PLTW Biomedical Science End of Course (EOC) Practice Test

DNA is shaped as a double helix—the two long strands twist around each other like a twisted ladder. The sugar-phosphate backbone forms the outside, while the bases pair on the inside (adenine with thymine, guanine with cytosine) held together by hydrogen bonds. The strands run in opposite directions (antiparallel), giving the familiar right-handed twist and a uniform width that fits together smoothly. This arrangement lets genetic information be copied accurately during replication and read by cellular machinery. While DNA can be circular in some organisms or plasmids, or exist as linear chromosomes in others, the classic and most recognizable form is the double helix.