A baby grows from a single fertilized egg to the 10 trillion cells that make up a person. Each cell has 23 pairs of structures called chromosomes. Each chromosome is comprised of a coil of DNA. The DNA in the gene - which is tightly wound around spools of proteins called histones, must be unwound or at least be loosely unwound to be used. This unwinding makes the genes accessible so they can be expressed.

Animation produced by Visible Productions Inc.