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.
At each point of juncture within a polynucleotide, the 5' end of one nucleotide attaches to the 3' end of the adjacent nucleotide through a connection called a phosphodiester bond (Figure 3). It is this alternating sugar-phosphate arrangement that forms the backbone of a DNA molecule.