The Nucleosome

The nucleosome is the basic structural unit of chromatin, which is the complex of DNA and proteins that make up the chromosomes in eukaryotic cells. It consists of DNA wrapped around a core of proteins called histones.

Here are the key features and components of the nucleosome:

1. Histones: Histones are a group of small, positively charged proteins that play a crucial role in packaging and organizing DNA within the nucleus. There are five main types of histones: H1, H2A, H2B, H3, and H4. Within the nucleosome, the core is made up of two copies each of histones H2A, H2B, H3, and H4.
2. DNA Wrapping: The DNA double helix wraps around the histone core in a left-handed manner, forming approximately 1.65 turns around the nucleosome. This wrapping of DNA around the histone core helps to compact and organize the long DNA molecule.
3. Histone Octamer: The histone core in the nucleosome consists of two copies each of histones H2A, H2B, H3, and H4, forming an octamer structure. The histone octamer provides a scaffold for DNA to wrap around.
4. Linker DNA: The DNA between adjacent nucleosomes, known as linker DNA, connects one nucleosome to the next. It is less tightly associated with histones compared to the DNA within the nucleosome core. Linker DNA length can vary, affecting the degree of compaction and accessibility of the chromatin.
5. Nucleosome Beads-on-a-String: When chromatin is decondensed, it appears as a “beads-on-a-string” structure, with nucleosomes spaced along the DNA molecule like beads on a string. The linker DNA connects adjacent nucleosomes, while the nucleosomes themselves are the “beads.”

The nucleosome plays a critical role in regulating the accessibility of DNA for various cellular processes. It helps to compact DNA, protecting it and enabling it to fit within the limited space of the nucleus. The structure of the nucleosome also influences gene expression and DNA replication by affecting the accessibility of regulatory proteins and enzymes to specific regions of DNA.

Understanding the nucleosome and its organization provides insights into how chromatin structure influences DNA packaging, gene regulation, and the overall functioning of the genome in eukaryotic cells.