How many nucleosomes are present in a turn of the 30-nm chromatin fiber?
The correct answer and explanation is:
Correct Answer: Approximately 6 nucleosomes
The 30-nanometer (nm) chromatin fiber represents a higher-order structure of DNA packaging found in eukaryotic cells. This level of organization comes after the formation of nucleosomes, which are the basic units of chromatin. Each nucleosome consists of DNA wrapped around a core of eight histone proteins. Roughly 147 base pairs of DNA are wrapped around each nucleosome, and a short segment of linker DNA connects adjacent nucleosomes.
To compact this structure further, the nucleosome string coils into a thicker fiber, known as the 30-nm fiber. This fiber is more condensed and provides additional regulation and protection for the genetic material. In this organization, nucleosomes are arranged in a helical or spiral configuration. Although the exact structure has been debated, one well-supported model is the solenoid model.
In the solenoid model, the nucleosomes are organized into a helical shape where around six nucleosomes make one complete turn. This means that in one loop or turn of the 30-nm fiber, approximately six nucleosomes are present. These nucleosomes are positioned such that their histone tails can interact with each other and with the linker histone H1, further stabilizing the compacted fiber.
The significance of this arrangement lies in its ability to efficiently package DNA into the nucleus while still allowing access to certain regions for transcription, replication, and repair. The structure is dynamic, and changes in chromatin compaction can regulate gene expression by making DNA more or less accessible to the necessary enzymes and transcription factors.
In summary, one turn of the 30-nm chromatin fiber contains about six nucleosomes. This organization plays a critical role in the hierarchical folding of DNA and the regulation of its accessibility for cellular processes.