D1.1 DNA replication

Before a cell divides it must copy its entire genome so that each daughter cell receives a complete set of instructions. DNA replication is the elegant process that achieves this, and its accuracy is the reason genetic information is conserved from one generation of cells to the next. The whole mechanism rests on a single structural feature of DNA — complementary base pairing — which means each strand already carries the information needed to rebuild its partner. For D1.1 you need the logic of semi-conservative copying, the named enzymes, and how the same principle is harnessed in the laboratory by PCR.

Semi-conservative replication and complementary base pairing

DNA replication is described as semi-conservative: each new DNA molecule is made of one original (template) strand and one newly synthesised strand. The two strands of the parent double helix separate, and each acts as a template for building a new partner strand. This was confirmed by the classic Meselson and Stahl experiment, but for the exam the key idea is the outcome — every daughter molecule conserves half of the parent.

Building the new strand depends on complementary base pairing: adenine pairs with thymine (A–T) and cytosine pairs with guanine (C–G), held together by hydrogen bonds. Because the pairing is fixed, the sequence of an existing strand exactly specifies the sequence of its new partner. This is why replication is so accurate and why the genetic information is conserved.

The enzymes: helicase and DNA polymerase

Replication is carried out by a team of enzymes; the syllabus highlights two:

• Helicase unwinds the double helix and breaks the hydrogen bonds between the base pairs, separating the two strands to form a replication fork and exposing the bases as templates.
• DNA polymerase adds free DNA nucleotides one at a time to the template strand, following complementary base pairing, and joins them with covalent (sugar–phosphate) bonds to build the new strand.

The free nucleotides used are deoxyribonucleoside triphosphates: the extra phosphate groups they carry are removed as the nucleotide is added, and this supplies the energy needed to link it into the growing strand. DNA polymerase can only extend a strand in one direction, which is why the two new strands are not made in identical ways — but at SL you are mainly expected to know that helicase separates the strands and DNA polymerase synthesises the new ones using free nucleotides.

Why accuracy matters and where errors come from

Because base pairing is complementary and specific, replication normally produces two molecules identical to the original. This accuracy is essential: every body cell of an organism carries the same genetic information precisely because DNA is copied faithfully before each cell division. Conserving the base sequence conserves the genes.

Occasionally an incorrect base is inserted. Most such errors are corrected by proofreading, but any that remain become mutations — permanent changes in the base sequence that can then be passed to daughter cells. The reliability of complementary base pairing keeps these rare, which is why mutations are infrequent relative to the enormous number of bases copied. This links D1.1 directly to mutation in D1.3.

Replication in the laboratory: PCR

The same principle is used artificially in the polymerase chain reaction (PCR), a technique that makes many copies of a specific DNA sequence — useful when only a tiny sample is available, for example in DNA profiling or diagnosis. PCR is carried out in a thermal cycler that repeats a cycle of temperature changes:

• Denaturation (about 95 °C): heat separates the two DNA strands by breaking their hydrogen bonds (doing the job of helicase).
• Annealing (about 50–60 °C): short primers bind to the target sequence on each strand.
• Elongation (about 72 °C): a heat-tolerant DNA polymerase (Taq polymerase) extends the new strands.

Each cycle doubles the number of target molecules, so the quantity rises exponentially. PCR shows the power of complementary base pairing: given one template strand, the matching strand can always be rebuilt.