Chromosome Duplication

The genetic material, the DNA, in a eukaryotic cell exists inside the nucleus as linear structures called the chromosomes.

Within the chromosomes, the double-stranded DNA primarily wraps around octameric nuclear proteins called histones to form nucleosomes. Several non-histone proteins also associate with the DNA, assisting in DNA packaging.

These nucleosomes and non-histone proteins arrange to form coils which further fold into loops, in an array, forming packed chromatin. The extent of chromatin compaction enables extremely long DNA to fit inside the nucleus. During M phase, chromatin further coils to form the condensed chromosomes.

Chromosome duplication involves duplicating the entire chromatin structure, so the DNA and chromatin-associated proteins are all duplicated. The production of histones is increased to generate the additional proteins required for packaging the newly synthesized DNA into chromosomes.

Replicating the exact chromatin structure of the duplicated chromosome is vital for gene regulation. 

Within the chromosomes, the level of chromatin packaging is not always uniform. Chromosomes contain regions with tightly packed chromatin called heterochromatin and regions with loosely packed chromatin called euchromatin.

Heterochromatin DNA is inaccessible to the transcription machinery, and hence genes in those regions are not regularly transcribed. In contrast, the DNA within the euchromatin is more accessible, and thus genes in these regions can be transcribed.   

This structural and functional significance of chromatin makes it crucial that the entire chromatin structure is reproduced accurately during chromosome duplication.