The human genome comprises a sequence of approximately 3 billion component parts, called nucleotides, which are organized into DNA molecules—the double helix. The nucleotides, which serve as the alphabet for the language of life, are represented by just four letters: A, C, G, and T, corresponding to adenine, cytosine, guanine, and thymine. The nucleotide alphabet codes for the sequence of amino acids the body will use to build proteins.
Combinations of three nucleotides indicate one of twenty possible amino acids (for example, CCT codes for the amino acid glycine), so sets of nucleotide triplets form the instructions that cells use to build proteins. These proteins perform the work of the cells from development throughout life, contributing to both our physical attributes and many of our less tangible features, such as behavior, learning, and predisposition to disease. A segment of a DNA molecule that codes for one complete protein is called a gene. The human genome is carried on 23 different chromosomes—or DNA molecules.
Genomes of other species contain more or fewer nucleotides and chromosomes but follow the same basic organizational scheme as the human genome.
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