Build a segment of the DNA consisting of Adenine, Thymine, Cytosine and Guanine. Color-coded pieces designate certain molecules making up the four nucleotides present in DNA.Kit includes: 12 packets of model parts, each with enough material to construct a segment of 6-paired bases. Instructions and reproducible student worksheets are also included. Chromosomes are structures in the nucleus of a cell and are composed of long strands of deoxyribonucleic acid (DNA). In 1953, two scientists, James D. Watson and F.H.C. Crick, proposed a model of the structure of the complex DNA. They described the molecule as a double helix or spiral, composed of nucleotides. Each nucleotide is composed of a phosphate unit joined to deoxyribose, a five-carbon sugar and a nitrogen-containing base. The DNA molecule is a double strand of possibly thousands of nucleotides bonded by synthesis in a cell.