The Structure of DNA — Study Notes

Every trait you have — eye colour, blood type, how a healing cut gets rebuilt — starts as an instruction stored somewhere inside your cells. That storage molecule is DNA, and O-Level Biology expects you to describe its physical shape precisely, not just wave at "genetic material."

How DNA Is Put Together

DNA is shaped like a twisted rope ladder, a form called a double helix: two long strands winding around a shared axis. Each strand isn't one continuous piece — it's a long chain of smaller repeating units, called nucleotides, threaded one after another like beads on a necklace. Every single nucleotide breaks down into three smaller bonded parts: a sugar, a phosphate unit, and one base chosen from four possible kinds. The bases point inward from each strand and lock onto a partner base from the strand opposite, forming the rungs of the ladder. Crucially, this locking isn't random — every base has exactly one partner it fits: adenine only ever bonds with thymine, and cytosine only ever bonds with guanine. Because there are just two valid combinations, knowing one strand's base order lets you deduce the second strand without ever seeing it directly — which is exactly the skill O-Level questions test.

Worked Example — Deducing the Opposite Strand

1. A DNA strand has the base sequence, reading left to right: G, C, C, T, A.
2. Swap each base for its one permitted partner, keeping the same left-to-right order: G → C, C → G, C → G, T → A, A → T.
3. Writing those partners out in order gives the second strand: C, G, G, A, T.

Work through every position in the sequence, one base at a time — skipping ahead under exam pressure is where most marks get dropped on this question type.

How this same molecule gets organised into genes and chromosomes, why base order ultimately decides which protein gets made, plus a full audio walkthrough and practice worksheet, are all waiting in the full lesson below.