In simple terms
A friendly intro before the formal notes — no formulas yet.
Photocopying the Blueprint of Life
DNA replication is the cell's way of making an exact copy of its DNA before it divides, so that each new cell receives a complete and identical set of genetic instructions. The trick that keeps the copy accurate is that each old strand acts as a template, and the base-pairing rules only allow one correct partner for every base.
Imagine a zip with two interlocking halves. To copy it you first unzip it into two separate rows of teeth. Each row is then used as a guide: a new matching row is built against it, tooth by tooth, so where one side has a bump the new side must have the exact matching dip. You end up with two complete zips, each made of one original half and one brand-new half — which is why replication is called 'semi-conservative': half of every new molecule is conserved from the parent.
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Helicase unwinds the double helix and breaks the hydrogen bonds between base pairs, separating the two strands.
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Each separated strand acts as a template; free nucleotides line up against it following complementary base pairing (A–T, G–C).
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DNA polymerase links these nucleotides into a new strand, working in the 5'→3' direction, and DNA ligase joins any fragments into a continuous strand.
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Two identical DNA molecules are formed, each with one original (parental) strand and one newly synthesised strand.
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Full topic notes
Formal explanation with the rigour you need for the exam.
The semi-conservative model
DNA replication is described as 'semi-conservative' because each new DNA molecule is built from one strand of the original molecule and one newly synthesised strand. The original strand is not consumed or discarded — it is conserved and used as a template that dictates the base sequence of its new partner through complementary base pairing. So each daughter molecule is half old and half new, which is exactly what 'semi' (half) conservative means. This single idea underpins everything else in the topic: because one parental strand survives intact in each copy, and because base pairing only permits one correct partner per base, the two daughter molecules are guaranteed to have the same sequence as the parent.
How we know: the Meselson–Stahl evidence
Before 1958 three models were plausible: conservative (the whole original helix stays together and an all-new helix is made), semi-conservative (one old + one new strand each), and dispersive (old and new DNA scattered along both strands). Meselson and Stahl distinguished them elegantly. They grew bacteria for many generations in a medium containing heavy nitrogen (¹⁵N) so that all the DNA was 'heavy', then switched the cells to normal light nitrogen (¹⁴N) and let them replicate. After one round of replication, all the DNA had a single intermediate density — heavier than pure light DNA but lighter than pure heavy DNA. That immediately rules out the conservative model (which predicts a heavy band and a separate light band). After a second round, two bands appeared: intermediate and light. This pattern is precisely what the semi-conservative model predicts — each molecule keeps one original heavy strand paired with a new light one — and it rules out the dispersive model too. Their result is the classic confirmation that replication is semi-conservative.
Each daughter molecule = one parental strand + one new strand (semi-conservative).
The parental strand acts as a template; complementary base pairing copies its sequence into the new strand.
Meselson–Stahl: after one round in light nitrogen, formerly-heavy DNA became a single INTERMEDIATE density — the signature of one heavy + one light strand.
After two rounds, intermediate and light bands appear, confirming semi-conservative and excluding the conservative and dispersive models.
The key enzymes of replication
Replication is carried out by a team of specific enzymes working together at high speed. Three roles are essential for D1.1 at both SL and HL: helicase unwinds and separates the strands, DNA polymerase builds the new strands, and DNA ligase joins fragments into a continuous strand. Being precise about what each enzyme does — and does not do — is where marks are won and lost in the exam.
Helicase: unwinds the double helix and separates the two strands by BREAKING the hydrogen bonds between complementary base pairs, creating a replication fork. It exposes the bases but adds nothing.
DNA polymerase: moves along each template strand and adds free complementary nucleotides to the 3' end of the growing strand, following the base-pairing rules (A–T, G–C) and forming phosphodiester bonds. It works only in the 5'→3' direction and proofreads as it goes.
DNA ligase: seals the gaps in the sugar–phosphate backbone, joining adjacent fragments into one continuous strand. It is essential for completing the lagging strand (HL).
The mechanism, step by step
Replication begins at specific sites called origins of replication. Helicase binds and unwinds the helix, breaking the hydrogen bonds so the two strands come apart to form a replication fork; each exposed strand now acts as a template. Free deoxyribonucleoside triphosphates (dNTPs) from the nucleoplasm supply both the building blocks and, through the cleavage of their extra phosphate groups, the energy to form each new bond. DNA polymerase reads the template and adds the one nucleotide whose base is complementary to the template base, extending the new strand at its 3' end. Because base pairing permits only one correct partner per base, the sequence of the new strand is fixed by the template — this is the molecular reason replication is so accurate. The result is two DNA molecules, each identical to the original and to each other, ready to be separated into the two daughter cells.
Why does this accuracy matter? Each daughter cell must inherit a faithful copy of every gene so that it can build the correct proteins and function like the parent cell. If replication introduced frequent errors, genes could be disrupted (mutations), potentially impairing the cell or, in a whole organism, being passed on to offspring. Accurate, semi-conservative replication is therefore the foundation of genetic continuity — the reliable passing of genetic information from one cell generation to the next.
HL: leading and lagging strands, and primers
This section is HL only. The two strands of DNA are antiparallel — they run in opposite directions (one 5'→3', the other 3'→5'). Since DNA polymerase can only extend a strand in the 5'→3' direction, the two new strands cannot both be made the same way. On the leading strand, synthesis runs continuously towards the advancing replication fork. On the lagging strand, which runs the opposite way, DNA polymerase must work away from the fork, so the strand is built in short pieces called Okazaki fragments as the fork keeps opening. These fragments are then joined together by DNA ligase to make one continuous strand. In addition, DNA polymerase cannot start a strand from scratch — it can only add to an existing 3'-OH end — so a short primer is laid down first to provide that starting point. The leading strand needs one primer; the lagging strand needs a new primer for each Okazaki fragment.
The strands are ANTIPARALLEL, and polymerase works only 5'→3', so the two new strands are made differently.
Leading strand: synthesised continuously towards the fork.
Lagging strand: synthesised discontinuously as Okazaki fragments, later joined by DNA ligase.
Primers: short sequences that provide the 3'-OH end polymerase needs to begin; the lagging strand needs one per fragment.
Common mistakes examiners penalise
Getting 'semi-conservative' wrong — it means one OLD + one NEW strand per daughter molecule, not 'half copied' and not 'the old molecule broken up and shared'. State the one-old-one-new point explicitly.
Swapping the enzyme roles — helicase unwinds and breaks hydrogen bonds; DNA polymerase adds nucleotides. Writing that helicase builds the new strand, or that polymerase separates the strands, loses the marks.
Saying helicase 'just separates the strands' — you must state that it BREAKS the hydrogen bonds between base pairs to get the specific mark.
Forgetting the direction — DNA polymerase works only 5'→3'; leaving this out (or writing 3'→5') costs a mark in HL strand questions.
Vague accuracy claims — the reason replication is accurate is that complementary base pairing allows only one correct partner per base (A–T, G–C), reinforced by polymerase proofreading. 'The enzyme is careful' scores nothing.
Confusing replication with transcription — replication copies the whole DNA into DNA using both strands; transcription copies a gene into RNA. Do not mention RNA, mRNA or ribosomes in a replication answer.
Bringing HL detail into an SL 'wrong' place, or omitting it in HL — leading/lagging strands, Okazaki fragments and primers are HL; know which paper you are sitting.
Model answer — marked the way our engine marks it
D1.1 explain questions are marked analytically: each distinct valid biological point is worth one mark, method marks (M) credit correct reasoning, answer/analysis marks (A) credit a correct concluding statement, equivalent wording is accepted, and error-carried-forward (ECF) means an early slip does not cost you later marks if your method is shown. Study how each mark below is tied to a specific, named idea rather than to loose phrasing.
Worked examples
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A diagram of a replication fork shows a molecule labelled X separating the two DNA strands, and a molecule labelled Y adding nucleotides to build a new strand. Identify X and Y and describe the function of each. [4]
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Model answer. X is helicase. Its function is to unwind the double helix and separate the two strands by breaking the hydrogen bonds between the complementary base pairs, exposing the bases as templates.
A single DNA molecule made entirely of a radioactive isotope undergoes three complete rounds of replication in a medium containing only non-radioactive nucleotides. What percentage of the resulting DNA molecules will contain any of the original radioactive material? Explain using the semi-conservative model. [3]
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Model answer. Replication is semi-conservative, so each new molecule keeps one original (radioactive) strand and gains one new (non-radioactive) strand. The two original radioactive strands are conserved throughout and simply become diluted among more and more molecules.
Explain why DNA replication is described as semi-conservative, and how complementary base pairing ensures the two new molecules are identical to the original. [4]
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Model answer. During replication the double helix is unwound and the two strands are separated (by helicase breaking the hydrogen bonds). Each original strand then acts as a template for building a new strand. Free nucleotides pair with the exposed bases according to complementary base pairing — adenine only with thymine and guanine only with cytosine — so the sequence of each new strand is fixed by its template. Because every base has exactly one correct partner, the new strand is an exact complement of the old one, and the resulting double helix has the same base sequence as the original. Each daughter molecule therefore consists of one original (conserved) strand and one newly made strand, which is why replication is called semi-conservative — and why both molecules are identical to the parent.
How it all connects
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Glossary
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Quick check
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Revision flashcards
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DNA replication
The process by which a double-stranded DNA molecule is copied to produce two identical daughter DNA molecules, ready to be passed to two daughter cells. Occurs in the S phase of interphase.
Key takeaways
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Each daughter molecule = one parental strand + one new strand (semi-conservative).
- ✓
The parental strand acts as a template; complementary base pairing copies its sequence into the new strand.
- ✓
Meselson–Stahl: after one round in light nitrogen, formerly-heavy DNA became a single INTERMEDIATE density — the signature of one heavy + one light strand.
- ✓
After two rounds, intermediate and light bands appear, confirming semi-conservative and excluding the conservative and dispersive models.
Practice — then mark it
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Get a Paper 2 question marked: explain semi-conservative replication, the enzyme roles and how base pairing ensures accuracy
Get a Paper 2 question marked: explain semi-conservative replication, the enzyme roles and how base pairing ensures accuracy
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