Here's why:
* DNA is very long: A single DNA molecule in a human cell is about 2 meters long!
* Nucleus is small: The nucleus, where the DNA is stored, is tiny.
* Chromatin is the solution: To fit within the nucleus, DNA is tightly packaged with proteins called histones. This complex of DNA and proteins is called chromatin.
Chromatin is a dynamic structure:
* Loose (euchromatin): When a gene needs to be transcribed (copied into RNA), the chromatin associated with that gene becomes less tightly packed. This loose form is called euchromatin, and it allows for access by the cellular machinery that reads the genetic code.
* Tight (heterochromatin): When a gene isn't needed, the chromatin associated with that gene becomes more condensed and tightly packed. This is called heterochromatin, and it helps to ensure that the DNA is protected and doesn't get transcribed accidentally.
In summary: The relaxed form of DNA in the nucleus is called chromatin. Chromatin can be either loose (euchromatin) or tightly packed (heterochromatin), depending on whether genes need to be accessed.
