In most cells of the human body, the genetic information is contained in 46 microscopic structures in the nucleus, called the chromosomes. The first 22 chromosomes are present in pairs, and the 23rd pair consists of either 2 X chromosomes (female) or an X and Y chromosome (male) (see figure 1A). The chromosomes are inherited from the parents, with each parent providing 1 set of 23 chromosomes. These chromosomes contain a large molecule called deoxyribonucleic acid (DNA) (see figure 1B). The DNA consists of four building blocks called nucleotides that are arranged in a specific order. This sequence of nucleotides encodes the genetic information necessary for the organism to develop and function. The DNA segments that determine those characteristics of an individual that are inherited from one generation to the next are called genes. Large areas of the DNA, however, do not appear to contain genes. Some of these regions help regulate the activity (i.e., expression) of genes; for other DNA segments the function is still unknown. Nevertheless, these “noncoding” DNA regions can provide important tools for the study of the genome, as described next.
