The nucleus is a large organelle that has a nuclear envelope consisting of two membranes. Nuclear pores in the nuclear envelope are often called nuclear pore complexes, which regulate what can enter or exit the nucleus.
Inside of the nucleus is a thread-like substance called chromatin, which consists of DNA and proteins. When a cell is getting ready to divide the chromatin coils up to form visible chromosomes.
A nucleus also contains a dark, non-membranous area structure called the nucleolus. The nucleolus is made of RNA and is responsible for making ribosomal RNA, which then combines with proteins to make ribosomes.