Differences Involved In Chromosomal Compaction

In an eukaryotic cell, DNA is tightly packaged into the nucleus. For a human cell, DNA measures approximately 2 meters, with its diameter being 2 nm. The total length of DNA present in one adult human is nearly 2.0 × 1013 meters, which is equivalent to nearly 70 trips from the earth to the sun and back. Since the size of the nucleus in human cell is about 6 μm, DNA should be tightly packaged to fit into it. At the same time, it should be easily accessible to express the genetic information. Inside a nucleus, such long strands of DNA are compacted in the form of chromosomes. Amazingly, although the DNA is very tightly folded, it is compacted in a way that allows it to easily become available to the many enzymes in the cell that replicate it, repair it, and use its genes to produce proteins [2].
There are number of steps in chromosomal compaction. In the first level of compaction, 147 base pair (bp) of negatively charged DNA wraps 1.65 times around eight positively charged histone core proteins, with two copies of the four histones H2A, H2B, H3, and H4, forming a nucleosome core particle (NCP). Each NCP is linked by a linker DNA with variable length in the range of 10-90 bp. In the second level of compaction, NCPs compact together into the form of 30 nm chromatin fiber. These fibers are condensed into chromosomes during mitosis, or the process of cell
In a nucleosome, each histone contains two regions, the ”histone fold” and the ”histone tails”. Those occupy approximately 70% and 30% of the histone masses and contribute nearly +90e and +44e charges respectively. Histone fold forms histone dimers (H2A-H2B, and H3-H4), which then form histone octamer by two H2A-H2B dimers and two H3-H4 dimers. Histone tails contributes to the binding of DNA around the histone octamer, linker DNA [19] and acidic patches of the neighboring nucleosomes