By: Anthony T. Annunziato, Ph.D. (Biology Department, Boston College) © 2008 Nature Education Citation: Annunziato, A. (2008) DNA packaging: Nucleosomes and chromatin. Nature Education 1(1)
Each of us has enough DNA to reach from here to the sun and back, more than 300 times. How is all of that DNA packaged so tightly into chromosomes and squeezed into a tiny nucleus?
The haploid human genome contains approximately 3 billion base pairs of DNA packaged into 23 chromosomes. Of course, most cells in the body (except for female ova and male sperm) are diploid, with 23 pairs of chromosomes. That makes a total of 6 billion base pairs of DNA per cell. Because each base pair is around 0.34 nanometers long (a nanometer is one-billionth of a meter), each diploid cell therefore contains about 2 meters of DNA [(0.34 × 10-9) × (6 × 109)]. Moreover, it is estimated that the human body contains about 50 trillion cells—which works out to 100 trillion meters of DNA per human. Now, consider the fact that the Sun is 150 billion meters from Earth. This means that each of us has enough DNA to go from here to the Sun and back more than 300 times, or around Earth's equator 2.5 million times! How is this possible? DNA, Histones, and Chromatin
The answer to this question lies in the fact that certain proteins compact chromosomal DNA into the microscopic space of the eukaryotic nucleus. These proteins are called histones, and the resulting DNA-protein complex is called chromatin. It may seem paradoxical that proteins are added to DNA to make it more compact. However, if you have ever tried to store a garden hose, you know that it is much easier to do so if you begin by coiling the hose. Of course, coiling requires work, and energy is needed to perform work. Thus, within the nucleus, histones provide the energy (mainly in the form of electrostatic interactions) to fold DNA. As a result, chromatin can be packaged into a much...