WHAT DO CHROMOSOMES LOOK LIKE AND HOW ARE THEY ANALYZED?

By Maimon M. Cohen, Ph.D., F.A.C.M.G.

All of our genetic information is included in structures called chromosomes. There are 23 pairs of chromosomes which are found in the nucleus of each cell of the body.  Chromosomes are composed of strands of DNA that make up the genes. There are about 30,000-50,000 pairs of genes that are responsible for growth, development, and all of the specific inherited traits such as hair color and height. We inherit one copy of each chromosome from our mother via the egg and one copy from our father via the sperm.  Everyone has approximately 4-5 genes that are altered or mutated in some way. In most cases, these gene mutations do not affect our health, and we are totally unaware of them.

To the left are chromosomes in what is known as a “metaphase spread," on a microscope slide as observed by the cytogeneticist.  The photo is the end result of a laboratory process in which the cells derived from amniotic fluid, blood, chorionic villi, bone marrow, or other tissue are grown in tissue culture.

The metaphase spread is only the beginning of chromosomal analysis.  The apparent random distribution of chromosomes must be presented in an easily interpretable format in order to evaluate the number and normality of each chromosome.  This labor-intensive procedure is computer-assisted in most labs. Cytogenetics technicians require special training and a great deal of experience to become skilled at recognizing each of the chromosomes and pairing them into a “karyotype”, the orderly array of the metaphase spread.  Below, the chromosomes from the metaphase spread in karyotype format:

Abnormalities in chromosome number (for example, Down syndrome, in which three copies of the number 21 chromosome are present) are readily discernible.  However, there are many other, more subtle, chromosomal abnormalities.  More information concerning the chromosomal complement in such abnormalities, as well as an explanation of terms utilized in human cytogenetics, is available at such sites as www.pathology.washington.edu/galleries/Cytogallery