In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.
Chromosomes are not visible in the cell’s nucleus—not even under a microscope—when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during cell division and is then visible under a microscope. Most of what researchers know about chromosomes was learned by observing chromosomes during cell division.
Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.” The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.
DNA and histone proteins are packaged into structures called chromosomes.
For more information about chromosomes:
Genetics Home Reference provides information about each human chromosome written in lay language.
The Centre for Genetics Education offers a fact sheet that introduces genes and chromosomes.
GeneEd also provides information about the basics of chromosomes.
The University of Utah’s Genetic Science Learning Center offers a description of chromosomes, including their size and how they are packaged in the cell.