AP BIOLOGY

THE MOLECULAR BASIS OF INHERITANCE

STRUCTURE OF NUCLEIC ACIDS

Question [CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
Which of the following describes the difference between a purine and a pyrimidine?
A
Purines are single ring structures while pyrimidines are double ring structures.
B
Pyrimidines are single ring structures while purines are double ring structures.
C
Purines are composed of single bonds while pyrimidines are composed of double bonds.
D
Pyrimidines are composed of single bonds while purines are composed of double bonds.
Explanation: 

Detailed explanation-1: -Purine has two rings and pyrimidines have one ring. DNA consists of purines adenine and guanine and pyrimidines cytosine and thymine.

Detailed explanation-2: -The purines (adenine and guanine) have a two-ringed structure consisting of a nine-membered molecule with four nitrogen atoms, as you can see in the two figures below. The pyrimidines (cytosine, uracil, and thymine) only have one single ring, which has just six members and two nitrogen atoms.

Detailed explanation-3: -There are two kinds of nitrogen-containing bases-purines and pyrimidines. Purines consist of a six-membered and a five-membered nitrogen-containing ring, fused together. Pyridmidines have only a six-membered nitrogen-containing ring.

Detailed explanation-4: -The pyrimidine bases have a 6‐membered ring with two nitrogens and four carbons. The purine bases have a 9‐membered double‐ring system with four nitrogens and five carbons.

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