The acronym of DNA stands for Deoxyribonucleic acids

The acronym of DNA stands for Deoxyribonucleic acids. I think everyone heard of DNA at least once in their lifetime. The acronym of DNA stands for Deoxyribonucleic acids. The majority of DNA mainly situated inside the cell nucleus. The size of one eukaryotic cell is an order of ~100 microns and the total number of such cell in human is around 10^13 (100 Trillion). Each eukaryotic cell has ~ 10-micron meter size of the nucleus and contains ~2 meters long and 2-nanometer width of DNA. The DNA is a long chain of hetero-polymer which is made of 4 nucleotide adenine (A), guanine (G), cytosine (C) and thymine (T). The polymer is called hetero because each nucleotide behaves differently. The total number of nucleotide in each cell is 3*10^9 base-pairs (3 billion comes from mom and 3 billion comes from dad). The structure of DNA exists in the double-helical form where one strand goes in right direction and other strand goes in left direction like a ladder. We need only one strand to know the whole DNA because one strand is just complimentary of another. 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 total content of DNA in a cell called genome. Our body is made up of more than ~200 different types of cell and each one is assigned to a different kind of job to perform. For example function of the heart, the cell is completely different from eye cell. The question is how each cell give different function while their genome is completely identical. The mechanism to tell each cell to behave differently is hidden in DNA. This hidden structure is made up of protein-DNA complex and we called them transcription factors (TFs). These TFs are responsible for different cell type activity and controlling the regulation of genes.

DNA does not found in the form of linear structure inside the nucleus because it needs a lot of space to accommodate and nature has given only 10-micron space to fill such structure. So DNA must have to fold in a way that it takes minimum space and also able to do the function. There are Histone proteins which help DNA to fold and make DNA-histone complex called nucleosome. The length of DNA wraps around histone protein is ~147 bp and the adjacent stretch of DNA (linker) between two nucleosome is between 10 to 80 bp and it varies according to species and tissue type. Condensed structure of DNA is called chromatin and functional genes are mostly found in open regions (linker DNA) of chromatin.

The compactness in chromatin works similar to zip (tar, rar, gz, bz etc) software files. The compression of the data depends on the algorithm and for chromatin, it depends on proteins. The condensation of chromatin 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 mRNA and these mRNA able to make proteins. If you think a country map is your genome then school, hospital, railway station, airports etc are your genes. So, the condensation of chromatin and the functions are correlated.

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. The final condensed form of chromatin is called a chromosome. The human genome (diploid) is divided into 46 chromosomes (22 autosome pairs and 2 sex chromosomes). Each cell goes through the process of cell cycle stage and divides, so X type structure of chromosome is visible only to a mitotic stage. For the rest of the stage e.g. interphase, the structure of the chromosome is unknown.

Each chromosome has a different size from largest chromosome 1 to smallest chromosome 21. The number of genes contained in each chromosome varies from most gene-rich chromosome 19 to most gene-poor chromosome 13. There is two popular theory for how chromosomes are organized inside the nucleus. First, based on size, smaller chromosomes have more interior positions than larger chromosomes. Second, based on gene density per chromosomes, gene-rich chromosomes are more interior than gene poor chromosomes. The chromosomes are situated non-randomly inside the nucleus and they generally form chromosome territories. A typical picture of chromosome territory is shown where each color depicts a different chromosome.