I guess most of the people have heard of term DNA at least once in their lifetime. The acronym of DNA stands is Deoxyribonucleic acids. The majority of DNA is situated in the nucleus and nucleus is placed inside the cell. So each eukaryotic cell has a nucleus and the number of cell in human is around 10^13 (100 Trillion). The size of the nucleus is the order of 10 micrometers and size of a cell is around 100 micrometer.
The DNA is a long chain of hetero-polymer which is made of adenine(A), guanine(G), cytosine(C) and thymine(T). The polymer is called hetero because its monomer has 4 possible possibilities A, C, G or T and the total number of such alphabets are roughly 3*10^9 base-pairs (3 billion from mom and 3 billion from dad) and total DNA content is called the Genome. The DNA exist in double-stranded helix form where one stranded goes in the right direction and other complementary strand goes in the left direction like a ladder. So basically you can say that the information is contained only on one strained. If you know the one stranded you will automatically know the other strand because A is complementary to T and G is complementary to C.
for e.g.
5'-A-G-C-C-T-G-T-A-C-3'
3'-T-C-G-G-A-C-A-T-G-5'
The DNA always does not found in linear double helical form inside the nucleus because you need a lot of space to accommodate linear structure and nature has given only 10-micron meter to accommodate 3 billion bp. So DNA has to fold such that it minimizes its structural space and also do function when its needed with the help of proteins. These proteins are called histones octamer and ~147 bp of DNA wrapped around this proteins. This DNA-histone structure is called Nucleosome and the distance between two nucleosomes is around ~200 bp. Chromatin is the combined folded structure of DNA and proteins. There are many proteins involved to make this structure compact but a majority of protein is histones.
The compactness in chromatin works like zip (tar,rar,gz,bz etc) software. The compression of the data depends on what algorithm have been used or similar to what proteins have been used in case of chromatin compactness. The compression should not be much that chromatin is unable to do any function. What i mean by function here is, Gene which is some part of DNA is able to express and express means able to make RNA and other necessary proteins. If you think a country map is your genome then school, hospital, railway station, airports etc are your genes. So, The compression and the functions both should be balanced.
This picture is taken from Molecular Biology of the cell 4th edition book.
file:///home/aagrawal/Screenshot%20from%202018-01-26%2023:19:50.png
In the picture, various level of compactness is shown. If the linear DNA is 1 fold then the beads-on-string form of chromatin is 3 fold, further 30-nm chromatin compact is 27 fold and the final chromosome is the 10,000 fold. Final highly condensed (or compact) structure of chromatin is called chromosome. The human genome is divided into 46 chromosomes (22 autosome pairs and 2 sex chromosomes). Each cell goes to cell cycle stage and divides, so X type structure of chromosome is visible only to a mitotic stage. For rest of cell cycle e.g. interphase stage, the structure of chromosome is not known.
Each chromosome has a different size from largest chromosome 1 to smallest chromosome 21. The number of gene contained in each chromosome varies from most gene-rich chromosome 19 to most gene-poor chromosome 13.
There is two popular theory for the organization of chromosomes.
First, based on size, smaller chromosomes have more interior positions than larger chromosomes.
Second, based on gene density per chromosomes, which says gene-rich chromosomes are more interior than gene poor chromosomes.
Overall the organization of chromsoomes is terrotrial means each chromosome has its own radial positions inside the nucleus.