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Genes are made of nucleotides and are segments of DNA that code for a specific kind of protein. The order of nucleotides in genes varies within DNA from individual to individual. It is the order of these nucleotides making up genes that determines what structures are made. The more similar two individuals' DNA and genes are, the more closely related they are.

DNA (deoxyribose nucleic acid) is the molecule that is found in all cells. It is floating in the cytoplasm of prokaryotic cells and found within the nucleus of eukaryotic cells. DNA stores all genetic information, that is, information contained within genes.

DNA is shaped like a double helix (spiral staircase). This structure of DNA is made of three substances. The sides of the staircase are made of alternating sugar (called deoxyribose- hence its name) and phosphate groups. The interior steps of the DNA are made of nucleotides (nucleic acids).

There are four types of nucleotides in DNA- adenine (A), thymine (T), cytosine (C), and guanine (G). Thymine and cytosine are single-ringed structures called pyrimidines. Guanine and adenine are double ringed structures called purines. A pyrimidine must always pair with a purine. Specifically, A always pairs with T while G always pairs with C. 

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