Chromosomal Abnormalities - Normal Number And Structure Of Human Chromosomes

A chromosome consists of the body's genetic material, the deoxyribonucleic acid, or DNA, along with many kinds of protein. Within the chromosomes, the DNA is tightly coiled around these proteins (called histones) allowing huge DNA molecules to occupy a small space within the nucleus of the cell. When a cell is not dividing, the chromosomes are invisible within the cell's nucleus. Just prior to cell division, the chromosomes uncoil and begin to replicate. As they uncoil, the individual chromosomes look somewhat like a fuzzy "X." Chromosomes contain the genes, or segments of DNA that encode for proteins, of an individual. When a chromosome is structurally faulty, or if a cell contains an abnormal number of chromosomes, the types and amounts of the proteins encoded by the genes is changed. When proteins are altered in the human body, the result can be serious mental and physical defects and disease.

Humans have 22 pairs of autosomal chromosomes and one pair of sex chromosomes, for a total of 46 chromosomes. These chromosomes can be studied by constructing a karyotype, or organized depiction, of the chromosomes. To construct a karyotype, a technician stops cell division just after the chromosomes have replicated using a chemical such as colchicine; the chromosomes are visible within the nucleus at this point. The chromosomes are photographed, and the technician cuts up the photograph and matches the chromosome pairs according to size, shape, and characteristic stripe patterns (called banding).