The most important structure to a cell is the nucleus. It is the nucleus that actually separates certain organisms into two separate and distinct domains of life: prokaryotic (without nucleus) and eukaryotic (with a nucleus).

Prokaryotic organisms (those without nucleus) are very simple, one-celled organisms: mainly bacteria. Their DNA floats around inside the cell and is not contained within a nucleus.

In a eukaryotic organisms, however, the DNA is contained within a double-membraned organelle called the nucleus. This much more complex type of cell allows for multicellular organisms.

Within the nucleus are the chromosomes, normally in an uncoiled fibrous form called chromatin. However, before the cell divides, the nucleus must first divide. For this to happen, the chromatin coils itself up into tight rods, which are the common form of chromosomes.

Within the nucleus, a strand of RNA is formed from the DNA through a process called transcription. These RNA strands can then leave the nucleus through small openings in the nuclear membranes, called nuclear pores. It is by sending these RNA strands outside that the nucleus can build proteins and control the cell, and ultimately control the organism of which it is a part.

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