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What is a Nucleosome?

A nucleosome is the structural unit of DNA packaging in eukaryotes. A nucleosome is basically DNA segments surrounded by histone protein octamers resembling a thread coiled around a spool. A nucleosome is the fundamental unit of chromatin.

The nucleosome is further folded into compact structures such that it can form chromosomes. The DNA has to be wrapped into a nucleosome so that it can fit inside the nucleus. A human cell contains almost 30 million nucleosomes each.

Nucleosomes were first observed by Don and Ada Olins in 1974 in an electron microscope as a particle. The structure was, however, later illustrated by Robert Kornberg. A nucleosome structure is made up of nucleosome core particles that are connected by a stretch of linker DNA.

Structure of Nucleosome

The nucleosome core particle is made up of 146 base pairs of DNA which accounts for 1.67 left hand superhelical turns around the histone octamer. Two copies of H2A, H2B, H3 and H4 make up the histone octamer.
Each nucleosome is joined to the other by linker DNA. Linker DNA are free stretches of DNA ranging from 10-80 base pairs in length.
The nucleosome core particle and linker DNA together generate a cylinder that is 11 nm in diameter and 5.5 nm in height.
While observing a dividing cell at the interphase stage, one can observe the unfolding chromatin as a beaded string. The string is the DNA segment, whereas the beads are the nucleosome core particles.

Nucleosome Model of Chromosome

Packaging of DNA into nucleosomes does not fully explain the compact structure of chromosomes found in the nucleus. Further compaction of the nucleosome is not fully understood. The current understanding states that the repeating nucleosomes with the linker DNA connections form a 10 nm fibre that is also described as beads on strings. A chain of nucleosomes can be arranged in a 30 nm fibre, with a packing ratio of almost 50. This compact packing of the nucleosome depends on H1 protein.

The structure of a 30 nm fibre has been proposed as a two-start helix by analysing the crystal structure of a tetranucleosome. This model is still not fully accepted as the results do not match with the electron microscopy scans. It is assumed that the 30 nm fibre folds into loops with a central protein as a scaffold to form euchromatin that is transcriptionally active. Further complications and folding in this structure forms heterochromatin.

Frequently Asked Questions

What is a nucleosome and its function?

A nucleosome is the functional unit of DNA packaging in eukaryotes. It reduces the size and volume of DNA strands so that it can fit inside the nucleus.

What is a nucleosome made of?

A nucleosome is made up of nucleosome core particles and linker DNAs. The nucleosome core particles are made of 146 base pairs of DNA wrapped around by histone octamers. The nucleosome core particles are linked together by linker DNAs.