Here, we can see, the DNA molecules, comprising of 4 nitrogenous bases (Adenine, Guanine, Thymine and Cytosine), deoxyribose sugars and phosphoric acid, are packed so compactly, so as to fit into the nucleus. They wrap around the histone proteins, to form nucleosomes, coil upon themselves to ultimately form chromosomes. The reverse of this is to occur, when a cell needs to multiply. To begin with, the heavily folded DNA molecules will need to be unwound first. This is done by enzymes called helicases. When this enzyme is defective (due to a mutation in the helicase gene), the DNA molecule can no longer be unwound. They can no longer be "read and copied" and even 'repaired' as the DNA needs to unfold, to show its faulty codes. Hence, in helicase deficiency, errors accumulate within the DNA molecule and replication is inefficient. This leads to accelerated aging, diseases called segmental progerias. Werner's syndrome (adult onset progerias, seen mostly in the Japanese; often presents with cancer) and Hutchinson-Gilford progeria (childhood onset progerias) are two common examples. In these diseases the persons age very fast; the skin wrinkles faster and there is rapid overall decline in health. There may also be a component of free radical injury or telomere shortening in these diseases. In fact, cells with much shorter telomeres have been observed in these diseases.
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