HESI A2 Biology 2024/2025 – 100% Verified
Answers with Expert Concept Review and
Clarifications
Instructions
The following 80 multiple-choice questions are designed to test your biology knowledge
for the HESI A2 exam, covering cellular biology, genetics, DNA/RNA, evolution, macro-
molecules, and ecosystems. Each question includes four answer options, with the correct
answer marked and a detailed explanation of the concept provided. Use these questions
to prepare effectively for your exam.
Question 1: What is the primary function of the mitochondria in a eukaryotic cell?
A. Protein synthesis
B. Energy production (Correct)
C. Cell division
D. Waste storage
Rationale: Mitochondria are the powerhouses of eukaryotic cells, producing ATP
through cellular respiration. Protein synthesis occurs in ribosomes, cell division
is managed by the nucleus and cytoskeleton, and waste storage is associated with
vacuoles.
Question 2: Which molecule carries genetic information from DNA to the ribosome for protein
synthesis?
A. mRNA (Correct)
B. tRNA
C. rRNA
D. snRNA
Rationale: Messenger RNA (mRNA) transcribes genetic information from DNA
in the nucleus and carries it to the ribosome for translation into proteins. tRNA
transfers amino acids, rRNA forms part of the ribosome, and snRNA is involved in
splicing.
Question 3: In a Punnett square for a monohybrid cross (Aa ΠAa), what is the probability of
an offspring being homozygous recessive (aa)?
A. 25% (Correct)
B. 50%
C. 75%
D. 100%
1
, Rationale: For a monohybrid cross (Aa ΠAa), the genotypic ratio is 1 AA : 2
Aa : 1 aa. The homozygous recessive (aa) phenotype occurs in 1 out of 4 offspring,
or 25%.
Question 4: Which macromolecule is primarily responsible for storing genetic information?
A. Carbohydrate
B. Lipid
C. Nucleic acid (Correct)
D. Protein
Rationale: Nucleic acids (DNA and RNA) store and transmit genetic informa-
tion. Carbohydrates provide energy, lipids form membranes and store energy, and
proteins perform diverse functions but do not store genetic data.
Question 5: What process allows populations to adapt to environmental changes over genera-
tions?
A. Natural selection (Correct)
B. Genetic drift
C. Gene flow
D. Mutation
Rationale: Natural selection drives adaptation by favoring traits that enhance
survival and reproduction in specific environments. Genetic drift is random, gene
flow involves migration, and mutation introduces new variations but is not the
primary adaptive mechanism.
Question 6: Which organelle is responsible for detoxifying harmful substances in liver cells?
A. Lysosome
B. Smooth endoplasmic reticulum (Correct)
C. Golgi apparatus
D. Peroxisome
Rationale: The smooth endoplasmic reticulum (SER) detoxifies drugs and toxins
in liver cells. Lysosomes degrade waste, the Golgi apparatus processes proteins,
and peroxisomes break down fatty acids and produce hydrogen peroxide.
Question 7: During DNA replication, what enzyme unwinds the double helix?
A. DNA polymerase
B. Helicase (Correct)
C. Ligase
D. Primase
2
, Rationale: Helicase unwinds the DNA double helix, separating the strands for
replication. DNA polymerase adds nucleotides, ligase seals gaps, and primase syn-
thesizes RNA primers.
Question 8: In an ecosystem, what term describes organisms that break down dead organic
matter?
A. Producers
B. Consumers
C. Decomposers (Correct)
D. Predators
Rationale: Decomposers, such as fungi and bacteria, break down dead organic
matter, recycling nutrients. Producers make food, consumers eat other organisms,
and predators hunt prey.
Question 9: What type of bond links amino acids in a protein?
A. Hydrogen bond
B. Ionic bond
C. Peptide bond (Correct)
D. Disulfide bond
Rationale: Peptide bonds link amino acids in proteins through a dehydration
reaction. Hydrogen bonds stabilize protein structure, ionic bonds involve charged
groups, and disulfide bonds form between cysteine residues.
Question 10: Which phase of meiosis results in the separation of homologous chromosomes?
A. Prophase I
B. Metaphase I
C. Anaphase I (Correct)
D. Telophase II
Rationale: In anaphase I of meiosis, homologous chromosomes separate, reducing
the chromosome number by half. Prophase I involves pairing, metaphase I aligns
chromosomes, and telophase II completes cell division.
Question 11: What is the primary source of energy for Earths ecosystems?
A. Geothermal heat
B. Sunlight (Correct)
C. Chemical reactions
D. Tidal energy
Rationale: Sunlight drives photosynthesis, the foundation of most ecosystems.
Geothermal heat and chemical reactions support specific ecosystems (e.g., hy-
drothermal vents), and tidal energy is not a primary source.
3
Answers with Expert Concept Review and
Clarifications
Instructions
The following 80 multiple-choice questions are designed to test your biology knowledge
for the HESI A2 exam, covering cellular biology, genetics, DNA/RNA, evolution, macro-
molecules, and ecosystems. Each question includes four answer options, with the correct
answer marked and a detailed explanation of the concept provided. Use these questions
to prepare effectively for your exam.
Question 1: What is the primary function of the mitochondria in a eukaryotic cell?
A. Protein synthesis
B. Energy production (Correct)
C. Cell division
D. Waste storage
Rationale: Mitochondria are the powerhouses of eukaryotic cells, producing ATP
through cellular respiration. Protein synthesis occurs in ribosomes, cell division
is managed by the nucleus and cytoskeleton, and waste storage is associated with
vacuoles.
Question 2: Which molecule carries genetic information from DNA to the ribosome for protein
synthesis?
A. mRNA (Correct)
B. tRNA
C. rRNA
D. snRNA
Rationale: Messenger RNA (mRNA) transcribes genetic information from DNA
in the nucleus and carries it to the ribosome for translation into proteins. tRNA
transfers amino acids, rRNA forms part of the ribosome, and snRNA is involved in
splicing.
Question 3: In a Punnett square for a monohybrid cross (Aa ΠAa), what is the probability of
an offspring being homozygous recessive (aa)?
A. 25% (Correct)
B. 50%
C. 75%
D. 100%
1
, Rationale: For a monohybrid cross (Aa ΠAa), the genotypic ratio is 1 AA : 2
Aa : 1 aa. The homozygous recessive (aa) phenotype occurs in 1 out of 4 offspring,
or 25%.
Question 4: Which macromolecule is primarily responsible for storing genetic information?
A. Carbohydrate
B. Lipid
C. Nucleic acid (Correct)
D. Protein
Rationale: Nucleic acids (DNA and RNA) store and transmit genetic informa-
tion. Carbohydrates provide energy, lipids form membranes and store energy, and
proteins perform diverse functions but do not store genetic data.
Question 5: What process allows populations to adapt to environmental changes over genera-
tions?
A. Natural selection (Correct)
B. Genetic drift
C. Gene flow
D. Mutation
Rationale: Natural selection drives adaptation by favoring traits that enhance
survival and reproduction in specific environments. Genetic drift is random, gene
flow involves migration, and mutation introduces new variations but is not the
primary adaptive mechanism.
Question 6: Which organelle is responsible for detoxifying harmful substances in liver cells?
A. Lysosome
B. Smooth endoplasmic reticulum (Correct)
C. Golgi apparatus
D. Peroxisome
Rationale: The smooth endoplasmic reticulum (SER) detoxifies drugs and toxins
in liver cells. Lysosomes degrade waste, the Golgi apparatus processes proteins,
and peroxisomes break down fatty acids and produce hydrogen peroxide.
Question 7: During DNA replication, what enzyme unwinds the double helix?
A. DNA polymerase
B. Helicase (Correct)
C. Ligase
D. Primase
2
, Rationale: Helicase unwinds the DNA double helix, separating the strands for
replication. DNA polymerase adds nucleotides, ligase seals gaps, and primase syn-
thesizes RNA primers.
Question 8: In an ecosystem, what term describes organisms that break down dead organic
matter?
A. Producers
B. Consumers
C. Decomposers (Correct)
D. Predators
Rationale: Decomposers, such as fungi and bacteria, break down dead organic
matter, recycling nutrients. Producers make food, consumers eat other organisms,
and predators hunt prey.
Question 9: What type of bond links amino acids in a protein?
A. Hydrogen bond
B. Ionic bond
C. Peptide bond (Correct)
D. Disulfide bond
Rationale: Peptide bonds link amino acids in proteins through a dehydration
reaction. Hydrogen bonds stabilize protein structure, ionic bonds involve charged
groups, and disulfide bonds form between cysteine residues.
Question 10: Which phase of meiosis results in the separation of homologous chromosomes?
A. Prophase I
B. Metaphase I
C. Anaphase I (Correct)
D. Telophase II
Rationale: In anaphase I of meiosis, homologous chromosomes separate, reducing
the chromosome number by half. Prophase I involves pairing, metaphase I aligns
chromosomes, and telophase II completes cell division.
Question 11: What is the primary source of energy for Earths ecosystems?
A. Geothermal heat
B. Sunlight (Correct)
C. Chemical reactions
D. Tidal energy
Rationale: Sunlight drives photosynthesis, the foundation of most ecosystems.
Geothermal heat and chemical reactions support specific ecosystems (e.g., hy-
drothermal vents), and tidal energy is not a primary source.
3
