What is a Chromosome?

Chromosomes, composed of protein and DNA, are distinct dense bodies found in the nucleus of cells. Genetic information is contained in the DNA of chromosomes in the form of linear sequences of bases (A,T,C,G). The DNA in an individual chromosome is one, long molecule which is highly coiled and condensed.The total number of bases in all the chromosomes of a human cell is approximately six billion and individual chromosomes range from 50 to 250 million bases. The DNA sequence for a single trait is called a gene. Each chromosome contains a few thousand genes, which range in size from a few thousand bases up to 2 million bases.

During most of the cell cycle, interphase, the chromosomes are somewhat less condensed and are not visible as individual objects under the light microscope. However during cell division, mitosis, the chromosomes become highly condensed and are then visible as dark distinct bodies within the nuclei of cells. The chromosomes are most easily seen and identified at the metaphase stage of cell division and most of the chromosome images in this gallery are pictures of metaphase chromomosomes.

The number of chromosomes in human cells is 46 with 22 autosomal pairs (one of each type contributed by the mother and one of each type from the father) and 2 sex chromosomes - 2 X chromosomes for females (one from father and one from mother) or an X and a Y chromosome for males (the X from the mother and the Y from the father). The characteristic banding of chromosomes you will see in pictures here is obtained by staining with various dyes. The banding of chromosomes by using dyes was discovered in the late 1960's and before that cytogeneticists depended on chromosome length and position of a constriction to identify the individual chromosomes. The band width and the order of bands is characteristic of a particular chromosome - a trained cytogeneticist can identify each chromosome (1,2,3...22, X and Y) by observing its banding pattern under the microscope.