Final answer:
Double-stranded DNA is held together by hydrogen bonds between the nitrogenous bases of each strand, forming a stable yet flexible double helix structure that allows for DNA replication.
Step-by-step explanation:
Double-stranded DNA is two complementary strands of DNA held together primarily by hydrogen bonds. These hydrogen bonds form between the nitrogenous bases of each strand creating the characteristic double helix structure of DNA. The sugar-phosphate backbone is on the exterior of the DNA molecule, and the bases connected by hydrogen bonds face each other in the middle.
The ease with which these hydrogen bonds can be broken allows for the two strands of DNA to 'unzip' down the middle during the replication process. Although hydrogen bonds are relatively weak individually, the cumulative effect of millions of them makes the DNA structure stable yet flexible enough to be unzipped for DNA replication.