The nucleus of a cell contains chromatin molecules and chromosomes; these are the important genetic material that the body uses to determine exactly how to function. For a multi-cellular organism like a human being, the entire body's genetic code is stored in the nucleus and found in every cell. Every cell of the body thus contains the instructions for building and utilizing every other cell in the body.
Chromatin is made of long strands of deoxyribonucleic acid, or DNA, that form chromosomes. DNA controls most of the functions of a cell, but does not leave the nucleus. Instead, when it needs to send a command to another part of the cell, it is transcribed into a protein string of ribonucleic acid called Messenger RNA, which is transported out of the nucleus and to the relevant organelle.
One important component of DNA is the nucleotides. Nucleotides contain the actual DNA information in the cell. They are molecules based around the amino acids known as adenine, guanine, cytosine, and thymine. The order of these molecules determines the actual instructions received from the DNA. Nucleotides are linked into pairs that form the central chain structure of the double helix. Adenine is always paired with thymine, and guanine is always paired with cytosine.