Nucleosome

To understand nucleosome properly, it is important to know that eukaryotic DNA is very long and cannot fit inside the nucleus unless it is packed in an organised way. This organised packaging starts with a basic unit known as the nucleosome. A nucleosome is the basic structural and functional unit of chromatin in eukaryotic cells. It plays a crucial role in DNA packaging by enabling long DNA strands to be compacted within the nucleus in a highly organised way. The term "nucleosome" was first introduced by Roger Kornberg in 1974, who also explained its "bead-like" appearance under the electron microscope. A nucleosome consists of DNA wrapped around histone proteins. This structure is essential for compacting DNA within the nucleus, ensuring its proper organisation for processes like transcription and replication.

Structure of Nucleosome

The nucleosome consists of a core particle and linker DNA. The core particle is an octamer of histone proteins, with two copies each of histones H2A, H2B, H3 and H4. Around this octamer, approximately 147 base pairs of DNA are wrapped in 1.65 left-handed superhelical turns. This DNA-histone complex is tightly coiled to form a structure that resembles "beads on a string." The core nucleosome is connected by linker DNA, which varies in length (approximately 20-80 base pairs) and connects adjacent nucleosomes. A fifth histone protein called H1 binds to the linker DNA between nucleosomes. It helps in stabilizing the structure and supports the formation of higher-order chromatin arrangements such as the 30 nm fiber. The H1 histone functions like a clamp and strengthens the DNA's firm attachment to the histone core.
The nucleosome consists of a core particle and linker DNA. The core particle is an octamer of histone proteins, with two copies each of histones H2A, H2B, H3 and H4. Around this octamer, approximately 147 base pairs of DNA are wrapped in 1.65 left-handed superhelical turns. This DNA-histone complex is



DNA-histone complex is tightly coiled to form a structure that resembles "beads on a string." The core nucleosome is connected by linker DNA, which varies in length (approximately 20-80 base pairs) and connects adjacent nucleosomes. A fifth histone protein called H1 binds to the linker DNA between nucleosomes. It helps in stabilizing the structure and supports the formation of higher-order chromatin arrangements such as the 30 nm fiber.

Functions of Nucleosome

The nucleosome performs multiple essential functions:
  • It helps in condensing and packaging the DNA into a compact form so that it fits inside the nucleus.
  • It acts as a protective unit for DNA, preventing it from physical damage and enzymatic degradation.
  • It plays a regulatory role in gene expression, as the wrapping of DNA influences which genes are active or silent.
  • It also helps in chromatin remodeling, DNA replication and repair processes by making specific regions accessible or inaccessible as needed.





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