DNA and RNA are made of nucleotides. Each unit is a three-part molecule with a phosphate at one end, a sugar in the middle and a variable nucleobase at the other end.
So, for example, hydrogen or fluorine they'll never be in the middle, they'll always be on the end of a molecule.
比如，氢或者氟，永远不会在中间，它们总是会在分子中最边缘的位置。
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The research team began testing of mice at one year, the mouse equivalent of middle-age, as that is when a small molecule drug mimicking dietary restriction might be given to humans.