BIOL 101 Exam 3 V3 | BIOL 101 Principles of Biology | Actual Q&A
with Rationale (BIOL101 Exam 3) | Liberty University
1. Which enzyme is responsible for unwinding the DNA double helix during the process of
replication?
A. Helicase
B. RNA Primase
C. DNA Polymerase
D. DNA Ligase
Correct Answer: A
Explanation: Helicase is the specific enzyme that breaks the hydrogen bonds between the
nitrogenous bases of the two DNA strands. This action creates the replication fork which
allows other enzymes to access the single strands for copying. Without helicase, the double
helix would remain tightly coiled and inaccessible for replication.
2. According to Chargaff’s rules, if a DNA sample contains 20% Adenine, what is the expected
percentage of Guanine?
A. 20%
B. 30%
C. 40%
D. 80%
Correct Answer: B
Explanation: Chargaff’s rules state that Adenine always pairs with Thymine, and Cytosine
always pairs with Guanine. If Adenine is 20%, then Thymine must also be 20%, totaling
40% for the A-T pairs. The remaining 60% is split equally between Guanine and Cytosine,
resulting in 30% Guanine.
3. In Mendelian genetics, what is the expected phenotypic ratio of a monohybrid cross
between two heterozygous parents (Aa x Aa)?
A. 1:2:1
B. 9:3:3:1
C. 1:1
D. 3:1
Correct Answer: D
,Explanation: A cross between two heterozygotes results in three genotypes: AA, Aa, and aa
in a 1:2:1 ratio. Because the ‘A’ allele is dominant, both AA and Aa individuals will display
the dominant phenotype. This results in a physical appearance ratio where 75% show the
dominant trait and 25% show the recessive trait.
4. Which molecule carries the genetic code from the nucleus to the ribosome for protein
synthesis?
A. tRNA
B. mRNA
C. DNA
D. rRNA
Correct Answer: B
Explanation: Messenger RNA (mRNA) is transcribed from the DNA template within the
nucleus of eukaryotic cells. It serves as the mobile copy of the genetic instructions that can
travel through the nuclear pores. Once in the cytoplasm, the mRNA interacts with
ribosomes to begin the translation process.
5. What is the primary function of the tRNA molecule during translation?
A. To unzip the DNA double helix
B. To provide the structural framework for the ribosome
C. To synthesize the mRNA strand from DNA
D. To transfer specific amino acids to the growing polypeptide chain
Correct Answer: D
Explanation: Transfer RNA (tRNA) acts as a bridge between the mRNA codons and the
amino acids they encode. Each tRNA molecule has an anticodon that matches a specific
mRNA codon and carries the corresponding amino acid. This ensures that the protein is
built with the correct sequence of amino acids as dictated by the genetic code.
6. A sequence of three nucleotides on mRNA that codes for a specific amino acid is called a:
A. Codon
B. Anticodon
C. Gene
D. Intron
Correct Answer: A
Explanation: A codon is a triplet of bases that represents one of the 20 standard amino
acids used in protein construction. The genetic code is read in these non-overlapping
, groups of three starting from a specific start point. This triplet system provides enough
combinations to account for all amino acids and stop signals.
7. Which of the following describes a mutation where a single nucleotide is replaced by a
different one, potentially changing one amino acid?
A. Frameshift mutation
B. Deletion mutation
C. Point mutation (substitution)
D. Insertion mutation
Correct Answer: C
Explanation: A point mutation occurs at a specific single location in the DNA sequence. If a
substitution occurs, it may result in a missense mutation (different amino acid), a silent
mutation (same amino acid), or a nonsense mutation (early stop codon). Unlike insertions
or deletions, a single substitution does not shift the reading frame of the entire gene.
8. What technique is used to amplify small segments of DNA into millions of copies?
A. Polymerase Chain Reaction (PCR)
B. DNA Fingerprinting
C. Gel Electrophoresis
D. Gene Therapy
Correct Answer: A
Explanation: PCR is a laboratory technique that mimics natural DNA replication in a
controlled environment. By using heat-stable DNA polymerase and specific primers,
scientists can target and multiply a specific region of DNA. This process is essential for
forensics, medical diagnostics, and genetic research where DNA samples are limited.
9. In a pedigree, a fully shaded square typically represents a:
A. Female with the trait
B. Male carrier
C. Male with the trait
D. Female carrier
Correct Answer: C
Explanation: Pedigrees use standardized symbols to track inheritance across generations.
Squares represent males, while circles represent females. Shading indicates that the
individual expresses the phenotype of the trait being studied.
with Rationale (BIOL101 Exam 3) | Liberty University
1. Which enzyme is responsible for unwinding the DNA double helix during the process of
replication?
A. Helicase
B. RNA Primase
C. DNA Polymerase
D. DNA Ligase
Correct Answer: A
Explanation: Helicase is the specific enzyme that breaks the hydrogen bonds between the
nitrogenous bases of the two DNA strands. This action creates the replication fork which
allows other enzymes to access the single strands for copying. Without helicase, the double
helix would remain tightly coiled and inaccessible for replication.
2. According to Chargaff’s rules, if a DNA sample contains 20% Adenine, what is the expected
percentage of Guanine?
A. 20%
B. 30%
C. 40%
D. 80%
Correct Answer: B
Explanation: Chargaff’s rules state that Adenine always pairs with Thymine, and Cytosine
always pairs with Guanine. If Adenine is 20%, then Thymine must also be 20%, totaling
40% for the A-T pairs. The remaining 60% is split equally between Guanine and Cytosine,
resulting in 30% Guanine.
3. In Mendelian genetics, what is the expected phenotypic ratio of a monohybrid cross
between two heterozygous parents (Aa x Aa)?
A. 1:2:1
B. 9:3:3:1
C. 1:1
D. 3:1
Correct Answer: D
,Explanation: A cross between two heterozygotes results in three genotypes: AA, Aa, and aa
in a 1:2:1 ratio. Because the ‘A’ allele is dominant, both AA and Aa individuals will display
the dominant phenotype. This results in a physical appearance ratio where 75% show the
dominant trait and 25% show the recessive trait.
4. Which molecule carries the genetic code from the nucleus to the ribosome for protein
synthesis?
A. tRNA
B. mRNA
C. DNA
D. rRNA
Correct Answer: B
Explanation: Messenger RNA (mRNA) is transcribed from the DNA template within the
nucleus of eukaryotic cells. It serves as the mobile copy of the genetic instructions that can
travel through the nuclear pores. Once in the cytoplasm, the mRNA interacts with
ribosomes to begin the translation process.
5. What is the primary function of the tRNA molecule during translation?
A. To unzip the DNA double helix
B. To provide the structural framework for the ribosome
C. To synthesize the mRNA strand from DNA
D. To transfer specific amino acids to the growing polypeptide chain
Correct Answer: D
Explanation: Transfer RNA (tRNA) acts as a bridge between the mRNA codons and the
amino acids they encode. Each tRNA molecule has an anticodon that matches a specific
mRNA codon and carries the corresponding amino acid. This ensures that the protein is
built with the correct sequence of amino acids as dictated by the genetic code.
6. A sequence of three nucleotides on mRNA that codes for a specific amino acid is called a:
A. Codon
B. Anticodon
C. Gene
D. Intron
Correct Answer: A
Explanation: A codon is a triplet of bases that represents one of the 20 standard amino
acids used in protein construction. The genetic code is read in these non-overlapping
, groups of three starting from a specific start point. This triplet system provides enough
combinations to account for all amino acids and stop signals.
7. Which of the following describes a mutation where a single nucleotide is replaced by a
different one, potentially changing one amino acid?
A. Frameshift mutation
B. Deletion mutation
C. Point mutation (substitution)
D. Insertion mutation
Correct Answer: C
Explanation: A point mutation occurs at a specific single location in the DNA sequence. If a
substitution occurs, it may result in a missense mutation (different amino acid), a silent
mutation (same amino acid), or a nonsense mutation (early stop codon). Unlike insertions
or deletions, a single substitution does not shift the reading frame of the entire gene.
8. What technique is used to amplify small segments of DNA into millions of copies?
A. Polymerase Chain Reaction (PCR)
B. DNA Fingerprinting
C. Gel Electrophoresis
D. Gene Therapy
Correct Answer: A
Explanation: PCR is a laboratory technique that mimics natural DNA replication in a
controlled environment. By using heat-stable DNA polymerase and specific primers,
scientists can target and multiply a specific region of DNA. This process is essential for
forensics, medical diagnostics, and genetic research where DNA samples are limited.
9. In a pedigree, a fully shaded square typically represents a:
A. Female with the trait
B. Male carrier
C. Male with the trait
D. Female carrier
Correct Answer: C
Explanation: Pedigrees use standardized symbols to track inheritance across generations.
Squares represent males, while circles represent females. Shading indicates that the
individual expresses the phenotype of the trait being studied.
