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.
See Fig. 1. The nucleotides within each strand are held together by the phosphodiester bonds between the 5' carbon of one nucleotide and the 3 ' carbon of the adjacent nucleotdie. These strong covalent bonds form the the glue that holds the sugar/phosphate backbone together and thus act to keep each strand together.The two strands of DNA are held together by weak hydrogen bonds (remember, something must hold them together!) between the nitrogenous bases.The interesting (and very important) aspect of the Watson-Crick model is that these hydrogen bonds are base specific.ames Watson and Francis Crick proposed a structure for the DNA molecule that suggested the basic mechanism of DNA replication. The model proposes that DNA is composed of two strands of DNA running anti-parallel (remember the 5' and 3' ends!!) to each other.