Chromatin Structure

Chromatin is the complex of DNA, proteins, and RNA that forms the genetic material in the nucleus of eukaryotic cells. It is responsible for organizing and packaging the DNA so that it can fit within the small confines of the nucleus. When viewed under an electron microscope, the structure of chromatin appears as beads-on-a-string, with the beads representing nucleosomes.

Nucleosomes

Nucleosomes are the basic repeating units of chromatin. Each nucleosome consists of DNA wrapped around a core of eight histone proteins, forming a structure known as a histone octamer. The histone octamer is composed of two copies each of four different types of histone proteins: H2A, H2B, H3, and H4. The DNA is wrapped around the histone octamer in about 1.65 turns, forming a compact structure.

Formation of Nucleosomes

The formation of nucleosomes is a crucial step in the packaging of DNA. The negatively charged DNA molecule is attracted to the positively charged histone proteins, causing the DNA to wrap around the histone octamer. This wrapping process helps to condense the DNA and protect it from damage. It also plays a role in regulating gene expression by controlling access to the DNA.

Beads-on-a-String Structure

The nucleosomes are connected by stretches of linker DNA, which appear as strings between the beads when viewed under an electron microscope. This gives the chromatin structure a "beads-on-a-string" appearance. The beads, or nucleosomes, are about 10 nm in diameter and are spaced approximately 20-30 nm apart. The linker DNA between nucleosomes is approximately 20-60 base pairs long.

Advantages of Beads-on-a-String Structure

The beads-on-a-string structure of chromatin provides several advantages. Firstly, it allows for efficient packaging of the DNA within the nucleus, as the compact nucleosomes can be easily folded and organized. Secondly, it provides accessibility to the DNA when needed, as the nucleosomes can be unwrapped to expose the DNA for transcription or replication. Lastly, it allows for the compaction of the DNA during cell division, as the nucleosomes can further condense into higher-order structures.

Overall, the beads-on-a-string structure of chromatin, with nucleosomes as the beads, plays a crucial role in the organization, packaging, and regulation of the genetic material within the nucleus of eukaryotic cells.

The negatively charged DNA is wrapped around the positively charged hi stone octamer to form the 'nucleosome'. A typical nucleosome contains 200 base pairs of a DNA helix. Nucleosomes constitute repeating unit lime structure in the nucleus called 'chromatin', thread-like stained (coloured) bodies seen in the nucleus.So the nucleosomes in the chromatin are seen as 'beeds-on -string' structure when viewed under electron microscope