Key Idea: Before a cell divides it must make an exact copy of all its DNA so each daughter cell inherits the full instructions. This copying is DNA replication. This topic follows the story of DNA copying from the molecule to the lab: how DNA is replicated (semi-conservatively, with one strand kept), which enzymes do the work (helicase and DNA polymerase), how scientists copy DNA artificially (PCR + gel electrophoresis), and what the whole set of DNA is called (the genome) and how its variable regions let us tell people apart (DNA profiling). It is a Paper 1A favourite for quick recall and matching items, and a regular on Paper 2 (describe replication, name an enzyme's role) and Paper 3 (read a PCR gel, explain the temperatures).
🧬 Semi-conservative replication
The double helix unzips into its two strands, and each old strand acts as a template for building a new partner strand by complementary base pairing (A–T, C–G). The result is two identical molecules, and each one keeps one old (parental) strand paired with one new strand. 'Semi' = half: half of each new molecule is conserved from the original.
The double helix as a flat ladder: two antiparallel sugar–phosphate backbones (the rails, running 5'→3' in opposite directions) held together by complementary base pairs — A–T (2 hydrogen bonds) and G–C (3). Replication starts by breaking these rungs.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Key Idea: Meselson and Stahl decided between three possible models — conservative, semi-conservative and dispersive — using density labelling. Bacteria were grown in heavy ¹⁵N (so all DNA was heavy), then switched to light ¹⁴N, and the DNA was spun in a centrifuge to sort it by density. Generation 1 = one intermediate band → rules out conservative. Generation 2 = an intermediate + a light band → rules out dispersive. Only semi-conservative fits both.
| Model | Each daughter molecule contains | Supported by the evidence? |
|---|---|---|
| Conservative | one all-old molecule + one all-new molecule | No — generation 1's single intermediate band rules it out |
| Semi-conservative | one old strand + one new strand | YES — fits both generations |
| Dispersive | both strands a patchwork of old and new | No — generation 2's light band rules it out |
Generation 1's intermediate band kills the conservative model. Generation 2's light band kills the dispersive model. Learn the pairing.
🔓 The two enzymes of replication
Two enzymes do the key work, and the exam wants you to keep their jobs — and the bonds they act on — completely separate. Helicase goes first, opening the helix. DNA polymerase follows, filling in the new strands.
Helicase unzips the helix by breaking the hydrogen bonds; DNA polymerase builds a new complementary strand on each template. Every daughter molecule keeps one old (blue) strand paired with one new (green) strand — this is semi-conservative replication.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
| Enzyme | Job | Bond it acts on | Detail |
|---|---|---|---|
| Helicase | unzips the double helix | BREAKS hydrogen bonds (between the bases) | separates the parental helix into two single template strands |
| DNA polymerase | builds the new strand | FORMS covalent bonds (along the backbone) | adds complementary nucleotides (A–T, G–C) to each template, working 5'→3' |
Helicase handles the Hydrogen bonds — and it breaks them. Polymerase Puts nucleotides together — forming the covalent bonds of the new backbone. Different enzyme, different bond, opposite action: one breaks, one forms.
🔥 PCR, Taq & gel electrophoresis
Scientists copy DNA artificially with PCR (the polymerase chain reaction), which amplifies a chosen piece of DNA by repeating a three-step thermal cycle. The amount of DNA roughly doubles each cycle, so a tiny sample becomes billions of copies after ~30 cycles. Afterwards, gel electrophoresis separates the fragments by size so they can be seen and compared.
| PCR step | Temperature | Why that temperature |
|---|---|---|
| 1. Denaturation | ~95 °C (hot) | breaks the hydrogen bonds, separating the helix into two single strands |
| 2. Annealing | ~55 °C (cool) | lets short primers bind (anneal) to their matching sequence on each strand |
| 3. Extension | ~72 °C (warm) | the optimum for Taq polymerase, which adds nucleotides to build the new strand |
Key Idea: Taq polymerase is heat-stable (thermostable) — it is not denatured by the ~95 °C step, so the same enzyme keeps working every cycle. (It comes from Thermus aquaticus, a hot-spring bacterium.) On a gel, DNA is negatively charged, so an electric field pulls it towards the positive electrode. The gel acts as a sieve: smaller fragments travel further, larger ones stay near the wells — giving a pattern of bands.
Temperatures in order — hot splits (95 °C denatures), cool sticks (55 °C anneals primers), warm builds (72 °C Taq extends). On a gel: small = fast = far, and no DNA = no band (that lane is the control).
📦 The genome & DNA profiling
The genome is the whole of an organism's genetic information — every base of all its DNA. On the nested scale, base ⊂ gene ⊂ chromosome ⊂ genome, so the genome is the outermost, all-containing level. Every nucleated body cell holds a complete copy of the genome, so even a tiny sample (a hair root, a cheek swab) carries all of a person's DNA.
The nested scale of genetic information: a base sits inside a gene, a gene inside a chromosome, and all the chromosomes together make up the genome — the whole of an organism's DNA.
🔒 Interactive diagram
Explore the labelled diagram, charts and maps for this topic in study mode.
Key Idea: Most of the genome is almost identical between people, so the coding genes are no use for telling individuals apart. DNA profiling instead reads short stretches of non-coding DNA that are repeated (short tandem repeats). The number of repeats at each place varies a lot between individuals, so a profile is essentially unique — useful to identify a person or test how closely two people are related (more shared patterns = more closely related).
Genome size does not match organism complexity — some simpler organisms (e.g. a lungfish) have far larger genomes than humans. A bigger genome ≠ a more advanced organism.
✍️ Worked examples
IB-style question — explain the Meselson–Stahl banding
Bacteria grown in heavy ¹⁵N were switched to light ¹⁴N. After one round of replication the DNA formed a single band of intermediate density; after a second round it formed an intermediate band and a light band. Explain how these results show replication is semi-conservative. [2]
How to score both marks:
Read generation 1. A single intermediate band means every molecule is half-heavy, half-light — one old (¹⁵N) strand + one new (¹⁴N) strand. This rules out the conservative model.
Read generation 2. An intermediate + a light band means some molecules are now fully light, which rules out the dispersive model. Only semi-conservative replication fits both results.
Generation 1's intermediate band = one old + one new strand (rules out conservative); generation 2's light band = some fully light molecules (rules out dispersive). Only semi-conservative fits.
IB-style question — which bonds does each enzyme act on?
During DNA replication, state which type of bond helicase breaks and which type of bond DNA polymerase forms. [2]
Model answer:
Helicase breaks the hydrogen bonds between the paired bases, separating the two strands.
DNA polymerase forms the covalent bonds that join the nucleotides of the new strand (its sugar–phosphate backbone).
Helicase breaks hydrogen bonds; DNA polymerase forms covalent bonds.
IB-style question — explain the PCR temperatures
Explain why the temperature is changed at each of the three steps of a PCR cycle. [3]
How to score all three marks:
Denaturation (~95 °C). The high heat breaks the hydrogen bonds, separating the double helix into two single strands so each can be a template.
Annealing (~55 °C). Cooling lets the primers bind (anneal) to their complementary sequences, marking where copying starts.
Extension (~72 °C). This is Taq polymerase's optimum, so it can add nucleotides to the primer and build the new complementary strand.
~95 °C breaks hydrogen bonds to separate the strands; ~55 °C lets primers anneal; ~72 °C is Taq's optimum so it can extend the new strand.
IB-style question — name the genome and the right cell
(a) State the term for the whole of the genetic information of an organism. (b) Identify a type of cell that could supply a complete copy of it, and explain why. [3]
Model answer:
(a) Name the term. The genome — all of an organism's DNA, not a single gene or chromosome.
(b) Name a suitable cell. Any nucleated body cell (e.g. a white blood cell, skin cell or cheek cell).
Explain why. Every nucleated body cell holds a complete copy of the whole genome, so one such cell carries all the DNA. (A mature red blood cell has no nucleus, so it would not work.)
(a) The genome. (b) A nucleated body cell (e.g. a white blood cell) — because every nucleated body cell contains a complete copy of the whole genome.
✅ Quick self-check
Tap each card to check yourself.
What does 'semi-conservative' replication mean? Each new DNA molecule is made of one original (parental) strand and one newly built strand — half of each copy is conserved from the original.
Which band ruled out which model in Meselson–Stahl? Generation 1's single intermediate band ruled out the conservative model; generation 2's light band ruled out the dispersive model — leaving only semi-conservative.
Helicase vs DNA polymerase — jobs and bonds? Helicase unzips the helix and BREAKS hydrogen bonds; DNA polymerase builds the new strand by adding complementary nucleotides and FORMS covalent bonds (5'→3').
Why is the temperature changed at each PCR step? ~95 °C breaks hydrogen bonds to separate the strands; ~55 °C lets primers anneal; ~72 °C is Taq's optimum so it adds nucleotides and extends the new strand.
Why is heat-stable Taq used, and how does a gel separate fragments? Taq is thermostable, so it is not denatured at ~95 °C and works every cycle. A gel pulls negatively charged DNA towards the positive electrode and acts as a sieve — smaller fragments travel further.
What is the genome, and why can DNA profiling identify a person? The genome is all of an organism's DNA (base ⊂ gene ⊂ chromosome ⊂ genome). Profiling reads the variable non-coding repeat regions, whose repeat numbers differ between individuals, making each profile essentially unique.
🧠 The whole topic at a glance
If you remember nothing else, remember these
- Replication is semi-conservative: each new molecule = one old + one new strand
- Meselson–Stahl: gen-1 intermediate band kills conservative; gen-2 light band kills dispersive
- Helicase unzips → breaks hydrogen bonds; DNA polymerase builds → forms covalent bonds (5'→3')
- PCR amplifies DNA: 95 °C splits, 55 °C anneals primers, 72 °C Taq extends; DNA doubles each cycle
- Taq is used because it is heat-stable (not denatured at 95 °C)
- Gel electrophoresis sorts fragments by size — smaller travels further; no DNA = no band
- Genome = all the DNA (base ⊂ gene ⊂ chromosome ⊂ genome); every nucleated cell holds a full copy
- DNA profiling reads the variable non-coding repeats — more shared patterns = more closely related
Exam Tips
- For a 'state the conclusion' mark, name it: replication is semi-conservative (one old + one new strand per molecule).
- Data 'Explain': describe EACH band and link it to the model it rules out — don't just name the answer.
- Bonds are a classic trap — helicase BREAKS hydrogen bonds; DNA polymerase FORMS covalent bonds. Never swap them.
- 'State the role of DNA polymerase' = it ADDS complementary nucleotides / builds the new strand — naming the enzyme alone scores nothing.
- Explain the PCR temperatures with one distinct reason each: 95 °C splits, 55 °C anneals, 72 °C is Taq's optimum.
- Taq is suitable because it is heat-stable (not denatured at 95 °C) — not just because 'it copies DNA'.
- On a gel: smaller fragments travel further, and the no-DNA control is the lane with no band.
- 'The whole of an organism's genetic information' = the genome (one word). Any NUCLEATED cell holds a full copy — a red blood cell does not.
- Genome-size trap: a bigger genome does NOT mean a more complex organism — quote a number that breaks the pattern.
Key Idea: DNA is copied semi-conservatively — helicase unzips it (breaking hydrogen bonds) and DNA polymerase builds a new complementary strand (forming covalent bonds), so each daughter molecule keeps one old + one new strand, as Meselson and Stahl proved. Scientists copy DNA in the lab with PCR (three-step thermal cycle, heat-stable Taq) and separate the fragments by size with gel electrophoresis. All of a cell's DNA together is the genome; its variable repeat regions let DNA profiling tell individuals apart and test relatedness.