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Purines and Pyrimidines
A pyrimidine is an example of an aromatic compound, meaning it enjoys greater stability because of the way electrons are shared around the ring. It's a six-member ring much like benzene but with nitrogen substituted for two of the carbons. A purine is a pyrimidine ring fused to an imidazole ring, which is a five-member aromatic ring also containing two nitrogen atoms. The pyrimidines and purines in DNA are often called "bases," and two bases in opposite strands are called a "base pair."
Hydrogen Bonding
Bases in a base pair form hydrogen bonds with each other. The shape of each base and the number of hydrogen bonds it can form determine which other base is complementary to it. Adenine, which is a purine, can form two hydrogen bonds and is always found opposite thymine, which is a pyrimidine. Guanine and cytosine are always found opposite each other; unlike adenine and thymine, they form three hydrogen bonds rather than two.
Transcription
You can read this sentence and understand it because each sequence of letters carries a specific meaning. Cells use DNA in much the same way. Each sequence of base pairs in a gene is a code for a specific series of amino acids in a protein. The cell makes RNA copies of the DNA genes and uses these RNA copies as a set of instructions to manufacture the protein. Consequently, the order of the base pairs in the DNA has a specific meaning for the cell, just as the order of the letters in a word have a specific meaning for you.
Regulation
Base pairs also have other functions besides coding for proteins. Many stretches of DNA are involved in gene regulation; regulatory proteins recognize and bind to specific sequences of base pairs, either activating a gene and turning it on or repressing it and turning it off. Some stretches of DNA also code for RNA molecules like transfer RNAs, which play a critical role in translating the RNA code into protein, and microRNAs, which can reduce the amount of protein produced by an activated gene.