... up of alternating units of deoxyribose and phosphate. The rungs of the ladder are composed of paired nitrogen bases. Adenine always pairs with thymine, and guanine always pairs cytosine. The bases are held together by hydrogen bonds. Watson and Crick's model also suggested a way in which DNA could make copies of itself. First, the ladder untwists. Then the bases break apart. Since and adenine nucleotide can only bond with thymine, and guanine can only bond with cytosine, new units are assembled in precisely the same order as old. When the splitting and pairing processes are competed, two identical DNA molecules stand in the place of one. The process by which DNA makes copies of itself is called replication. DNA replication DNA replication begins with a partial unwinding of the double helix at an area known as the replication fork. This unwinding is accomplished by an enzyme known as DNA helicase. This ...

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