Which characteristic best describes heterochromatin?

Heterochromatin is best described as always highly condensed. This state of chromatin is distinctly different from euchromatin, which is more loosely packed and accessible for transcription and gene expression. The high level of condensation in heterochromatin makes it less transcriptionally active, as the tightly wound DNA is not easily accessible to the transcription machinery.

Heterochromatin typically contains repetitive sequences and is often found at the centromeres and telomeres of chromosomes. Its structure plays crucial roles in maintaining chromosome integrity and regulating gene expression, particularly by silencing certain genes. By remaining in a condensed form, heterochromatin can also help protect these regions from rearrangements and mutations.

In contrast, characteristics such as being loosely packed for transcription, varying in density, or constituting most of the DNA do not accurately reflect the nature of heterochromatin. These features are more characteristic of euchromatin or do not define heterochromatin specifically.