In the structure of DNA, how are the nucleotide units linked together?

In the structure of DNA, the nucleotide units are linked together by covalent bonds known as phosphodiester bonds. Each nucleotide consists of a sugar, a phosphate group, and a nitrogenous base. The phosphodiester bond forms between the phosphate group of one nucleotide and the sugar of the next nucleotide in the sequence. This creates a strong backbone of alternating sugar and phosphate units that holds the individual nucleotides together in a chain, which is essential for maintaining the stability of the DNA structure.

In contrast, hydrogen bonds play a crucial role in stabilizing the double helix structure of DNA by forming connections between the complementary bases on the two DNA strands, but they do not link the nucleotides themselves. Additionally, ionic bonds and peptide bonds do not participate in the linking of nucleotides in DNA; ionic bonds typically occur between charged particles, while peptide bonds are involved in linking amino acids together in proteins. Thus, the presence of phosphodiester bonds is key to the structural integrity and function of DNA molecules.