Molecular Biology Multiple Choice Questions on “Alternative Helical Structures of DNA”.
1. What is the condition required to observe B form of DNA?
A. Low humidity
B. High humidity
C. Low temperature
D. High temperature
Answer: B
Explanation: X – ray diffraction studies of DNA shows two different kinds of structures of DNA, A and B forms. Of the two, B form of DNA is generally observable at high humidity and closely resembles to the structure of the DNA in physiological condition.
2. What is the condition required to observe A form of DNA?
A. Low humidity
B. High humidity
C. Low temperature
D. High temperature
Answer: A
Explanation: A form of DNA is generally observed at low humidity. The conformation is usually taken by DNA – RNA and RNA – RNA complexes. They have a much more compact structure than any another form of DNA.
3. How many bases are there in per turn of the B form of DNA?
A. 8 bp
B. 9 bp
C. 10 bp
D. 11 bp
Answer: C
Explanation: B form of DNA has 10 base pairs per turn. The height of one complete turn, that is, the pitch is 2.46 nm.
4. Which of the following is not a character of A DNA?
A. Short and broad
B. Left handed helix
C. Narrow minor groove
D. Broad and shallow major groove
Answer: B
Explanation: A form of DNA is known to have a right handed helical structure. The left handed helical conformation is the characteristic form of the Z form of DNA.
5. How many bases are there in per turn of the A form of DNA?
A. 11 bp
B. 10 bp
C. 12 bp
D. 8 bp
Answer: A
Explanation: A form of DNA has 11 base pairs per turn. The height of one complete turn, that is, the pitch is 3.32 nm.
6. Which of the following is not a character of B form of DNA?
A. Longer and thinner
B. Right-handed helix
C. Narrow minor groove
D. Flat major groove
Answer: D
Explanation: B form of DNA has a wide and intermediate major groove. Flattened major groove is normally found Z form of DNA.
7. In a right handed DNA the glycosidic bond is in syn configuration.
A. True
B. False
Answer: B
Explanation: Glycosidic bond connects the base to the 1’ position of 2’-deoxyribose. This bond can be in one of the two conformations called syn and anti. In right handed DNA, the glycosidic bond is always in the anti configuration.
8. In the Z form of DNA the fundamental repeating unit is __________
A. Purine – pyrimidine nucleotide
B. Purine – pyrimidine dinucleotide
C. Purine – pyrimidine tri-nucleotide
D. Purine – pyrimidine tetra-nucleotide
Answer: D
Explanation: As we know Z form of DNA is the left–handed DNA. Thus it has the anti–conformation at the pyrimidine and the syn–conformation at the purine and is usually a dinucleotide unit.
9. What is the factor responsible for the left-handed helical conformation in Z form of DNA?
A. Syn conformation at pyrimidine
B. Anti conformation at pyrimidine
C. Syn conformation at purine
D. Anti conformation at purine
Answer: C
Explanation: It is the syn conformation at the purine nucleotides that is responsible for the left-handed helical conformation. The change to the syn position in the purine residues to alternating anti – syn conformation gives the backbone of the left-handed DNA a zig-zag look.
10. In higher salt concentration the DNA helix assumes the right-handed conformation.
A. True
B. False
Answer: B