Before a cell can divide it has to copy every letter of its DNA, and it has to get the copy right. The trick the cell uses is beautifully simple: keep one old strand as a guide, and build the partner strand against it.
📘 What you need to know
DNA replication is semi-conservative: each new molecule keeps one original strand and one newly made strand.
The kept strand acts as the template, and the new strand is built by complementary base pairing.
Helicase unwinds the double helix and breaks the hydrogen bonds between base pairs.
DNA polymerase joins free nucleotides together to form the new strand, building the sugar-phosphate backbone.
Replication happens in the nucleus, before mitosis, so both daughter cells get a full copy.
Keeping one original strand gives genetic continuity — the copy is almost identical to the original.
What “semi-conservative” means
Take the two words apart. Conservative means something is kept. Semi means half. So half of every new DNA molecule is kept from the old one.
Start with one double-stranded molecule. Separate the two strands. Each old strand is now a template, and the cell builds a fresh partner strand alongside it. You end up with two double-stranded molecules, and every one of them is half old and half new.
Two molecules out, and neither of them is brand new all the way through.
A quick check you can do in your head: after replication, how many of the original strands still exist? Two — the same two you started with. They have just been split up and given new partners.
Complementary base pairing does the proofreading
The reason this works is that the bases only fit together one way.
Base on the template
Base added to the new strand
Number of hydrogen bonds
Adenine (A)
Thymine (T)
2
Thymine (T)
Adenine (A)
2
Cytosine (C)
Guanine (G)
3
Guanine (G)
Cytosine (C)
3
Free nucleotides are floating around in the nucleus. When one drifts up against the exposed template, hydrogen bonds only form if the bases match. Any nucleotide carrying the wrong base simply will not stick, so it drifts away again.
That is why the copy is so accurate. The template does not just tell the cell what to build — the chemistry itself rejects most mistakes.
The pairing rule
A pairs with T · C pairs with G so the template determines the new strand completely
The two enzymes you must name
Helicase
Helicase unwinds the double helix and breaks the hydrogen bonds holding the base pairs together. The two strands come apart, exposing the bases on both.
Two analogies that stick: helicase untwists a rope ladder, then unzips it. Notice that only the hydrogen bonds between bases are broken. The covalent bonds along each backbone stay intact, which is why each strand survives in one piece.
DNA polymerase
DNA polymerase joins the free nucleotides together. It catalyses condensation reactions between the deoxyribose sugar of one nucleotide and the phosphate group of the next, building the sugar-phosphate backbone of the new strand.
DNA polymerase can only build in one direction, described as 5′ to 3′. It attaches to the 3′ end of the template and reads along it in the 3′ to 5′ direction. Because the two strands of DNA run in opposite directions — they are antiparallel — the new strand ends up being built 5′ to 3′.
Helicase opens the molecule; polymerase builds the new strands behind it.
📝 The process, in order
Helicase unwinds the double helix and breaks the hydrogen bonds between the base pairs.
The two strands separate, exposing the bases. Each acts as a template.
Free nucleotides line up against the template by complementary base pairing.
DNA polymerase catalyses condensation reactions, joining nucleotides into a sugar-phosphate backbone in the 5′ to 3′ direction.
Hydrogen bonds form between the template bases and the new bases.
The result is two identical DNA molecules, each with one old and one new strand.
Why the cell bothers
Replication happens in the nucleus during interphase, before mitosis. If DNA were not copied first, each daughter cell would end up with only half the genetic material.
Growth — a multicellular organism gets bigger by making more cells.
Repair and replacement — damaged or worn-out cells are replaced.
Reproduction — gametes and offspring need a full set of genes.
Keeping one original strand is what gives genetic continuity. The instructions that worked in the parent cell are handed on, letter for letter, to both daughter cells.
A note on terms. A parent cell is the cell that divides into two daughter cells. It has nothing to do with a parent organism, and no reproduction is involved.
How we know it is semi-conservative
When Watson and Crick worked out the double helix in 1953, they suggested that the structure hinted at how DNA might copy itself. It was only a hypothesis; they had no evidence for it.
The evidence came from Meselson and Stahl, who grew bacteria in a medium containing a heavy isotope of nitrogen so that their DNA was heavier than normal, then moved them to a normal medium and tracked the density of the DNA over successive rounds of replication. The pattern they saw matched the semi-conservative prediction and ruled out the alternatives.
This is a lovely nature-of-science example. A hypothesis on its own is not enough, however elegant it looks. Someone has to design an experiment whose results would come out differently if the hypothesis were wrong.
WORKED EXAMPLE
A single DNA molecule has both of its strands labelled with a heavy isotope. It is allowed to replicate twice in a medium containing only the normal light isotope. Deduce how many molecules are produced and how many of them contain a heavy strand. [4]
Step 1: after the first round1 molecule gives 2 molecules; each keeps one heavy strand and gains one light strandStep 2: after the second roundeach of those 2 gives 2, so there are 4 molecules in totalStep 3: track the two original strandsthere were only ever 2 heavy strands, and each ends up in a different moleculeStep 4: conclude4 molecules, of which 2 contain a heavy strand and 2 are entirely lightthe number of original strands never changes — that is the whole point of semi-conservative
💡 Exam tip
Say helicase breaks hydrogen bonds, not “breaks the DNA”. The backbones stay whole.
Say DNA polymerase forms covalent bonds in the sugar-phosphate backbone by condensation.
Use the word template for the original strand. It is worth a mark on its own.
If a question gives a base sequence, write the complementary one carefully — and check A goes with T, not U.
Mention 5′ to 3′ and antiparallel if the question asks about direction.
Link accuracy to genetic continuity between generations of cells.
⚠ Common mix-up
Semi-conservative does not mean half the molecules are old. It means half of each molecule is old.
Helicase and DNA polymerase do different jobs. One opens, the other builds. Never swap them.
DNA replication is not protein synthesis. No RNA and no ribosomes are involved here.
Uracil does not appear in DNA. That is RNA. DNA uses thymine.
Replication happens before mitosis, in interphase, not during it.
Parent cell is not parent organism. Check which one the question means.
Up next: Electrophoresis & PCR — now that you know how a cell copies DNA, let us look at how scientists copy it in a tube, and how they sort the fragments afterwards.
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