Final answer:
Barriers are DNA elements that prevent the spreading of heterochromatin into euchromatin, ensuring euchromatic regions remain transcriptionally active.
Step-by-step explanation:
The component that can prevent heterochromatin spreading into an adjacent euchromatic region of the chromosome is a barrier. Barriers, also known as insulators, are DNA elements that can block the interaction between enhancers and promoters, and they also prevent the spread of heterochromatin into euchromatin. This helps to maintain the euchromatic region's ability to be actively transcribed, ensuring proper gene expression.