A nucleosome is a section of DNA wrapped around a core of eight histone proteins. An additional histone protein is associated with the linker DNA between nucleosomes, helping to organize DNA into chromatin.
The Reasoning and Structure
DNA molecules are extremely long, so they must be packaged efficiently inside the nucleus. Histones are proteins around which DNA can wrap, producing repeating DNAandprotein units called nucleosomes.
The core of each nucleosome contains eight histone proteins, forming a histone octamer. DNA wraps around the outside of this core. Linker DNA connects one nucleosome to the next, and an additional histone protein is attached to this linker region.
| Component | Structure and role |
|---|---|
| Histone core | Eight histone proteins grouped into an octamer |
| Wrapped DNA | DNA associated with and wrapped around the histone core |
| Linker DNA | DNA connecting adjacent nucleosomes |
| Additional histone | Histone associated with the linker DNA, helping stabilize the arrangement |
This repeating organization produces chromatin, which allows a long DNA molecule to be packaged within the nucleus. Nucleosomes therefore contain both DNA and protein; they are not made from DNA alone.
A common misconception is that a nucleosome is the same as a chromosome. A nucleosome is one small DNA/histone unit, whereas a chromosome consists of a much longer DNA molecule associated with many proteins and containing many nucleosomes.
Exam Technique
This is Additional Higher Level (AHL) content in subtopic A1.2, Nucleic acids. For a structure question, state all three essential features: DNA wraps around a core of eight histones, linker DNA connects adjacent nucleosomes, and an additional histone is associated with the linker DNA. Do not merely write that histones “package DNA”; that statement lacks the structural detail needed for full credit.
