Adenine and cytosine cannot form stable pairs due to their incompatible shapes, which limits effective hydrogen bonding. While they can bond, the bond strength is weak, making it unstable compared to the stronger pairing of adenine with thymine or uracil. The discussion also touches on the significance of molecular duplication, emphasizing that successful molecules must replicate to proliferate and survive in their environments. This replication process is crucial for the continuation of life, despite nature's indifference to the complexity of these molecules. Understanding these concepts clarifies the fundamental principles of DNA stability and replication.