DNA is a double helix of nucleotides whose complementary base pairing allows semi-conservative replication.

What this dot point is asking

You need to describe the molecular structure of DNA, explain how complementary base pairing works, and explain the semi-conservative model of replication including the roles of the key enzymes.

The structure of DNA

DNA (deoxyribonucleic acid) is a polymer of nucleotides. Each nucleotide has three parts:

• a deoxyribose sugar (a 5-carbon pentose),
• a phosphate group, and
• one of four nitrogenous bases: adenine (A), thymine (T), cytosine (C) or guanine (G).

Nucleotides join through covalent phosphodiester bonds between the phosphate of one nucleotide and the sugar of the next, forming a sugar - phosphate backbone. Two such strands wind around each other in a double helix, first described by Watson and Crick using Rosalind Franklin's X-ray diffraction data.

Antiparallel strands and the 5' to 3' direction

Each strand has a direction set by its carbon numbering: one end is the 5' (five-prime) end (a free phosphate) and the other is the 3' end (a free hydroxyl). The two strands run in opposite directions - they are antiparallel. One runs 5' to 3' and its partner runs 3' to 5'. This matters during replication because enzymes can only build a new strand in the 5' to 3' direction.

Complementary base pairing

The two strands are held together by hydrogen bonds between bases that face each other in the centre of the helix:

• Adenine pairs with thymine (A - T) via two hydrogen bonds.
• Cytosine pairs with guanine (C - G) via three hydrogen bonds.

A purine (A or G, double-ring) always pairs with a pyrimidine (T or C, single-ring), keeping the helix a constant width. This base-pairing rule means the two strands are complementary: knowing one strand's sequence lets you deduce the other.

Semi-conservative replication

Before a cell divides, it must copy its entire genome. DNA replication is semi-conservative: each new double helix contains one original (parental) strand and one newly synthesised strand. This was confirmed by the Meselson - Stahl experiment.

The steps and enzymes

1. Helicase unwinds the double helix and breaks the hydrogen bonds between base pairs, separating the two strands at the replication fork.
2. Each separated strand acts as a template. Free nucleotides in the nucleus pair with the exposed bases following the complementary base-pairing rules.
3. DNA polymerase adds the free nucleotides to the new strand, forming phosphodiester bonds. It works only in the 5' to 3' direction.
4. Because the strands are antiparallel, one new strand (the leading strand) is made continuously, while the other (the lagging strand) is made in short fragments (Okazaki fragments) that are later joined.
5. DNA ligase joins the fragments of the lagging strand together to complete a continuous strand.

The result is two identical DNA molecules, each able to be passed to a daughter cell.

Why accuracy matters

Replication is remarkably accurate because base pairing is specific and DNA polymerase has a proofreading function that removes incorrectly added nucleotides. High fidelity ensures genetic information is faithfully passed to daughter cells; errors that escape repair become mutations.

Linking structure to function

The elegance of DNA is that its structure explains its function. Complementary base pairing means each strand carries the information to rebuild its partner, which is exactly what makes faithful copying possible. The double-helix and base-pairing rules underpin everything else in this topic - transcription, the genetic code, and how mutations arise.