Final answer:
The 3' end of a DNA strand is characterized by a hydroxyl group on the sugar molecule, not by the presence of any particular nitrogenous base such as adenine, thymine, guanine, or cytosine.
Step-by-step explanation:
The polarity in a DNA strand is indicated by the 3′ and 5′ ends, referring to the carbon numbers in the DNA's sugar backbone. The 3′ end has a free hydroxyl (-OH) group on the 3' carbon of the sugar, while the 5′ end has a free phosphate group attached to the 5' carbon. In terms of the nitrogenous bases—which are adenine, thymine, cytosine, and guanine—none of these is specifically connected to the 3′ end, as they are part of the structure and pair with complementary bases across the two strands which run antiparallel. Thus, the nitrogenous bases are not a structural feature that differentiates the 3′ end from the 5′ end.
The 3′ and 5′ ends of DNA refer to the directionality imposed by the structure of the sugar-phosphate backbone. In the double-helical structure of DNA, the two strands are antiparallel, meaning one strand runs 5′ to 3′ and the other runs 3′ to 5′. The 3′ end of a DNA strand is characterized by having a hydroxyl group attached to the third carbon of the deoxyribose sugar, while the 5′ end has a phosphate group attached to the fifth carbon. Nitrogenous base pairs (adenine-thymine and cytosine-guanine) are located between the sugar-phosphate backbones and are not the distinguishing feature of the 3' or 5' ends.