DNA makes copies of itself with astonishing accuracy — and the machinery to do it has been working since LUCA.
Every sub-topic below feeds at least one of these questions.
How is new DNA produced?
How has knowledge of DNA replication enabled applications in biotechnology?
The required syllabus content for D1.1, in order. Each card is one lesson-sized checkpoint.
DNA replication as production of exact copies of DNA with identical base sequences
Semi-conservative nature of DNA replication and role of complementary base pairing
Role of helicase and DNA polymerase in DNA replication
Polymerase chain reaction and gel electrophoresis as tools for amplifying and separating DNA
Note: The sequence of subtopics have been changed for the SL section of this topic, to improve clarity.
Before any cell divides — for growth, tissue replacement, or reproduction — it must first copy its DNA precisely. Each daughter cell needs the full genome.
DNA replication produces two daughter molecules with sequences identical to the original. This precision is what allows genetic information to pass faithfully from cell to cell, generation to generation.
When does it happen? Before mitosis (for growth and tissue replacement) and before meiosis (for sexual reproduction). The S phase of the cell cycle.
Each daughter helix has one old strand and one new — proven by Meselson & Stahl in 1958. Each strand serves as the template for its partner.
Result: two identical double helices, each with one parent strand and one new strand. Accuracy comes from the obligate complementary base pairing — no other combinations fit.
PCR amplifies a chosen DNA sequence in vitro, using cycles of heating and cooling. The single technique that powers forensics, diagnostics, and most molecular biology.
Normal DNA polymerases denature at 95 °C. The whole technique would be impractical if you had to add fresh enzyme every cycle. Taq polymerase from Thermus aquaticus (a bacterium that lives in Yellowstone's hot springs) is heat-stable up to ~98 °C. One addition lasts all 30 cycles. The breakthrough that made PCR practical.
DNA fragments cut by restriction enzymes can be separated by length using an electric field through an agarose gel.
PCR + gel electrophoresis enables DNA profiling, used in forensics, paternity testing, evolution studies, disease diagnosis, and more.
Tiny DNA samples from a crime scene (hair, blood, skin) are amplified by PCR, then profiled by gel electrophoresis. The pattern is compared with profiles from suspects. All bands must match for a confident identification.
A child shares half of its DNA with each parent. Every band in the child's profile must appear in either the mother's or the father's profile. If they do, parentage is confirmed.
The reliability of a DNA profile depends on the number of markers used. More markers → smaller probability of a chance match → fewer false identifications. Modern forensic profiling uses 13+ markers.
An extra 4 sub-topics for HL — same syllabus, deeper mechanism.
Directionality of DNA polymerases
Differences between replication on the leading strand and the lagging strand
Functions of DNA primase, DNA polymerase I, DNA polymerase III and DNA ligase in replication
DNA proofreading
DNA polymerases add new nucleotides only to the 3' end of an existing strand — they synthesise 5' → 3'. This constraint shapes everything about how replication works.
Each nucleotide's 5' phosphate attaches to the previous nucleotide's 3' hydroxyl — so the strand grows from the 5' end outward. Since the two parent strands are antiparallel (5'→3' one way; 3'→5' the other), the new strands are also antiparallel — and they get synthesised differently on the two sides of the replication fork.
Because polymerase only adds in the 5' → 3' direction, one new strand can be made continuously and the other must be made in pieces.
Synthesised in the same direction as the replication fork moves. Polymerase keeps adding nucleotides without interruption. Just one RNA primer needed at the start.
Synthesised away from the fork — only short stretches can be made before having to restart. Each stretch is an Okazaki fragment. Multiple primers are required — one for each fragment. Fragments are later joined by DNA ligase.
Prokaryotic DNA replication is the IB's reference case. Five enzymes do the job.
| Enzyme | Function |
|---|---|
| Helicase | Unwinds the double helix at the replication fork by breaking H-bonds. |
| Gyrase (topoisomerase) | Relieves the supercoiling strain that builds up ahead of the fork. |
| DNA primase | Synthesises short RNA primers to initiate replication. |
| DNA polymerase III | Adds DNA nucleotides to the 3' end of each primer (5' → 3' direction). Main polymerase. |
| DNA polymerase I | Removes RNA primers and replaces them with DNA nucleotides. |
| DNA ligase | Joins Okazaki fragments on the lagging strand. |
DNA polymerase III doesn't just add nucleotides — it also reads back what it just added, and corrects mistakes on the spot.
After adding each nucleotide, DNA polymerase III checks for correct base pairing. If a mismatch is detected at the 3' terminal, the polymerase backs up, removes the mis-matched nucleotide, and replaces it with the correct one before resuming. This proofreading lowers the error rate from about 1 in 10⁴ to about 1 in 10⁶ — roughly 100× more accurate. Additional repair pathways after replication push the final error rate to about 1 in 10⁹.
If you can't define one of these in a sentence, that's where to revise next.
“How is genetic continuity ensured between generations?”
“What biological mechanisms rely on directionality?”