DNA in human cells is organized into chromosomes. Each chromosome has one very long DNA molecule, with hundreds or thousands of genes arranged along its length. The DNA of chromosomes replicates as a cell prepares to divide. Each of the two cellular offspring inherits a complete set of genes. In each cell, the genes along the length of DNA molecules encode the information for building the cell’s other molecules. DNA thus directs the development and maintenance of the entire organism. Most genes program the cell’s production of proteins. Each DNA molecule is made up of two long chains arranged in a double helix. Each link of a chain is one of four nucleotides, encoding the cell’s information in chemical letters. The sequence of nucleotides along each gene codes for a specific protein with a unique shape and function. Almost all cellular activities involve the action of one or more proteins.
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