The human genome is the full complement of genetic material in a human cell. The genome, in turn, is distributed among 23 sets of chromosomes, which, in each of us, have been replicated and re-replicated since the fusion of sperm and egg that marked our conception. The source of our personal uniqueness, our full genome, is therefore preserved in each of our body's several trillion cells. At a more basic level, the genome is DNA, deoxyribonucleic acid, a natural polymer built up of repeating nucleotides, each consisting of a simple sugar, a phosphate group, and one of four nitrogenous bases. In the chromosomes, two DNA strands are twisted together into an entwined spiral -- the famous double helix -- held together by weak bonds between complementary bases, adenine (A) to thymine (T) and cytosine to guanine (C-G); structurally the molecule resembles a twisted ladder. In the language of molecular genetics, each of these linkages constitutes a base pair. All told, if we count only one of each pair of chromosomes, the human genome comprises about three billion base pairs.
The specificity of these base-pair linkages underlies all that is wonderful about DNA. First, replication becomes straightforward. Unzipping the double helix provides unambiguous templates for the synthesis of daughter molecules: One helix begets two with near-perfect fidelity. Second, by a similar template-based process, a means is also available for producing a DNA-like messenger known as messenger @RNA (or @mRNA). This faithful complement of a particular DNA segment transports its information to the cell's cytoplasm where it directs the synthesis of a particular protein. Many subtleties are entailed in the synthesis of proteins, but in a schematic sense, the process is elegantly simple.
Every protein is made up of one or more polypeptide chains, each a series of (typically) several hundred molecules known as amino acids, linked by so-called peptide bonds. Remarkably, only 20 different kinds of amino acids suffice as the building blocks for all human proteins in nature. The synthesis of a protein chain, then, is simply a matter of specifying a particular sequence of amino acids. Each linear sequence of three bases (both in RNA and in DNA) corresponds uniquely to a single amino acid. The RNA sequence AAU (RNA uses the base uracil instead of thymine) thus dictates that the amino acid asparagine should be added to a polypeptide chain, GCA specifies alanine -- and so on. A segment of the chromosomal DNA that directs the synthesis of a single type of protein constitutes a single gene.
As we have seen before, one of the central goals of the Human Genome Project is to produce a detailed "map" of the human genome.
