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.
How are nucleotides held together vs how DNA strands in a DNA molecule are held together? The nucleotides are linked together covalently by phosphodiester bonds through the 3'-OH group of one sugar and the 5'-PO3 of the next, into polynucleotide chains. A DNA molecule is composed of two of these polynucleotide chains (DNA strands) which are held together by hydrogen bonds between the paired bases. DNA is wound into a _______ _______.