This set of Molecular Biology Multiple Choice Questions & Answers (MCQs) focuses 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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
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
View Answer
Explanation: In higher salt concentration the DNA helix assumes the left-handed conformation. This is because the higher concentration of the positively charged ions is required to shield the negatively charged phosphate groups to maintain the stability of the conformation.
Sanfoundry Global Education & Learning Series – Molecular Biology.
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