Biology DNA Test 2026 – Complete Practice on Structure, Function, and Replication

The basic structure being tested is that DNA is a double helix built from nucleotides. Each nucleotide has a sugar (deoxyribose), a phosphate group, and a nitrogenous base. The two strands form a twisted ladder, running in opposite directions, and are held together by hydrogen bonds between complementary bases: adenine pairs with thymine, and guanine pairs with cytosine. The sugar–phosphate backbone is the outer “sides” of the ladder, while the bases form the rungs on the inside. This arrangement not only makes DNA a stable carrier of genetic information but also enables accurate replication, since each strand can serve as a template for a new partner strand. The other descriptions don’t fit: a protein complex stores genetic information, a circular molecule mainly of lipids describes membranes rather than DNA’s structure, and a single strand lacking a helix describes RNA or a non-helix structure, not DNA.