Inside every cell of our bodies is a long, thin molecule called DNA. DNA is your own personal instruction manual and it tells your body everything it needs to know! DNA determines your eye color, skin tone, how tall you are, and even whether your muscles are better at sprinting or running a marathon. Just like a real instruction manual, the instructions in DNA are written in a series of letters. In DNA, there are just four letters–A, T, G, and C. These letters are combined to spell out the instruction for proteins. Proteins are the building blocks of cells. Your brain, heart, and all other organs are made of lots of different proteins. The DNA letters needed to make one protein are referred to as a "gene." Can you guess how many genes a human being has?
Over 20,000!

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Go to source: https://kids.frontiersin.org/article/10.3389/frym.2018.00037