DNA Structure: How is DNA Packed into a Nucleus?

The DNA length varies in different organisms. The DNA of a virus may be as small as a few thousand base pairs. However, the DNA of E. coli, a bacterium has 4 million base pairs and the human genome contains 6 billion base pairs. In general, the more complex the organism, the greater the DNA content in its cells. But, how is such a long DNA structure arranged inside the cell nucleus? To give a sense of perspective, there are 46 chromosomes in a human cell, which are packed into a nucleus millions of times smaller than the length of the chromosomes.

In order to pack large chromosomes into a small nucleus, an elaborate packing procedure is needed. In the case of eukaryotic cells, DNA attaches to proteins called histones, which are rich in arginine and lysine – these amino acids bear positive charges and form an ionic bond with the negative phosphate group of the DNA helix. This protein-DNA structure is called chromatin.

There are four kinds of histones attached to the DNA: H2A, H2B, H3, and H4. All these proteins form an octamer protein complex called a nucleosome core around which the DNA wraps two turns and then links to another nucleosome (10 nm diameter). This kind of arrangement condenses the DNA. There is another kind of histone protein called H1 that takes part in the DNA packaging. However, its function is still not known.

Now the 10 nm nucleosomes further condense to form a fibre. The nucleosome arranged in this fibre structure (30 nm diameter) is often called a solenoid arrangement. Moreover, there are other models also and one of them is a zigzag arrangement. In this condition, the condensation of the DNA is approximately 100-fold. Further, the 30nm diameter nucleosome fibres form a long loop and get attached to the central chromosomal protein scaffolding – this looped structure again gets denser.

Following are the levels of organization:

• · DNA winding around nucleosomes
• · Nucleosomes packed into a 30nm fibre
• · Fibres form loops, thousands of nucleosomes long
• · The loops form into coils

However, in the case of prokaryotes, there is no nuclear membrane and there is no well-established nucleosome structure. But proteins reminiscent of histones are found in the bacterium E.coli, and are thought to play a similar role.