DNA Model Refill, Single Packet - 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. Instructions DNA Model Refill, Single Packet - 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. Instructions