image credit: U.S. Department of Energy Human Genome Program, http://www.ornl.gov/hgmis.

caption: The four nitrogenous bases of DNA are arranged along the sugar- phosphate backbone in a particular order (the DNA sequence), encoding all genetic instructions for an organism. Adenine (A) pairs with thymine (T), while cytosine (C) pairs with guanine (G). The two DNA strands are held together by weak bonds between the bases.

A gene is a segment of a DNA molecule (ranging from fewer than 1 thousand bases to several million), located in a particular position on a specific chromosome, whose base sequence contains the information necessary for protein synthesis.

For more on the science behind the Human Genome Project, see our Website.

This image originally appeared in the 1992 U.S. DOE Primer on Molecular Genetics.