Under the Human Genome Project funded by the Department of Energy and the
National Institutes of Health, various sequencing tools have been used to determine the
order of the building blocks of genes. Genes are made of millions of deoxyribonucleic
acid (DNA) molecules. A DNA molecule is constructed like a spiral staircase, or a
double helix. The rails of the staircase are made of a backbone structure of phosphates
and sugars, and the steps are pairs of four nitrogen-containing bases—adenine (A),
cytosine (C), guanine (G), and thymine (T). Through hydrogen bonds the two rails of
the staircase are kept together, A and T pair together, and C and G are partners (see
image below). For example, a sequence of TACAT would bond specifically with a
sequence of ATGTA.
A gene is made of a unique sequence of DNA bases; it is like a message containing a
unique combination of letters. This message is translated into information for protein
production. A protein is a folded chain of amino acids in a specific order; up to 20
different amino acids exist. The message for each amino acid within a protein is dictated
by a sequence of three DNA bases. If the key word in the message provided by the
gene is misspelled—say, part of it is supposed to be CATTAG but instead is spelled
CATGAG—then it will have a point mutation (G substitutes for the T that should be
there). As a result of this altered DNA base, the gene may produce a protein that has an
incorrect shape so it won't dock with another protein (e.g., a receptor), leading to a
mistake in the resulting message. In other words, if the message in the gene is
misspelled, the protein it encodes may be wrong and its function in the body may be
changed, sometimes for the worse.
It was once believed that each gene codes for a single
protein. However, experimental and computational evidence
(partly obtained at ORNL) shows that many genes produce
an average of three different proteins and as many as ten
protein products. Genes have protein-coding regions (exons)
interspersed with non-coding regions (introns). Through
"alternative splicing," a gene's exons can be combined in
different ways to make variants of the complete protein.
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