Molecular Biology Questions and Answers – Double Stranded DNA Forms a Double Helix

This set of Molecular Biology Multiple Choice Questions & Answers (MCQs) focuses on “Double Stranded DNA Forms a Double Helix”.

1. The first X-ray diffraction patterns of DNA were taken in 1938 by ___________
a) William Asbury
b) Rosalind Franklin
c) Francis H. Crick
d) Linus Pauling
View Answer

Answer: a
Explanation: The first X–ray diffraction patterns of DNA of low quality taken in 1983 by William Asbury. He used using DNA supplied by Ola Hammarsten and Torbjorn Caspersson.

2. In early 1950s high quality X-ray diffraction photographs of DNA suggesting the DNA being double helix and composed of two nucleotide strands. Who took those photographs?
a) Rosalind Franklin
b) William Asbury
c) Francis H. Crick and James D. Watson
d) Rosalind Franklin and Maurice Wilkins
View Answer

Answer: d
Explanation: The high quality X-ray diffraction photographs were taken by Rosalind Franklin and Maurice Wilkins. It suggested that DNA structure was helical and might be composed of more than one nucleotide strand in early 1950s.

3. In 1952, an unambiguous established work in the laboratory of Alexander Todd led to the discovery of _____________
a) Chemical nature of DNA
b) X-ray diffraction structure of DNA
c) 3’-5’ phosphodiester linkage regularly links the nucleotides of DNA
d) Nucleic acid strands are held together by hydrogen bonds
View Answer

Answer: c
Explanation: Chemical nature of DNA was discovered by Watson and Crick in 1952. They also found that nucleic acid strands are held together by hydrogen bonds. Again the first X-ray diffraction structure was discovered by William Asbury in 1938.

4. What should be the complementary strand of 3’….ATGGCTTGA….5’?
a) 3’….TACCGAACT….5’
b) 5’….TACCGAACT….3’
c) 3’….TAGGCAAGT….5’
d) 5’….TAGGCAAGT….3’
View Answer

Answer: b
Explanation: The complementary of A is T and for G is C, therefore the complementary sequence should be ….TACCGAACT…. And the two strands are antiparallel as the given strand is in 3’ to 5’ direction the complementary strand should be in 5’ to 3’ direction.

5. The hydrogen bonds formed during A and T bonding occurs between C6NH2 of A and __________
a) C4O of T
b) PO3
c) C6O of A
d) C1O of T
View Answer

Answer: a
Explanation: As the sugar molecule is connected to N1 in T the C4O and C1O are free. Also the C1O is farthest from C6NH2 thus the bond forms between C6NH2 and C4O. This stabilizes the model because of being in preferred tautomeric state.
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6. In a DNA double helix the bases are held together by hydrogen bonds. These hydrogen bonds are _________
a) Covalent bonds
b) Non-covalent bonds
c) Ionic bonds
d) Van der Waals forces
View Answer

Answer: b
Explanation: Hydrogen bond is a weak non covalent bond. This occurs between two molecules resulting from an electrostatic attraction between a proton and an electronegative atom of the other molecule. No share of electron takes place which is the criteria for non-covalent bonding.

7. It is easy to break the bond between A and T than in between G and C.
a) True
b) False
View Answer

Answer: a
Explanation: It is easy to break the bond between A and T than in between G and C. This is because A and T has a double bond whereas G and C has triple bond connecting them. Thus, the energy required to break a double bond is lower compared to a triple bond.

8. With respect to the importance of hydrogen bonding and DNA double helix stability, which of the following statements is false?
a) Favorable tautomeric form of nucleotide bases
b) Contributes to the thermodynamic stability
c) Decreases the entropy
d) Specificity of base pairing
View Answer

Answer: c
Explanation: When the two complementary strands of DNA comes together in an aqueous solution the water molecules attached to the polynucleotide strands are displaced from the bases. This creates disorder and this increases the entropy. Therefore, it finally increases the stability of the DNA duplex.

9. In the late 1970s non double helical form of DNA was also suggested but was discarded on the basis of certain factors. Which of the following is not a factor responsible?
a) X-ray crystallography
b) Nuclein
c) Topoisomerase
d) Nucleosome core particle
View Answer

Answer: b
Explanation: Nuclein, later named as nucleic acid and eventually DNA was first discovered by Swiss physiological chemist Friedrich Miescher inside the nuclei of human WBCs in 1869. Thus nuclein is not a term related to the structural orientation and chemical nature of DNA.

10. Levene investigated and found that the nucleic acid is composed of poly-nucleotides and each nucleotide is composed of one base, a sugar molecule and a phosphate. This was performed on the genome of________________
a) Bacteria
b) Fungi
c) WBCs
d) Yeast
View Answer

Answer: d
Explanation: Levene placed his initial proposal of nucleic acids being composed of nucleotides and nucleotides being composed of one of the four bases, a phosphate and a sugar molecule in 1919. He gathered all his evidence regarding the structure were from the repeated experiments of hydrolysis and analysis of the yeast nucleic acid.

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Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

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