III.

The Human Genome

A genome is the complete collection of an organism’s genetic material. The human genome is composed of about 20,000 to 25,000 genes located on the 23 pairs of chromosomes in a human cell. A single human chromosome may contain more than 250 million DNA base pairs, and scientists estimate that the entire human genome consists of about 3 billion base pairs.

The DNA analyzed in the Human Genome Project came from small samples of blood or tissue obtained from many different people. Although the genes in each person’s genome are made up of unique DNA sequences, the average variation in the genomes of two different people is estimated to be 0.05 to 0.1 percent. That is, approximately 1 in 1,000 to 1 in 2,000 nucleotides will be different from one individual to another. Thus the differences between human DNA samples from various sources are small in comparison to their similarities.