Question

The number of bases in each turn of Z-DNA and B-DNA helices respectively is
A. 9 and 11
B. 12 and 10
C. 10 and 13
D. 12 and 11

Answer 1

Hint: DNA molecule exists in different forms in different conditions. The DNA exists in three different forms- A-DNA, B-DNA, Z-DNA. Under ordinary conditions, B- DNA exists in the cell predominantly. In human cells a huge volume of DNA is twisted and stored in a very small volume.

Complete Answer:
- DNA occurs as a double helical structure. A, B, Z DNA are the forms of DNA in which the nucleotides(A, G, C, T) are linked together in a long chain and form amino acids. The bond between the two chains is formed by the complementary bases.
- Adenine bonds with Thymine and Cytosine bonds with Guanine and forms a double helix.
- B-DNA: In this the nitrogenous base is perpendicular to the axis of helix and the base twists at 36 degrees with the adjacent bases. Each complete turn has around 10 base pairs.
- Z-DNA: Z DNA is a left handed helical structure with 4.4nm turn length and 12 bases in each turn. The presence of Z-DNA occurs in some enhancers.

Thus the correct answer is (B). The number of bases in each turn of Z-DNA and B-DNA helices respectively is 12 and 10.

Additional Information: A DNA is the most stable form of DNA as it has an additional –OH group in the ribose. It occurs during the transcription.

Note: Generally under the ordinary condition, DNA exists in the B form. Under low humidity and presence of certain salts the B DNA gets converted to Z form.