The chromosomes are composed of DNA and protein. Generally DNA is about 40% and protein 60%. The DNA consists of nucleotides while the protein is histone which consists of amino acids argine and lysine in abundance besides other amino acids. The presence argine and lysine amino acids make the histone protein, a positively charged particle. The DNA show negative charge due to phosphate. A polynucleotide part consisting of 200 nucleotides base pairs of the DNA takes two turn on the core of eight histone proteins forming a complex known as nucleosome. The negatively charged coil of polynucleotide is strongly attracted by the positively charged histone protein. The nucleosome appears as a beaded structure on the string under electron microscope. Two nucleosomes are joined by linker DNA consisting of up to 200 nucleotides. Each nucleosome units further wrap up into higher order coil called super coil. A typical human chromosome consist of about 140 million nucleotides in its DNA which are repeatedly arranged as beaded structures called nucleosomes on a string. Highly condensed portions of chromatin are called heterochromatin. If these remain permanently condensed, they do not transcribe. The chromatin which is condensed only during cell division but remains in open configuration in an undividing cell is transcribed and thus genes are expressed. It is euchromatin.
