The term �chromosomes� is used to refer to the nucleic acid molecule that is the respiratory of genetic information in a virus, a bacterium, a eukaryotic cell or an organelle. It also refers to the densely colored bodies seen in the nuclei of dye stained eukaryotic cells, as visualized using a light microscope.
In Prokaryotes
The DNA of a bacterial cell, such as Escherichia coli, is a circular double-stranded molecule often referred to as the bacterial chromosome. This DNA molecule contains about 4.6 millions base pairs. The circular DNA is packaged into a region of 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. Within this structure, the DNA is actually not a circular double stranded molecule, but it is a negatively super coiled, that is, it is twisted upon itself and is also complexed with several DNA binding proteins, the most common of which are HU, HLP-1, and H-NS. These are histone like proteins.
In Eukaryotes
The genomic DNA of a eukaryotic cell is contained within a specialized organelle, the nucleus. The very large amount of eukaryotic nuclear DNA is tightly packaged in chromosomes. 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. The mitochondria and chloroplast of eukaryotic cells also contain DNA but, unlike the nuclear DNA, this consists of double stranded circular molecules resembling bacterial chromosomes.
In the nucleus, each chromosome contains a single linear double stranded DNA molecule. The length of the packaged DNA molecules varies. In humans, the shortest DNA molecule in a chromosome is about 1.6 cm and the longest is about 8.4 cm. During the metaphase stage of mitosis, when the chromosomes align on the mitotic spindle ready for segregation, they are at their most condensed and range in size from only 1.3�m to 10 �m long. Thus, the packaging ratio that is the ratio of the length of the linear DNA molecule to the length of the metaphase chromosome is about 104. In the time period between the end of one mitosis and the start of the next (i.e. interphase), the chromatin is amorphous and appears to be more disperse throughout the nucleus. Here the packing ratio is in the range 102-103. Overall, 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 histone on a weight basis is approximately 1:1.
If the nuclei are lysed very gently, the chromatin is seen to exist as 30nm diameter fiber. This diameter is larger than single nucleosomes and suggests that the nucleosomes are organized into a higher order structure. The fiber is formed by a histone H1 molecule binding to the linker DNA of each nucleosome at the point where it enters and leave the nucleosomes.
When chromosomes are depleted of histone, they are seen to have a central fibrous protein scaffold to which the DNA is attached in loops
