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.
A chain of DNA contains two different strands that contain stacks of nucleotides. These two strands combine down the middle to form the entire structure of DNA. Within each strand, the nucleotides are bound to the one above and below it using regular covalent single bonds between phosphorus and oxygen.