The loosely packed chromatin form that contains active genes is called ______.

• A. Nucleosome
• B. Euchromatin
• C. Barr body
• D. Chromatin

Answer: (B)

Chromatin structure controls gene expression: when chromatin is loosely packed, the DNA is accessible to RNA polymerase and transcription factors, so genes can be actively transcribed. This accessible, active form is called euchromatin. It tends to be less condensed during interphase and is enriched for genes being expressed. In contrast, tightly packed chromatin is generally transcriptionally silent. Nucleosomes are the basic units of DNA wrapped around histones found in all chromatin states, Barr bodies are highly condensed inactive X chromosomes, and chromatin is the general term for DNA–protein complexes at varying condensation levels. Thus, the loosely packed form that contains active genes is euchromatin.