The chromosomal theory of inheritance, proposed by Walter Sutton and Theodor Boveri in 1902-1903, is considered a milestone in the science of genetics (Martins , 1999). The theory established chromosomes as carriers of hereditary information (genes). This also implied that chromosomes (not genes) underwent independent segregation and assortment as proposed by Gregor Johann Medel (1865–1866) in his laws of inheritance. A chromosome can contain more than one or more genes. Although contemporary researchers were very skeptical of this idea, Thomas Hunt Morgan (1915) showed a linear arrangement of genes in chromosomes, providing convincing evidence for Sutton-Boveri's work. The aim of this essay is to study the structure of a chromosome and how external factors such as environment and epigenetics cause an alteration in its structure and therefore its function. Chromosomes are only visible during the metaphase of cell division and are seen as a condensed thread. -structures similar to those observed under a microscope. Most of the time, they are scattered throughout the nucleus of a cell in the form of fine fibers, called chromatin. Chromosomes differ from chromatin in that they are more compact and more coiled. Extensive studies have been carried out on the structure of chromosomes. (Ris & Kubai, 1970, Weintraub & Van Lente, 1974, Gillies, 1975, Paulson & Laemmli, 1977, Comings, 1978, Howell & Hsu, 1979, Marsden & Laemmli, 1979, Lima-de-Faria, 2003). These studies show that the structure of chromosomes varies considerably between viruses, prokaryotes and eukaryotes. Viral chromosomes consist of a nucleic acid surrounded by a protein coat while in prokaryotes they tend to be simply a copy of a circular DNA molecule arranged within the nucleoid...... middle paper ......8. doi: 10.1016/0092-8674(77)90280-XPilegaard, H., Ordway, GA, Saltin, B., and Neufer, PD (2000). Transcriptional regulation of gene expression in human skeletal muscle during recovery from exercise. American Journal of Physiology-Endocrinology and Metabolism, 279(4), E806-E814. Retrieved from http://ajpendo.physiology.org/content/279/4/E806.shortRis, H. and Kubai, D.F. (1970). Chromosomal structure. Annual Review of Genetics, 4(1), 263-294. doi: 10.1146/annurev.ge.04.120170.001403University of Tennessee (nd). Epigenesis and epigenetic cascade. Retrieved May 3, 2014, from https://notes.utk.edu/Bio/greenberg.nsf/0/f0820b0a18b7994185256d270047a586?OpenDocumentWeintraub, H., and Van Lente, F. (1974). Dissection of chromosome structure with trypsin and nucleases. Proceedings of the National Academy of Sciences, 71(10), 4249-4253. doi: 10.1073/pnas.71.10.4249