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Chromatin and Chromosomes
DNA, the biological molecule that is used to store genetic information in the nucleus, is actually a very long molecule. The human DNA, if extended over a straight line in a linear fashion, will be over 5 feet (1.5 meters) in length. How does the cell actually fit such a long molecule (actually 46 of them) into the nucleus of the cell (which is so very small)? It does this by using special proteins to condense the DNA into an extremely dense structure known as the chromosome. The DNA is first wrapped around proteins called histones and eight of these histones are then grouped together to form the nucleosome. The nucleosomes are joined together in a helical fashion to form coils called solenoids. These coils are coiled even further to form supercoils, which are condensed into structures called chromatin fibers or simply chromatin. The chromatin is used to form helical structures, which are themselves condenses into structures called chromosomes. A chromosome consists predominately (by mass) of protein, followed by DNA and a tiny bit of RNA. In the human somatic cell, there are 23 pairs of homologous chromosomes, in which each one of the individual chromosomes in a given pair comes from each one of the two parents. The chromosome pairs are joined together by using special proteins and the location where they are joined is known as the centromere. Somatic cells are said to be diploid cells because they contain the homologous pairs. Certain cells, such as germ cells (gametes) are haploid because they do not have the homologous pairs. Chromosomes in their very condensed form cannot be transcribed because RNA polymerase cannot get to the genes. In order for the DNA to be transcribed into RNA, the chromosome must uncoil into less-tightly packed fibers called euchromatin. Within the somatic cell, the majority of the DNA exists in its euchromatin state.
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