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During DNA replication, the two strands separate as the hydrogen bonds connecting the parent strands are broken by an enzyme called helicase. In the DNA molecule (double strand) complementary bases are joined by hydrogen bonds; that is; Adenine paired to thyamine and guanine to cytosine; during replication the enzyme helicase separates the double helix by breaking the hydrogen bonds between the complementary bases.
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The cells of living things are made mainly of four elements: carbon, hydrogen, oxygen and nitrogen. They make up 96% of the atoms that are in living things, so they would be considered major chemicals. However, depending on how you define major, other elements that only make up a few percent of cells can top the list.
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