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They hold the two strands of DNA together. The nitrogenous bases in DNA are A (adenine), C (cytosine), G (guanidine), and T (thymine). In each of these bases, the nitrogen of the secondary amine bonds to the deoxyribose in the backbone. DNA consists of two strands running in opposite directions. The hydrophobic nitrogenous bases are on the inside, hidden from water. Hydrogen bonds between the nitrogenous base hold the two strands of DNA together. These hydrogen bonds are base specific. That is, A (adenine) can form hydrogen bonds only with T (thymine). C (cytosine) can form hydrogen bonds only with G (guanine). The thymine-adenine pair interacts through two hydrogen bonds.
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 _______ _______.