DNA, found in the nucleus of plant and animal cells, is organized into structures called chromosomes. Cell biologists in the nineteenth century saw the rod shaped structures within cell nuclei and named the structures “chromosomes” because of the way they absorb colored stain. Later it was determined that those rod shaped structures held the genetic information (DNA) that was transferred from generation to generation. This DNA provides the instruction manual for life, informing an animal or plant how to grow, develop, and function. A single, continuous strand of DNA is tightly coiled into the chromosome structure. Proteins such as histones help package the DNA and aid in its function.

A human typically has 46 chromosomes (23 pairs) in every cell of his or her body. One copy of each chromosome pair is inherited from the mother, while the other copy of each chromosome is inherited from the father.  Twenty-two of the twenty-three chromosome pairs are called autosomes (non-sex chromosomes) which carry the same types of genetic information in both men and women. The final pair of chromosomes are called the sex chromosomes and determine gender, among other traits. A female typically has two X sex chromosomes while a male has a X and a Y. As mentioned previously, chromosomes come in matching (homologous) pairs. Homologous is a word that describes two chromosomes that carry the same types of information. While the sequence on the two homologous chromosomes may not be identical, the chromosomes have variations of the same genes. Variations of a specific gene are also known as alleles.

There are several important chromosome structures with which you should be familiar. A chromosome telomere is the region at each end of the chromosome. A chromosome centromere is the region of the chromosome where the chromosome attaches to its matching homolog during replication. This centromere can be in the middle of the chromosome or closer to an end.  The centromere divides the chromosome into two segments called "arms."  The shorter segment is called the "short arm" (p arm) while the longer segment is called the "long arm" (q arm). 

The Dolan DNA Learning Center has a good movie about chromosome structure on their DNAinteractive website: http://www.dnai.org.
Click on "genome, then on “tour,” then on “chromosome close-up,” then on “3D animation, chromosome coiling.”

“DNAi.” Dolan DNA Learning Center. 2003. Cold Spring Harbor Laboratory. 10 December 2008. http://www.dnai.org/c/index.html

The PBS Nova Online website also has a good animation about chromosomes: http://www.pbs.org/wgbh/nova/genome/dna.html#
Click on “Go to Journey into DNA.”

“Journey into DNA.” Nova Online. 2003. Public Broadcasting Service. 10 December 2008. http://www.pbs.org/wgbh/nova/genome/dna.html#