Final answer:
The statement is true; adenine and guanine are larger molecules than cytosine and thymine because they have a double-ring structure, classifying them as purines, while cytosine and thymine have a single-ring structure, classifying them as pyrimidines.
Step-by-step explanation:
The sentence in question is true. Adenine (A) and guanine (G) are indeed larger molecules than cytosine (C) and thymine (T) because their structure consists of two carbon-nitrogen rings, making them purines. In contrast, cytosine and thymine have a single carbon-nitrogen ring, classifying them as pyrimidines. These structural differences are significant in molecular biology as they influence how these bases pair up in the DNA double helix. Adenine pairs with thymine with two hydrogen bonds, and guanine pairs with cytosine with three hydrogen bonds, maintaining a uniform shape of the DNA molecule.