Question

What accounts for the uniform diameter of the DNA molecule? A.The two sides of the DNA molecule are held together by hydrogen bonds.
B.A purine always bonds with a pyrimidine.
C. One side of the DNA molecule has an unconnected 5' phosphate group and the opposite end has an unconnected 3' hydroxyl group.
D. The 3' carbon of one deoxyribose and the 5' carbon of another deoxyribose bond together.
E.The alternating sugar and phosphate backbone coils around the outside of the helix.

Answer 1

Answer:

B. A purine always bonds with a pyrimidine.

Explanation:

DNA is a polymer of deoxyribonucleotides each of which has a nitrogenous base, a pentose sugar and phosphate group. There are two types of nitrogenous bases present in DNA. Adenine and guanine are the purine bases and have two rings in their structure while the cytosine and thymine are the pyrimidine bases with a single ring structure.

A purine base of one DNA strand always pairs with the pyrimidine base of another DNA strand. Adenine (purine) makes two hydrogen bonds with thymine (pyrimidine) while guanine (purine) makes three hydrogen bonds with cytosine (pyrimidine). The pairing of a purine with a pyrimidine base keeps the diameter of a DNA double helix uniform throughout the length. For instance, the diameter of B-DNA is around 20 angstrom.