CHEMICAL COMPOSITION OF CHROMOSOMES:

The major chemical components of chromosomes are DNA, RNA, histone proteins and non-histone proteins. Calcium is also present in addition to these constituents.

DNA -Deoxyribonucleic Acid:
DNA is the most important of chemical components of chromatin, since it plays the central role of controlling heredity.

Histone proteins:
These are Lysine And Arginine rich, basic and stable proteins in chromosomes.
They are classified into five types called HI, H2A, H2B, H3 and H4. 

Non-histone proteins:
Non-histone proteins provide the scaffolding structure.

DNA PACKAGING:

The diploid human genome contains 6 billion base pairs of DNA per cell.This is enough to make some very lengthy journeys like, to the sun and back.300 times.In order for this massive amount of DNA to exist in a cell ,it has to be made very compact.In short, it needs to be packaged into a very small surface area to volume ratio.
Patterns of packaging are facilitated by associated proteins, which in eukaryotes, could be histones or non-histones.

The unit of compaction is nucleosome consisting of a segment of DNA wound in sequence around four histone protein cores.

• Histone proteins provide the spools about which DNA winds thus compressing 1.8 meters of dna into 90 mm.The negatively charged coil of polynucleotide (due to phosphate) is strongly attracted by the positively charged histone protein which helps in compaction.

• The nucleosomes are linked together by the long strand of DNA called linker DNA. 

• The beads-on-string structure in chromatin is packaged to form chromatin fibres which then fold together into large looped domain.that are further coiled and condensed at metaphase stage of cell division to form chromosomes.

• The packaging at higher level requires non-histone proteins. 

HETEROCHROMATIN AND EUCHROMATIN:

In a typical nucleus, some region of chromatin are loosely packed (and stains light) and are referred to as euchromatin.Euchromatin is said to be transcriptionally active
The chromatin that is more densely packed and stains dark is called as heterochromatin. Heterochromatin is transcriptionally inactive.