AP Biology: Mutations and DNA Repair Practice Questions
Test yourself on AP Biology: Mutations and DNA Repair with 12 original AP Exams practice questions. Pick an answer to see instant feedback and a full explanation.
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1. Sequencing shows that one DNA base pair has changed from A-T to G-C, with no change in the total number of base pairs. Which description fits this change?
Explanation. One base pair has been replaced by another, so this is a substitution. An insertion or deletion would change sequence length. The sequence change alone does not tell us whether an encoded protein or an observable trait changes.
2. A DNA substitution changes an internal mRNA codon from GCU, encoding alanine, to GUU, encoding valine. What type of coding effect is shown?
Explanation. The supplied codon meanings show that alanine is replaced by valine. This is a missense change. No base was added or removed, and neither stated codon is a stop signal. Its effect on protein function requires additional evidence.
3. Exactly one complete internal codon is deleted from a coding sequence. The start codon and every remaining base are unchanged. What is the direct consequence for the downstream reading frame?
Explanation. Removing a complete three-base codon removes one encoded amino acid while preserving the grouping of the later bases. The deletion may still affect protein function. Preserving a reading frame does not imply that the protein remains fully functional.
4. A new mutation is present in a sperm cell that fertilizes an egg. A different mutation is confined to a skin-cell lineage in the same human parent. Under ordinary sexual reproduction, which mutation can enter the offspring's genome through fertilization?
Explanation. The fertilizing sperm contributes DNA to the zygote, so a mutation it carries can be inherited. A mutation confined to a skin-cell lineage is somatic and does not enter that offspring through the gametes. The location of the mutation matters for inheritance.
5. A DNA lesion is detected and repaired so that the original sequence is restored before replication. What does this example demonstrate?
Explanation. A lesion can be corrected before it becomes a lasting sequence change. Here the stated restoration preserves the original sequence; damage and mutation are therefore not interchangeable outcomes.
6. A replicative DNA polymerase inserts an incorrect nucleotide, removes it from the growing strand's 3' end, and then resumes synthesis. Which activity is being described?
Explanation. Proofreading removes a recently misinserted nucleotide during replication. It acts on the growing DNA strand, whereas translation builds a polypeptide and splicing processes RNA.
7. An incorrectly paired nucleotide escapes proofreading during replication. A later system identifies the newly made strand and replaces a segment containing the error. Which process is described?
Explanation. Mismatch repair corrects replication errors that remain after proofreading. Identifying the new strand allows replacement of the incorrect sequence using the parental strand as a guide.
8. A DNA glycosylase removes an abnormal base from DNA, leaving the sugar temporarily without its base. Which repair pathway has begun?
Explanation. Base excision repair can begin with a glycosylase removing a damaged or inappropriate base. Other enzymes then process the resulting site, replace missing material, and seal the strand.
9. A repair complex removes a short DNA segment containing a UV-induced thymine dimer by cutting the damaged strand on both sides of the lesion. Which pathway does this describe?
Explanation. Nucleotide excision repair removes a stretch containing a bulky lesion. After that segment is removed, DNA synthesis fills the gap and ligation restores strand continuity.
10. During a repair reaction, an intact DNA strand provides the template 3'-GCTA-5' opposite a four-nucleotide gap. Which sequence correctly fills that gap, written 5'-to-3'?
Explanation. DNA base pairing gives G with C, C with G, T with A, and A with T. The antiparallel complement is therefore 5'-CGAT-3'. The U-containing option is not the DNA sequence requested, and reversing the complement without accounting for strand ends gives the wrong orientation.
11. Researchers compare UV-treated repair-proficient yeast with UV-treated yeast lacking one repair gene. They also include an unexposed group of each genotype. What is the main purpose of those two unexposed groups?
Explanation. Unexposed controls reveal baseline measurements for each genotype. This helps distinguish an exposure-associated change from a difference already present without UV. Controls do not guarantee equal baselines or remove the need for replication and matched growth conditions.
12. In a fictional yeast experiment, 8 of 200,000 surviving cells in a control sample and 32 of 200,000 surviving cells in a treated sample carry a scored mutation. What is the treated-to-control ratio of the observed mutant frequencies?
Explanation. The denominators are equal, so the ratio is 32 divided by 8, which is 4. This compares observed mutant frequencies among survivors. It does not by itself estimate a mutation rate per cell division, establish statistical significance, or identify a molecular mechanism.
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FAQ
Does every mutation change a protein?
No. A synonymous coding substitution can preserve an amino acid, and many changes occur outside protein-coding sequences. Effects on gene regulation, RNA processing, or traits depend on the location and context of a change.
How should I approach a DNA repair pathway question?
Follow the sequence of events in the stem: identify what is wrong, what is removed, and what is rebuilt. Distinguish a base from a nucleotide or a longer segment, and keep strand orientations explicit in sequence problems.
Why is mutant frequency different from mutation rate?
Mutant frequency is the fraction carrying a scored mutation in a measured population. A mutation rate concerns how often new changes arise per specified opportunity, such as a cell division. Growth, survival, and sampling can affect a measured frequency.