Chromosomes are made of long aggregates of genes formed from condensed chromatin. Chromatin is made up of DNA, proteins, RNA and other macromolecules. It is located in the nucleus of a cell and becomes a chromosome during the prophase stage of mitosis in eukaryotic cell division.
One of the main functions of chromatin is to reduce the size of DNA so it can fit in the cell. It does this by wrapping a DNA molecule tightly around proteins called histones. The histones are there to maintain the structure of the chromosome. Other functions of chromatin include reinforcing DNA molecules for mitosis, preventing DNA damage, and helping to control DNA replication.
A fully formed chromosome is made up of two chromatin sections that join together at a central point called the centromere. One of the chromatin sections, or arms, is generally shorter than the other. The shorter section is referred to as the "p arm" of the chromosome, and the longer section is called the "q arm."
Before cell division occurs, chromosomes cannot be seen inside the nucleus, even with the help of a microscope. It is only during cell division that the DNA becomes compact enough to be observed, so most of what scientists know about chromosomes comes from observing cell division.