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The Structure Of Chromosome And The Influence Of Epigenetic Factors

1546 words - 6 pages

The Chromosomal Theory of Inheritance, proposed by Walter Sutton and Theodor Boveri in 1902-1903 is considered a landmark in the science of genetics (Martins, 1999). The theory established chromosomes as the carriers of hereditary information (genes). It also implied that chromosomes (and not genes) undergo segregation and independent assortment as proposed by Gregor Johann Medel (1865-1866) in his Laws of Inheritance. A chromosome can contain more than one gene(s). Although the contemporary scholars were highly skeptical of this idea, Thomas Hunt Morgan (1915), showed linear arrangement of genes in chromosomes providing a convincing evidence for Sutton-Boveri’s work. The aim of this essay is to investigate the structure of a chromosome and how external factors such as the environment and epigenetics causes alteration in its structure and therefore its function.
Chromosomes are only visible during the metaphase stage of cell division and are seen as condensed thread-like structures when observed under a microscope. Most of the time they are dispersed in the nucleus of a cell in the form of thin fibers, called chromatin. Chromosomes differ from chromatin due to the fact that they are more highly compact and coiled. Extensive study has been done on the structure of chromosome. (Ris & Kubai, 1970, Weintraub & Van Lente, 1974, Gillies, 1975, Paulson & Laemmli, 1977, Comings, 1978, Howell & Hsu, 1979, Marsden & Laemmli, 1979, Lima-de-Faria, 2003). It can be noted from these studies that chromosome structure varies greatly among viruses, prokaryotes and eukaryotes. Viral chromosomes are made of a nucleic acid surrounded by a protein coat while in prokaryotes they tend to be just a copy of a circular DNA molecule arranged in the nucleoid. In eukaryotes they are made of linear DNA molecules attached to a protein core. One of the key features of a chromosome is its compactness, which allows a large amount of DNA to be packed in the nucleus of a cell. The packaging takes place through several hierarchies. At the first level, the DNA (2nm in diameter) is wound around the histone protein that resembles beads on a string. This structure is known as a nucleosome (11nm) which coil and stack upon one another forming a solenoid (30nm), the second level of packing. This further coils to form looped domains and then chromatin fibers (300 nm). The chromatin fibers further coil to form chromatids which are the longitudinal subunits of chromosomes (Klug, Cummings, Spencer & Palladino, 2014).
When the nuclei of cells are examined, it is observed that the structure of chromosome is not uniform. For example, during interphase, there are parts of the chromatin fiber that remain highly stained. In 1928, Emil Heitz used the term heterochromatin to represent these regions of strong stain/dark banding. He also used the term euchromatin to represent the regions of light staining/banding. It has now been found that heterochromatin represents regions of high, repetitive,...

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