Final answer:
Nucleotides in DNA are held together by phosphodiester bonds forming the backbone and hydrogen bonds enabling base pairing between adenine and thymine, and guanine and cytosine.
Step-by-step explanation:
In a strand of DNA, the nucleotides are held together by phosphodiester bonds and hydrogen bonds. The phosphodiester bonds link the 5' phosphate group of one nucleotide to the 3' hydroxyl group of the next to form the sugar-phosphate backbone of DNA. Additionally, complementary base pairing occurs between the nitrogenous bases of the two strands of DNA.
Each adenine pairs with a thymine through two hydrogen bonds, and each guanine pairs with a cytosine through three hydrogen bonds. These hydrogen bonds are crucial for the DNA's double helix structure and allow the DNA to unzip for replication, as they are strong enough to hold the strands together but weak enough to be separated without breaking the stronger covalent bonds of the DNA backbone.