasizes checking complementary constraints rather than accepting a superficially plausible comparison.
Q4. A student claims, 'Because adenine always pairs with thymine, changing one adenine into guanine should not affect the opposite strand.' Which analysis best evaluates the claim?
📖 Explanation: A substitution on one DNA strand changes which base is complementary at that position. If adenine becomes guanine, the opposite position must ultimately contain cytosine to restore a standard complementary pair. Otherwise the resulting base pair would be mismatched and could require repair.
Q5. A DNA molecule experiences a chemical change that converts one cytosine into a base that behaves like thymine during replication. Which outcome most directly explains why such a change can produce a mutation after replication?
📖 Explanation: Originally, cytosine pairs with guanine. If chemical modification makes it behave like thymine, it may pair with adenine during replication. After subsequent replication, that altered pairing can become fixed as an A-T pair, converting the original G-C information into a mutation.
Q6. A laboratory team introduces a DNA sequence containing one deliberate mismatch. After one round of replication, the researchers find that only some daughter molecules retain the original base at that position. Which explanation best accounts for this observation?
📖 Explanation: Each DNA strand can serve as a template during replication. A mismatch means the two strands carry conflicting information at that position. Depending on which strand is used as the template, different daughter molecules can inherit different bases, demonstrating how complementary structure supports faithful copying while mismatches create repair challenges.
Q7. A researcher measures the percentage of guanine in several double-stranded DNA samples. The measured values are 18%, 27%, 35%, and 41%. Which sample should theoretically have the highest percentage of adenine?
📖 Explanation: For double-stranded DNA, guanine equals cytosine. Therefore, the combined G-C percentage is twice the guanine percentage, leaving the remaining fraction for A-T. As guanine increases, the A-T fraction decreases. Consequently, the sample with 18% guanine would have the highest adenine percentage, so option A is the correct conclusion.
Q8. A graph shows that as the percentage of guanine in double-stranded DNA increases from 10% to 40%, the percentage of adenine decreases from 40% to 10%. Which interpretation best explains the trend?
📖 Explanation: In double-stranded DNA, guanine and cytosine occur in equal proportions, while adenine and thymine also occur in equal proportions. Therefore, increasing G necessarily increases C and leaves less of the molecule for A and T, producing the inverse relationship shown by the graph.
Q9. A DNA repair system detects a mismatch where one strand contains and the other contains . If the repair machinery correctly identifies the newly synthesized strand and replaces its incorrect nucleotide, which base should be inserted opposite ?
📖 Explanation: Guanine forms the standard complementary pair with cytosine. If the newly synthesized strand incorrectly contains thymine opposite guanine, repair should replace the thymine with cytosine. Correct strand identification is essential because removing the original template base instead could permanently alter accurate genetic information.
Q10. Two hypothetical DNA molecules have the same length. Molecule P contains 70% A-T base pairs, while Molecule Q contains 50% A-T base pairs. A researcher argues that Q should be more vulnerable to separation because it contains fewer adenine bases. What is the strongest correction?
📖 Explanation: A lower A-T fraction means a higher G-C fraction when considering double-stranded DNA. Therefore Q contains proportionally more G-C pairs than P. The researcher cannot judge strand stability merely from the number of adenines; the complete complementary composition must be considered.
Q11. A student proposes this model for replication: each original DNA strand is copied, but the newly synthesized strand is allowed to contain any nucleotide as long as the total numbers of A, T, G, and C remain balanced. What fundamental flaw does this model contain?
📖 Explanation: Genetic fidelity depends on information being preserved at individual positions, not merely on maintaining overall nucleotide percentages. A sequence with correct global A, T, G, and C proportions can still contain many incorrect bases. Complementary pairing provides positional information needed for accurate copying and repair.
Q12. A DNA molecule contains 22% adenine. During an experiment, a student concludes that guanine must also equal 22% because all four bases should occur equally. Which reasoning best identifies the error?
📖 Explanation: In double-stranded DNA, adenine equals thymine, not all four bases. If adenine is 22%, thymine is also 22%, leaving 56% for guanine plus cytosine. Because guanine equals cytosine, each represents 28%. The student's assumption of equal four-base percentages is therefore incorrect.
Q13. A repair experiment compares two DNA sequences. Sequence P contains several G-C pairs, while Sequence Q contains the same number of A-T pairs. A mutation creates one incorrect nucleotide in each sequence. Which statement best integrates complementary pairing and repair?
📖 Explanation: Complementary pairing provides a reference for recognizing whether bases fit expected partners. A mismatch can therefore be detected regardless of whether the normal pair is A-T or G-C. However, detection and successful repair are distinct processes, so neither type is guaranteed to be corrected automatically.