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Essential Principles of Gene Expression

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The DNA double helix transmits genetic information from generation to generation and is the repository of information required to guide an organism's development and interaction with the environment. The role of DNA in storing and transferring hereditary information depends on the innate properties of its four constituent bases. There are two purine bases, adenine (A) and guanine (G), and two pyrimidine bases, cytosine (C) and thymine (T). Within the DNA double helix, A is complementary to T, and G is complementary to C. Each block of DNA that codes for a single RNA or protein is called a gene, and the entire set of genes in a cell, organelle, or virus forms its genome. Cells and organelles may contain more than one copy of their genome. There are 46 chromosomes in a typical human cell; when "unraveled," the total DNA of a single cell is approximately 1 m in length. The 46 human chromosomes consist of 22 pairs of autosomes and 2 sex chromosomes, either XX for females or XY for males. Such a large amount of genetic material is effectively packaged into a cell nucleus, which is also the site of DNA replication and transcription. Only a small percentage of chromosomal DNA in the human genome is responsible for encoding the genes that act as a template for RNA strands; there are approximately 20,000–25,000 genes total, of which about 10,000–15,000 genes are expressed in any individual cell.

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