You treat a sample of DNA with DNase and run the digested fragments out on an electrophoretic gel. You found mostly long fragments. What can you conclude about this sample of DNA

the sample contains mostly heterochromatin  

Explanation:

Heterochromatin refers to a highly condensed state of DNA, whereas euchromatin is lightly packed. It is for that reason that heterochromatin is associated with a transcriptionally inactive state, whereas euchromatin regions are active for transcription. Heterochromatin regions resist DNases because these enzymes don't cut DNA randomly, but they show sequence preference. In consequence, highly repetitive DNA heterochromatin regions (e.g., centromeres, telomeres, etc) may exhibit resistance to DNA cleavage.

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