Creatures do have maximum DNA caps per Creature Level based on their rarity. When the cap is reached, any additional DNA acquired in-game will not be added. The only exception for this is for any DNA acquired through purchased incubators. 

• Common: 250,000 DNA 
• Rare: 100,000 DNA
• Epic: 25,000 DNA 
• Legendary: 10,00 DNA 
• Unique: 4,000 DNA