What does the DNA in your cells look like right now? If you imagine that it is a long, out-stretched, extended double helix—than you would be wrong.
In reality, the double helical DNA is wound tightly, over and over again, around proteins called histones and many histones together is referred to as chromatin which is, in large part, so that the meters of DNA can fit into the cells. For humans, packing DNA up in this manner is vital for packing an entire human genome inside just one cell. Each of our 70 trillion or so cells (minus cells without genomic information like red blood cells) must each pack 6 feet of DNA!
A second, role of chromatin structure is in determining how much or little the genes on the DNA are expressed.
Not all the DNA is wound around histones to the same extent. When the DNA-histone complexes are tightly packaged, the chromatin is called “heterochromatin.” When the complexes are loosely associated, it is called “euchromatin.”
The general rule is that the more tightly DNA is wound around the proteins, the more condensed the chromatin becomes and the less those genes are expressed. This is because the areas where the proteins need to bind to the DNA to start the process of expression are blocked because the chromatin is too compact.
Think of heterochromatin like a rope that is continually wound up, eventually becoming tightened to the point where it would start to wind up on itself. What if a message was written along that rope that you had to read? The letters found on the portion of the rope that was tightly wound would be inaccessible. However, you could read the letters found on a loosely wound section and access the information. This is, indeed, how the information in the DNA is managed as well.
The amount of compactness of the chromatin is fluid, however, and the rope can be wound and unwound by different modifications being made to both the DNA and the proteins. Therefore, it is possible for the cell to alter the amount of heterochromatin in that area to make that gene more accessible. The changes in the chromatin from tight to loose (or vice versa) are called chromatin remodeling. Like other epigenetic factors, chromatin remodeling has a huge influence on which genes are being expressed and the extent to which they are expressed.
Some evidence has suggested that environmental triggers (like diet or stress) can influence the arrangement of histones and therefore our environment may be able to influence the arrangement of chromatin. It is also possible that some of these modifications may be passed onto to the next generation, some evidence from nematodes suggests transgenerational inheritance of histone modifications can occur.