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📝 DNA nucleotides A T C G genetic code (7 MCQs)

📖 From Campbell Biology • 1. Evolution and the theme of Biology and Scientific Inquiry • 7 questions available

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What is DNA nucleotides A T C G genetic code?

Definition:
DNA nucleotides are the basic building blocks of DNA and contain a sugar, phosphate group, and nitrogenous base. The four DNA bases are adenine $A$ , thymine $T$ , cytosine $C$ , and guanine $G$ . Their sequence stores biological information.

Working:
DNA bases pair specifically, with $A$ pairing with $T$ , and $C$ pairing with $G$ . During gene expression, nucleotide sequences provide information used to determine protein sequences.

Example:
For a DNA sequence $A-C-G-T$ , the complementary sequence is $T-G-C-A$ . The pairing follows $A-T$ and $C-G$ rules.

Reason:
The order of nucleotides acts like biological information because different sequences can encode different instructions for producing functional molecules.

Easy

Medium

Hard

📝 All DNA nucleotides A T C G genetic code MCQs

Q1. A researcher identifies a DNA segment containing adenine, thymine, cytosine, and guanine. Which conclusion best explains why changing one nucleotide can sometimes alter a protein while another nucleotide change has little or no effect?

A.Each nucleotide directly represents one amino acid, so every substitution must change the protein

B.Nucleotides form codons, and some codon changes can specify the same amino acid because the genetic code is redundant ✅

C.Only adenine and thymine can influence protein structure because they form complementary pairs

D.A nucleotide substitution always changes the reading frame regardless of its location

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: DNA nucleotides are read in groups of three during translation, producing codons that specify amino acids. Because multiple codons can encode the same amino acid, some substitutions are synonymous and produce no protein sequence change.

Q2. A DNA template contains the sequence $3'-TAC-GGA-CTT-5'$ . A student claims that the corresponding coding strand must read $3'-ATG-CCT-GAA-5'$ . What is the most accurate evaluation of the student's reasoning?

A.Correct, because the coding strand is identical to the template strand

B.Incorrect, because the complementary coding strand is

5&#039;-ATG-CCT-GAA-3&#039;

✅

C.Correct, because DNA strands are always written from

3&#039;

5&#039;

D.Incorrect, because thymine must be replaced by uracil in the coding DNA strand

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The coding strand is complementary to the template strand but runs antiparallel. Therefore, a template written $3'$ to $5'$ corresponds to a coding strand written $5'$ to $3'$ as $5'-ATG-CCT-GAA-3'$ .

Q3. A mutation changes an mRNA codon from $5'-GAA-3'$ to $5'-GAG-3'$ . Both codons specify glutamic acid. Which prediction is most reasonable if this is the only mutation?

A.The protein must become longer because GAG contains more guanine

B.The protein sequence may remain unchanged because different codons can specify the same amino acid ✅

C.The mutation must shift the reading frame because two nucleotides changed

D.The mutation must replace an amino acid with a stop signal

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The two codons differ at their third nucleotide but both encode glutamic acid. This illustrates redundancy in the genetic code, meaning a nucleotide substitution does not necessarily alter the amino acid sequence or protein structure.

Q4. A laboratory compares two DNA variants from the same gene. Variant X differs from the original at one nucleotide, while Variant Y differs at three nucleotides. Surprisingly, Variant X produces a different amino acid but Variant Y produces the same protein sequence. Which explanation best accounts for these results?

A.The number of nucleotide changes alone determines protein differences

B.Variant X may contain a nonsynonymous substitution, whereas Variant Y may contain synonymous substitutions ✅

C.Variant Y cannot be a real DNA sequence because three mutations always destroy a gene

D.Variant X must contain three codons because one nucleotide represents one amino acid

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Protein consequences depend on where nucleotide changes occur and how they alter codons, not simply on mutation count. One substitution can change an amino acid, while several substitutions may be synonymous and leave the protein sequence unchanged.

Q5. A scientist records the percentage of mutations that change an amino acid in four regions of a gene: Region A = 72%, Region B = 48%, Region C = 21%, and Region D = 19%. Which interpretation is most defensible from these data?

A.Regions C and D necessarily contain no functional sequences

B.Regions A and B may contain codons where nucleotide substitutions are more likely to alter amino acids, but mutation location and codon structure must also be considered ✅

C.Region A must contain only adenine and thymine

D.Regions C and D cannot undergo mutations because their percentages are low

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: A higher fraction of amino-acid-changing mutations can reflect differences in codon composition and substitution positions. However, the data alone cannot establish that a region is nonfunctional or mutation-free.

Q6. A mutation inserts one nucleotide near the beginning of a coding sequence. Several downstream codons are then interpreted differently from the original sequence. Why can the effect be much greater than that of a single nucleotide substitution?

A.An inserted nucleotide can shift the reading frame, changing how downstream nucleotides are grouped into codons ✅

B.An inserted nucleotide always becomes a stop codon immediately

C.Insertions cannot affect protein synthesis because DNA contains only four nucleotide types

D.The insertion changes every nucleotide into a different nucleotide through complementary pairing

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Codons are interpreted as consecutive groups of three nucleotides. Adding one nucleotide changes the grouping of nearly all downstream bases, potentially altering many amino acids and creating a premature stop signal.

Q7. Two researchers propose models for predicting the effect of a single-base substitution. Model 1 considers only which nucleotide was changed. Model 2 considers the original codon, the new codon, and the reading frame. Which model should generally provide more reliable predictions, and why?

A.Model 1, because nucleotide identity alone determines the amino acid

B.Model 2, because the effect depends on codon context and how the altered codon is interpreted ✅

C.Model 1, because all substitutions involving guanine are harmful

D.Both models are equally reliable because every nucleotide has exactly one amino-acid meaning

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: A nucleotide has no fixed amino-acid meaning by itself. Its effect depends on its position within a codon and the resulting codon sequence. Considering codon context therefore provides a biologically stronger prediction.

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