The Packaging of DNA helix - The Nucleosome

 

• The nucleosomes were first observed by Don and Ada  under the electron microscope.

• The role of the nucleosome was demonstrated by Lorch I vitro and by Han and Grunstein in vivo respectively.

DNA Packaging in Prokaryotes.

• In prokaryotes, there is a single and coiled genetic material. This genetic material is open and not bounded by the nuclear membrane. Therefore such a structure is termed a nucleoid. 

• In Prokaryotes like bacteria, DNA is not scattered in cell but negative charged DNA is held by the postive charged protein in form of large loop. This type of DNA is called as nucleoid.

DNA Packaging in Eukaryotes.

• In Eukaryotes, DNA is packed in the nucleosome. This is the basic structural unit of DNA packaging and  a fundamental subunit of chromatin material.

• Inside the chromatin material or chromosome, DNA in the form of a nucleosome is presented as beads of string. 

• Their presence of DNA as histone octamer in the nucleosome was proposed by Roger Kornberg.

• A nucleosome is the basic structural unit of DNA packaging in eukaryotes.

• Inside the Nucleus of Eukaryotes, there are thread-like structures called chromatin material. A typical nucleosome is also a segment of DNA.

Where can we find the Nucleosome 

• A nucleosome is part of DNA that is wrapped around a core of proteins. Inside the nucleus, DNA is present in the form of a complex structure called chromatin.

• When the chromatin is extended and viewed under a microscope, the structure resembles beads on a string. A Single bead is called a Nucleosome.

• Beads on string in chromatin is packed as chromatin fibre which is further coiled at metaphase stage of cell divison.

• The packaging of Chromatin require high level of additional protein called Non histone protein.

Structure of nucleosome

• A  Nucleosome is prominently consisting of DNA and histone protein.

• A DNA molecule  is negatively charged  and wrapped around  histone proteins like  Wool is wrapped around a woollen spool.

• Every nucleosome has two turns of DNA  that is wrapped around the  histone  proteins.

• The histone protein is basic in nature and positively charged. The basic nature of histone  is due to the presence of two amino acids named Arginine and lysine. 

• This histone protein has eight proteins hence named as  histone octamer. Each histone octamer is composed of two copies of each histone protein i.e., H2A, H2B, H3, and H4.

• A typical nucleosome has 200 base pairs in DNA that are wrapped around the histone octamer.

• A nucleosome is linked to another nucleosome by means of H1 Proteins hence called linker protein. H1 protein does not make a part of histone octamer.

Heterochromatin and Euchromatin 

• The chromatin that are densely packed and gives dark color on staining, called Heterochromatin.

• The chromatin that are loosely packed and gives light color on staining, called Euchromatin.

• Heterochromatin are transcriptionally inactive whereas Euchromatin are active.