Humans carry a copy of all of their genetic information in every cell in their body. In order to pack all of this information into the nucleus of each cell, the genetic material must be packaged in a very efficient way. This is done using packaging proteins to tightly coil DNA around proteins called histones. Nucleosomes are histones with DNA wrapped around them. Many nucleosomes joined together become chromatin fiber. These fibers end up looking like beads on a string. Chromatin fiber is condensed once again to make up chromatin. Chromatin is small enough to fit inside the nucleus of the cell. During cell division chromatin is even further condensed into structures called chromosomes that are visible under a microscope.
Chromosomes are comprised of 2 sister chromatids. These chromatids are joined at a central point called the centromere. The location of the centromere is often used to describe differences in the chromosomes. The portion of the chromatid below the centromere is termed the “q arm” whereas the portion above the centromere is termed “p arm”. The location of the centromere changes the size of the arms.
Image courtesy of U.S. National Library of Medicine
