Final answer:
DNA has a purine across from a pyrimidine to maintain a consistent and stable double helix structure. The larger purines pair with the smaller pyrimidines to create uniform steps in the helix, and hydrogen bonding between pairs adenine-thymine and cytosine-guanine imparts additional stability, which is critical for accurate genetic information replication.
Step-by-step explanation:
The structure of DNA is optimized for stability and uniformity. Purines, which include adenine (A) and guanine (G), are larger molecules with a double-ringed structure. In contrast, pyrimidines, including cytosine (C) and thymine (T), are smaller and have a single-ring structure. A key aspect of DNA's structure is that a purine on one strand always pairs with a pyrimidine on the complementary strand.
This purine-pyrimidine pairing is essential because it maintains a consistent width of the DNA double helix. If two purines were to pair, or two pyrimidines, the width of the helix would be irregular, disrupting the structure. Think of it like a staircase where every step must be the exact same width for the stairs to work properly. Moreover, the pairing allows for the hydrogen bonding that holds the two strands of DNA together. Adenine pairs with thymine forming two hydrogen bonds, while cytosine pairs with guanine forming three hydrogen bonds, further stabilizing the structure.
These pairings are the basis for Chargaff's rules, which state that the amount of adenine always equals thymine, and the amount of cytosine always equals guanine in a DNA molecule. Thus, the purine and pyrimidine pairing not only maintains the DNA structure but also ensures the accurate replication and transmission of genetic information.