Why Do We Age?

– Ines, 17, Tunisia

Dear Ines,

I have gray fur. But sometimes I think it’s getting grayer—like my human friends’ hair does as they get older. I asked my friend Jiyue Zhu about that. He’s a biochemist at Washington State University. He told me it’s a mystery. “We still don’t completely comprehend this,” Zhu said. “It’s an active area of research.”

Aging seems to be related to the way cells duplicate. DNA is a set of instructions for making your body. You keep a copy of your DNA folded up inside each of your cells. All those cells have different life spans. Some, like your skin cells, get replaced often. Some, like your heart cells, last your whole life or only duplicate rarely. Your cells copy their DNA every time they duplicate. One protein unzips your DNA. Another prote

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