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Deoxyribonucleic acid (DNA) is called a double helix because it is a double-stranded molecule. DNA contains three components: deoxyribose (a five-carbon sugar), a series of phosphate groups, and four nitrogenous bases, (nitrogen compounds that contain bases). The four bases in DNA are adenine (A), thymine (T), guanine (G), and cytosine (C). The deoxyribose combines with phosphates to form the backbone of DNA. Thymine and adenine always come in pairs. Likewise, guanine and cytosine bases come together too.
Every human has his/her unique genes. Genes are made up of DNA, therefore the DNA sequence of each human is unique. However, surprisingly, the DNA sequence of all humans are 99.9% identical, which means there is only 0.1% difference.