Page 1 of 98
HOLECHEK BIO 340 EXAM 1,2 ,3 ACTUAL
2026
Q1. Who is credited with discovering the double-helix structure
of DNA using X-ray diffraction data?
✅ Answer: James Watson and Francis Crick.
🧠 Rationale: Watson and Crick integrated Rosalind Franklin‘s
X-ray images to propose the double-helix model in 1953.
Franklin’s Photo 51 was critical, but the Nobel Prize was
awarded to Watson, Crick and Wilkins.
Q2. The “Central Dogma” of molecular biology describes the
flow of genetic information as:
✅ Answer: DNA → RNA → Protein.
🧠 Rationale: The Central Dogma, first stated by Francis Crick,
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holds that genetic information flows from DNA to RNA
(transcription) and RNA to protein (translation). Reverse
transcription (RNA → DNA) occurs only in retroviruses, not in
normal cells.
Q3. Which of the following experiments proved that DNA, not
protein, is the genetic material in bacteriophages?
✅ Answer: Hershey–Chase experiment.
🧠 Rationale: In 1952, Hershey and Chase used radioactive
isotopes (³²P to label DNA, ³⁵S to label protein) to track which
molecule entered bacteria during phage infection. Only ³²P was
transferred, confirming DNA as the genetic material.
Q4. Chargaff’s rules state that in a DNA molecule:
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✅ Answer: A + G = T + C (and A = T, G = C).
🧠 Rationale: Chargaff’s parity rules: the amount of adenine
equals thymine, and the amount of guanine equals cytosine,
because A pairs with T (two hydrogen bonds) and G pairs with C
(three hydrogen bonds).
Q5. A DNA sample contains 28% thymine. What is the
approximate percentage of guanine?
✅ Answer: 22%.
🧠 Rationale: If T = 28%, then A = 28% (A = T). Therefore,
A+T = 56%, leaving 44% for G+C (since G = C). Thus, G =
44% / 2 = 22%.
Q6. Which type of RNA carries the genetic code from DNA to
the ribosome for protein synthesis?
, Page 4 of 98
✅ Answer: mRNA (messenger RNA).
🧠 Rationale: mRNA is transcribed from a DNA template and
serves as the intermediate molecule that directs the order of
amino acids during translation.
Q7. The sequence of amino acids in a protein is determined by:
✅ Answer: The sequence of codons in mRNA.
🧠 Rationale: Each set of three nucleotides (codon) in mRNA
corresponds to one specific amino acid (or a start/stop signal),
thanks to the nearly universal genetic code.
Q8. The Hershey–Chase experiment incorporated ³²P into DNA
and ³⁵S into protein. Why was ³²P used to label DNA?
✅ Answer: Because phosphorus is abundant in DNA but absent
in the typical amino acids of proteins.
HOLECHEK BIO 340 EXAM 1,2 ,3 ACTUAL
2026
Q1. Who is credited with discovering the double-helix structure
of DNA using X-ray diffraction data?
✅ Answer: James Watson and Francis Crick.
🧠 Rationale: Watson and Crick integrated Rosalind Franklin‘s
X-ray images to propose the double-helix model in 1953.
Franklin’s Photo 51 was critical, but the Nobel Prize was
awarded to Watson, Crick and Wilkins.
Q2. The “Central Dogma” of molecular biology describes the
flow of genetic information as:
✅ Answer: DNA → RNA → Protein.
🧠 Rationale: The Central Dogma, first stated by Francis Crick,
,Page 2 of 98
holds that genetic information flows from DNA to RNA
(transcription) and RNA to protein (translation). Reverse
transcription (RNA → DNA) occurs only in retroviruses, not in
normal cells.
Q3. Which of the following experiments proved that DNA, not
protein, is the genetic material in bacteriophages?
✅ Answer: Hershey–Chase experiment.
🧠 Rationale: In 1952, Hershey and Chase used radioactive
isotopes (³²P to label DNA, ³⁵S to label protein) to track which
molecule entered bacteria during phage infection. Only ³²P was
transferred, confirming DNA as the genetic material.
Q4. Chargaff’s rules state that in a DNA molecule:
,Page 3 of 98
✅ Answer: A + G = T + C (and A = T, G = C).
🧠 Rationale: Chargaff’s parity rules: the amount of adenine
equals thymine, and the amount of guanine equals cytosine,
because A pairs with T (two hydrogen bonds) and G pairs with C
(three hydrogen bonds).
Q5. A DNA sample contains 28% thymine. What is the
approximate percentage of guanine?
✅ Answer: 22%.
🧠 Rationale: If T = 28%, then A = 28% (A = T). Therefore,
A+T = 56%, leaving 44% for G+C (since G = C). Thus, G =
44% / 2 = 22%.
Q6. Which type of RNA carries the genetic code from DNA to
the ribosome for protein synthesis?
, Page 4 of 98
✅ Answer: mRNA (messenger RNA).
🧠 Rationale: mRNA is transcribed from a DNA template and
serves as the intermediate molecule that directs the order of
amino acids during translation.
Q7. The sequence of amino acids in a protein is determined by:
✅ Answer: The sequence of codons in mRNA.
🧠 Rationale: Each set of three nucleotides (codon) in mRNA
corresponds to one specific amino acid (or a start/stop signal),
thanks to the nearly universal genetic code.
Q8. The Hershey–Chase experiment incorporated ³²P into DNA
and ³⁵S into protein. Why was ³²P used to label DNA?
✅ Answer: Because phosphorus is abundant in DNA but absent
in the typical amino acids of proteins.
