The nucleus is the main organelle of the eukaryotic animal and plant cells, as it contains genetic information in the form of DNA, and is where DNA replication and the synthesis of all RNA takes place.
The nucleus looks very different depending on where it is in the cycle. Thus, it is distinguished:
- Interphase nucleus when the cell is not dividing.
- Mitotic nucleus when chromosomes differentiate, and the cell is going to divide.
The nucleus is an organelle surrounded by a double membrane, the nuclear envelope, which surrounds the genetic material (DNA) of the cell, separating it from the cytoplasm. The internal environment is called nucleoplasm, which in addition to DNA, contains one or more corpuscles very rich in RNA, called nucleolus.
The interphase nucleus, as we will see, has a double nuclear membrane and the DNA is part of unwound chromatin fibers. Although the interphase nucleus has been erroneously called the resting nucleus, it is at this time when its activity is highest, since the DNA fibers (chromatin) are extended to allow their transcription into RNA, and to duplicate before cell division.