The nucleosome represents a fundamental structural unit within the cells of complex organisms. It plays an important role in organizing genetic material inside the nucleus. This structure ensures the lengthy DNA molecule is compactly stored and managed. Understanding nucleosomes helps comprehend how genetic information is maintained and utilized.
The Essential Building Blocks
The nucleosome is assembled from two components: DNA and proteins called histones. DNA carries the cell’s genetic instructions, forming long, linear strands. Histones are small, positively charged proteins important for DNA organization, binding tightly to negatively charged DNA.
Four core histone types exist: H2A, H2B, H3, and H4. Two copies of each type form an eight-protein complex, a histone octamer. This octamer serves as the central spool around which DNA winds, initiating compaction.
Constructing the Nucleosome
The formation of a nucleosome involves the precise winding of a segment of DNA around the histone octamer. Approximately 146 to 147 base pairs of DNA wrap nearly twice (about 1.67 left-handed turns) around this protein core. This arrangement creates a compact particle that resembles a “bead on a string” when viewed under an electron microscope, with nucleosomes as “beads” and connecting DNA as the “string.”
Linker DNA, a short stretch, connects one nucleosome to the next, with its length varying between 10 to 80 base pairs depending on the species and tissue. Another histone protein, H1, often associates with this linker DNA, helping to further stabilize the nucleosome structure and promote higher levels of compaction.
The Primary Function: DNA Packaging
The primary function of nucleosomes is the efficient packaging of DNA. Without this organizational unit, the 2-meter length of DNA within each human cell would not fit into the microscopic nucleus (only a few micrometers in diameter). Nucleosomes provide the initial level of condensation, reducing DNA length by a factor of about six.
These nucleosomes then undergo further coiling and folding into more complex arrangements, forming chromatin fibers. These fibers can further condense into loop domains, eventually forming compact structures, chromosomes, during cell division. This hierarchical packaging ensures the DNA is both manageable and protected within the cell.
Beyond Packaging: Gene Regulation
Beyond their role in packaging, nucleosomes are active participants in regulating gene expression, influencing gene activation. Nucleosome positioning along DNA determines whether specific genetic sequences are accessible to cellular machinery for “reading” genes, such as transcription factors. When DNA is tightly wrapped around nucleosomes, it can prevent these proteins from binding, silencing the associated genes.
Conversely, looser packing or changes in nucleosome position can expose DNA regions, allowing genes to become active. Chemical modifications to the histone proteins, such as acetylation or methylation, play a significant role in this regulatory process. These modifications can alter how tightly DNA is wound or recruit other proteins that influence gene activity, highlighting the nucleosome’s dynamic involvement in cellular processes.