A unit of chromatin consisting of a DNA strand wrapped around histone proteins is called what?

• A. Solenoid
• B. Nucleosome
• C. Fiber
• D. Nucleoid

Answer: (B)

DNA packaging into nucleosomes is the concept at play here. A nucleosome is the basic unit of chromatin and consists of DNA wrapped around a core of histone proteins (two copies each of H2A, H2B, H3, and H4). About 146 base pairs of DNA coil around this histone octamer, creating a bead-like structure that repeats along the chromosome. This arrangement is the primary way eukaryotic DNA is compacted and organized, and it also helps regulate access to genes. Higher-order folding of these nucleosome beads can lead to more condensed forms, like the 30-nm fiber often described as a solenoid, but the unit defined by DNA wrapped around histones is the nucleosome. The term nucleoid refers to the region in bacteria where their chromosome resides, not to eukaryotic chromatin.