Heterochromatin is not transcribed because ______.

• A. it contains only coding regions of DNA
• B. its tightly packed structure prevents access of transcription machinery to DNA
• C. it is actively acetylated, promoting gene silencing
• D. it is located exclusively in the nucleolus

Answer: (B)

Verified step by step guidance

1. Understand the nature of heterochromatin: it is a form of chromatin that is densely packed compared to euchromatin.
2. Recall that transcription requires access of transcription machinery (like RNA polymerase and transcription factors) to DNA sequences.
3. Recognize that the tightly packed structure of heterochromatin physically restricts the binding of transcription machinery to the DNA.
4. Note that heterochromatin is generally gene-poor or contains genes that are transcriptionally inactive due to this inaccessibility.
5. Conclude that the reason heterochromatin is not transcribed is because its compact structure prevents transcription machinery from accessing the DNA.