Genetic information is stored in the nucleus as DNA. Humans have 46 molecules of DNA in each cell; each molecule known as a chromosome. When the cell is in interphase of the cell cycle, the DNA is in the chromatin form: very loosely packaged on histone proteins and almost impossible to distinguish between separate strands of DNA by using a microscope. During interphase, the DNA is replicated, resulting in 2 copies of each DNA strand. In prophase of mitosis, each DNA strand condenses down to become much shorter and thicker by winding up much more tightly, in a process called supercoiling. This allows each strand of DNA to become visible, which is when it resembles the sausage shape that most would associate as being a chromosome. In prophase, each chromosome is visible as an X shaped sausage as the DNA has just been replicated. The two halves of the X are identical to each other as one is a copy of the other. These are known as sister chromatids, and are held together by a centromere. Each chromosome has a homologous pair which contains genes which code for the same proteins but with different alleles of the genes as one chromosome of the pair is inherited from each parent.
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