The DNA of a bacterial cell, such as Escherichia coli, is a circular double-stranded molecule often referred to as the bacterial chromosome.
The circular DNA is packaged into a region of the cell called the nucleoid where it is organized into 50 or so loops or domains that are bound to a central protein scaffold, attached to the cell membrane.
The DNA is negatively supercoiled, that is, it is twisted upon itself.
It is complexed with several DNA-binding proteins, the most common of which are proteins HU, HLP-1 and H-NS. These are histone-like proteins.
The large amount of genomic DNA in a eukaryotic cell is tightly packaged in chromosomes contained within a specialized organelle, the nucleus.
With the exception of the sex chromosomes, diploid eukaryotic organisms such as humans have two copies of each chromosome, one inherited from the father and one from the mother.
Chromosomes contain both DNA and protein.
Most of the protein on a weight basis is histones, but there are also many thousands of other proteins found in far less abundance and these are collectively called non- histone proteins (NHP).
This nuclear DNA–protein complex is called chromatin.
In the nucleus, each chromosome contains a single linear double-stranded DNA molecule.
The length of the packaged DNA molecule varies. In humans, the shortest DNA molecule in a chromosome is about 1.6 cm and the longest is about 8.4 cm.
The extensive packaging of DNA in chromosomes results from three levels of folding involving nucleosomes, 30 nm filaments and radial loops.
The first level of packaging involves the binding of the chromosomal DNA to histones.
Overall, in chromosomes, the ratio of DNA to histones on a weight basis is approximately 1:1.
There are five main types of histones called H1, H2A, H2B, H3 and H4.
Histones are very basic proteins; about 25% of their amino acids are lysine or arginine so histones have a large number of positively charged amino acid side-chains.
These positively charged groups therefore bind to the negatively charged phosphate groups of DNA
30 nm fiber
If nuclei are lysed very gently, the chromatin is seen to exist as a 30 nm diameter fiber.
The fiber is formed by a histone H1 molecule binding to the linker DNA of each nucleosome at the point where it enters and leaves the nucleosome.
The histone H1 molecules interact with each other, pulling the nucleosomes together.
When chromosomes are depleted of histones, they are seen to have a central fibrous ‘protein scaffold’ (or nuclear matrix) to which the DNA is attached in loops.
Therefore, in vivo it seems likely that the next order of packaging involves the attachment of the 30 nm fiber to multiple locations on this central protein scaffold in a series of radial loops.
The mitochondria and chloroplasts of eukaryotic cells also contain DNA but, unlike the nuclear DNA, this consists of double-stranded circular molecules resembling bacterial chromosomes.
David Hames and Nigel Hooper (2005). Biochemistry. Third ed. Taylor & Francis Group: New York.
Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S .Chand and company Ltd.