Final answer:
Highly condensed regions within chromosomes are known as heterochromatin. These are typically found in regions like the centromere and telomeres and are associated with genes that are not actively transcribed.
Step-by-step explanation:
Regions within the chromosomes where the chromatin fibers are highly condensed are known as heterochromatin. During the interphase of the cell cycle, eukaryotic chromosomes exhibit two distinct regions distinguishable by their staining properties.
The highly condensed heterochromatin generally comprises regions of the genome that are not active in transcription, such as the centromere and telomeres, and appears more intensely stained due to its densely packed state.
In contrast, euchromatin represents regions where chromatin is less condensed, typically involving genes that are actively transcribed, and this structure allows for easier access by enzymes involved in replication and transcription.