Inside every living cell, DNA directs vital activities such as growth, division, movement, respiration, and even death. It does so by providing the instructions cells use to build proteins.

The chemical language for this code is stunningly simple. It consists of just four letters, which correspond to the four functional molecules (called nucleotides) that, in addition to sugar and phosphate molecules, form DNA. These nucleotides are adenine (A), thymine (T), cytosine (C), and guanine (G). Various combinations of these four molecules make up genes, the discrete sequences of DNA that provide instructions for one or more proteins. Human cells are thought to contain between 20,000 and 25,000 genes that provide the code to build many more different types of proteins.

The DNA molecule resembles a twisted ladder, with pairs of nucleotides forming the ladder's rungs. As a rule, adenine always pairs with thymine and cytosine with guanine. When a cell requires a particular protein, an activation signal stimulates the release of an enzyme called RNA polymerase that causes the DNA to unzip between nucleotide pairs in the region of the appropriate gene. As the RNA polymerase molecule moves along one of the unzipped DNA strands, it assembles a similar nucleic acid molecule, known as messenger RNA (mRNA), using free nucleotides found inside the nucleus. The mRNA molecule contains the appropriate paired nucleotide in each position, and thus matches the sequence of the part that was unzipped, except that the nucleotide uracil (U) is substituted for thymine. After this process, called transcription, is complete, the mRNA is transported outside the nucleus to the cytoplasm, where it can be translated into a protein.

Structures found in the cytoplasm, called ribosomes, perform the process of translation. Reading the mRNA's nucleotides three at a time, these structures assemble strands of amino acids, the molecules that make up proteins. Each nucleic acid triplet corresponds to a particular amino acid. Thus, the order of amino acids in a strand corresponds directly to the mRNA sequence, as well as to the DNA sequence from which it was transcribed.

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