The instructions in the nucleus are carried in long, two stranded molecules called deoxyribonucleic acid. The DNA molecule looks like a twisted ladder. The two strands, or sides, of the DNA ladder wrap around each other in a spiral shape that scientists call a double helix. The word "helix" comes
from a Greek word meaning to wrap. The sides of the DNA ladder are made of sugar and phosphate. The steps of the ladder are made of four nitrogen bases, which are represented by the letters A (adenine), G (guanine), C (cytosine), and T (thymine).