DNA replication — IB Diploma Biology HL
IB Biology D1.1: semi-conservative DNA replication, helicase and DNA polymerase, PCR and gel electrophoresis, DNA profiling, and HL replication enzymes.
Revision notes
6 short notes, in the order of the specification. Each one in short:
DNA replication is the production of exact copies of DNA with identical base sequences. It is semi-conservative: each new double helix contains one original strand and one new strand. Complementary base pairing (A with T, C with G) means each old strand acts as a template, so the base sequence is copied with a high degree of accuracy.
Helicase is the enzyme that unwinds the DNA double helix and breaks the hydrogen bonds between the two strands, separating them so each can act as a template. DNA polymerase then links free DNA nucleotides, matched to the template by complementary base pairing, into a new strand by forming covalent bonds in the sugar–phosphate backbone.
PCR and gel electrophoresis
PracticalThe polymerase chain reaction (PCR) amplifies a chosen DNA sequence. Cycles at about 95 °C, 55 °C and 72 °C separate the strands, bind primers and let heat-stable Taq polymerase build new strands, doubling the DNA each cycle. Gel electrophoresis then separates fragments by length, as negatively charged DNA moves towards the positive electrode.
DNA profiling is the main application of PCR and gel electrophoresis: regions of DNA that vary in length between people are amplified and separated, giving a pattern of bands. Profiles are compared in paternity tests and forensic investigations. Using more markers makes the test more reliable, because it reduces the probability that two unrelated people match by chance.
DNA polymerases only add the 5' end of a new nucleotide to the 3' end of a growing strand, so new DNA is built in the 5' to 3' direction. At a replication fork the leading strand is therefore made continuously, from one RNA primer, while the lagging strand is made discontinuously as Okazaki fragments, each needing its own primer.
In prokaryotic replication, DNA primase makes short RNA primers, DNA polymerase III adds DNA nucleotides to them, DNA polymerase I removes the primers and replaces them with DNA, and DNA ligase seals the remaining gaps in the sugar–phosphate backbone. DNA polymerase III also proofreads, removing a mismatched nucleotide from the 3' end and replacing it.
Exam questions
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Quick check questions
Why is DNA replication described as semi-conservative?
Show answer
Each new DNA molecule contains one strand from the original molecule and one newly synthesised strand.
What is the role of helicase?
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It unwinds the double helix and breaks hydrogen bonds between bases so the strands separate.
Why is Taq polymerase used in PCR?
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It comes from a hot-spring bacterium and is not denatured at 95 °C, so it survives the denaturation step of every cycle.
Why do smaller DNA fragments travel further in gel electrophoresis?
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They pass more easily through the mesh of the gel, so they move faster towards the positive electrode.
HL only Why are many RNA primers needed on the lagging strand?
Show answer
It is made discontinuously as Okazaki fragments, and each fragment must start from its own primer.
Frequently asked questions
Why is DNA replication semi-conservative?
DNA replication is semi-conservative because the two strands of the original molecule separate and each acts as a template for a new strand. Each daughter molecule therefore keeps one original strand and gains one new strand. Complementary base pairing makes the new strand match the old partner, so both copies have identical base sequences.
What are the three stages of PCR?
The three stages of PCR are denaturation, annealing and extension. At about 95 °C hydrogen bonds break and the strands separate; at about 55 °C primers bind to the ends of the target sequence; at about 72 °C heat-stable Taq polymerase builds new strands. Each cycle doubles the number of copies.
How does gel electrophoresis separate DNA fragments?
Gel electrophoresis separates DNA fragments by length. DNA is negatively charged because of its phosphate groups, so when a voltage is applied it moves through the gel towards the positive electrode. Smaller fragments move through the gel mesh more easily, so they travel further than larger fragments in the same time.
How is DNA profiling used in paternity tests?
In a paternity test, DNA profiles of the child, mother and possible father are compared. A child inherits one copy of each STR region from each parent, so every band in the child's profile must match a band in the mother or the father. A man lacking the child's non-maternal bands is excluded.
What is the difference between the leading and lagging strand?
HL only The leading strand is replicated continuously in the same direction as the replication fork moves, needing only one RNA primer. The lagging strand is replicated discontinuously, away from the fork, as short Okazaki fragments that each need a primer and are later joined by DNA ligase. Both are built 5' to 3'.
Written and checked against the IB Biology HL specification · Updated October 2026