DNA is all the same chemical

The stringy stuff in the test tube is DNA. But you can't tell which one of these organisms it came from just by looking at it. That's because DNA looks exactly the same in every organism on Earth.

 

All living things have DNA. And whether it comes from you, a pea plant, or your pet rat, it's all the same molecule. It's the order of the letters in the code that makes each organism different.

 

The human genome

The order of building blocks in a strand of DNA makes up a "sequence." We can read a DNA sequence like letters in a book. In fact, we know the sequence of the entire human genome—all 3 billion letters. That's enough information to fill roughly 1,000 200-page books!

 

Contained within the 3 billion letters of the human genome are about 21,000 genes. Most of our known genes code for proteins, but some code for RNA molecules.

 

Your DNA makes you unique

All humans have the same genes arranged in the same order. And more than 99.9% of our DNA sequence is the same. But the few differences between us (all 1.4 million of them!) are enough to make each one of us unique. On average, a human gene will have 1-3 bases that differ from person to person. These differences can change the shape and function of a protein, or they can change how much protein is made, when it's made, or where it's made.