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.
Hydrogen bonds hold DNA strands together. Nitrogenous bases are between these two strands that link together in a specific manner with different types of hydrogen bonds. A DNA molecule is made up of the sugar deoxyribose, a phosphate group and complementary nitrogenous base pairs. The double strand of the helix consists of the sugar-phosphate backbone. The nitrogenous base pairs, which are linked by hydrogen bonds that also hold the strands together, are between this backbone. However, the nitrogenous bases, which are adenine, thymine, cytosine and guanine, pair up so that cytosine only pairs with guanine and adenine links up with thymine.