If you do not have an SSN and are not eligible to obtain an SSN, but you have a requirement to furnish a federal tax identification number or file a federal income tax return, you must apply for an ITIN. By law, an alien individual cannot have both an ITIN and an SSN.
DNA bases pair up with each other, A with T and C with G, to form units called base pairs. Each base is also attached to a sugar molecule and a phosphate molecule. Together, a base, sugar, and phosphate are called a nucleotide. Nucleotides are arranged in two long strands that form a spiral called a double helix.
The phosphate group (PO3) of one nucleotide links to the hydroxyl group (OH) of the following nucleotide. Hundreds of millions of these linkages occur within the DNA polymer. However, only four bases: A (adenine), C (cytosine), G (guanine), and T (thymine) constitute the backbone of the molecule. The DNA molecule is made up of two polynucleotide strands intertwined together to form a long helical structure, the DNA double helix.