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A coiled DNA molecule within the cell’s nucleus that carries an individual’s GENETIC CODE. Most of the time the chromosome’s structure is loose and indistinguishable. Only in the stage of cell division immediately before the cell divides (the metaphase) does the chromosome draw itself into a compact, rodlike structure the geneticist can see under a microscope after applying a special dye to the cell that the chromosomes absorb. It is this ability to absorb a colored dye that gives the chromosome its name, which means “colored body.”
The nucleus of every diploid cell, also called a SOMATIC CELL, contains the full complement of 46 chromosomes arranged in 23 pairs. One pair contains the sex chromosomes that establish gender, paired either as XX (female) or XY (male). The other 22 pairs are autosomes. The haploid cells, the gametes (spermatozoa and OVA), contain one half the chromosome complement. When gametes merge in CONCEPTION the diploid cell they form, the ZYGOTE, acquires the full chromosomal complement. The only cells in the body that do not have chromosomes are the erythrocytes, which do not have nuclei.
Autosomes carry the bulk of genetic code. Thousands of genes line each autosome, each in its ordained position. The sex chromosomes carry several hundred genes. The GENE positions, called loci (in the singular, each position is a locus), are constant. For example, the gene loci for the ABO BLOOD TYPE are always on chromosome 9, those for the rhesus (Rh) blood type are on chromosome 1, and those for EYE color on chromosomes 15 and 19.
The HUMAN GENOME PROJECT, completed in 2003, revealed the structure of chromosomes to be much larger and more complex than scientists previously had theorized. Chromosome 1, the largest CHROMOSOME, contains 2,968 genes. The smallest chromosome, the Y chromosome, contains 231 genes.
Geneticists designate the normal female chromosome complement as 46,XX and the normal male chromosome complement as 46,XY. Deviations from the norm are CHROMOSOMAL DISORDERSgeneticists designate according to the deviation, for example 47,XY,+21 denotes AUTOSOMAL TRISOMY21 (DOWN SYNDROME) in a male. The designation 45,X denotes TURNER SYNDROME, a monosomy disorder (missing chromosome) affecting the SEX CHROMOSOME in a female. A comprehensive standard of nomenclature (naming) exists so all geneticists can use a common “language” when describing chromosomal and genetic configurations.
A chromosome’s structure consists of two telomeres (end segments), a CENTROMERE (waistlike indentation), and two arms (the segments above and below the centromere). The centromere is somewhat off-center, such that each chromosome has a short arm (designated “p” for petite) and a long arm (designated “q” because scientific nomenclature is alphabetical). The regions of each arm are numbered. Geneticists identify a gene’s locus relative to its placement on the chromosome. The gene responsible for CYSTIC FIBROSIS, for example, is identified as CFTR 7q31.2—cystic fibrosis transmembrane conductance regulator located in band 31, region 2, on the long arm of chromosome 7.
For further discussion of chromosomes within the context of the structures and functions of genetics, please see the overview section “Genetics and Molecular Medicine.”