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molecule chemical strand thread

DNA (deoxyribonucleic acid), informational molecules contained in the nucleus of every living cell that, along with ribonucleic acid (RNA), transmit all genetic information. The instructions of the nucleic acids are finally expressed by proteins, which form many of the structural and mechanical components of living systems and act as catalysts in the chemical activity of cells. The DNA molecule is an extremely long chemical thread made up of 2 strands that are held as a pair forming a spiral, or double helix. Each strand consists of a long chain of the sugar deoxyribose and phosphate residues. Two purines—adenine (A) and guanine (G)—and 2 pyrimidines—thymine (T) and cytosine (C)—are also commonly found in DNA. In the DNA thread the purine and pyrimidine bases lie opposite one another in the structure, and the pattern is always one in which a T on one strand is faced by an A, and a G by a C. There is always one molecule of A to every molecule of T and one G for every C. The base pairs are stacked on top of one another like a pile of pennies. One millimeter of DNA contains about 5 million base pairs. The DNA of a human cell is about 16 in (41 cm) long.

See also: Genetics.

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