Difficulty: Intermediate | Prerequisites: Basic DNA structure, base pairing rules
DNA replication is the process by which a cell copies its entire genome before dividing, so each daughter cell gets a complete set of genetic instructions. The process is semiconservative: each new double helix keeps one original strand and pairs it with a freshly built one. In bacteria this starts at a single origin; in eukaryotes it fires from thousands of origins at once to get the job done in time.
Semiconservative replication
The model of DNA replication in which each daughter molecule retains one parental strand and one newly synthesised strand. Confirmed experimentally by Meselson and Stahl (1958) using density-gradient centrifugation with heavy nitrogen (15N).
Think of it as unzipping a jacket and sewing a new half onto each side, so both new jackets still have one original half.
Origin of replication (ori)
A specific DNA sequence where replication begins. Bacteria typically have a single origin on their circular chromosome; human cells have roughly 10,000 to 100,000 origins spread across their linear chromosomes.
In simple terms, this is the "start here" signal on the DNA.
Replication fork
The Y-shaped region of DNA that forms when the double helix unwinds during replication. Two forks travel in opposite directions from each origin (bidirectional replication).
Picture a zip being pulled open from the middle: the point where the teeth separate is the fork.
Leading strand
The new DNA strand synthesised continuously in the 5'-to-3' direction, moving toward the replication fork.
In simple terms, this strand is built in one smooth run because its direction of synthesis matches the direction the fork is moving.
Lagging strand
The new DNA strand synthesised discontinuously, in short pieces called Okazaki fragments, because it runs in the opposite orientation to the fork's movement.
Think of it as having to keep starting over in short bursts because you can only build in one direction while the fork moves the other way.
Okazaki fragments
Short stretches of newly synthesised DNA on the lagging strand (roughly 1,000 to 2,000 nucleotides in bacteria, 100 to 200 in eukaryotes). They are later joined together by DNA ligase.
DNA polymerase III (bacteria)
The primary enzyme responsible for synthesising new DNA strands during bacterial replication. It adds nucleotides in the 5'-to-3' direction and has 3'-to-5' exonuclease (proofreading) activity.
Primase
An RNA polymerase that synthesises short RNA primers on the template strand. DNA polymerase cannot start a new strand from scratch; it needs these primers as a starting point.
DNA ligase
The enzyme that seals the nicks (breaks in the sugar-phosphate backbone) between Okazaki fragments, producing a continuous lagging strand.
Think of it as the glue that joins the short pieces into one unbroken strand.
Helicase
An enzyme that unwinds the double helix at the replication fork by breaking the hydrogen bonds between complementary base pairs.
Single-stranded DNA-binding proteins (SSBs)
Proteins that coat and stabilise the separated single strands of DNA, preventing them from re-annealing or being degraded before replication is complete.
Replicon
A unit of DNA that replicates from a single origin. The bacterial chromosome is one replicon; a eukaryotic chromosome contains many replicons that fire at different times during S phase.
DNA polymerase I (bacteria)
Removes RNA primers and replaces them with DNA. Also possesses 3'-to-5' exonuclease activity for proofreading.
Polymerase α (eukaryotes)
Initiates DNA synthesis at each origin and at each Okazaki fragment by laying down a short RNA primer followed by a short stretch of DNA. Primase activity is part of the Pol α complex.
Polymerase δ (eukaryotes)
The main lagging-strand polymerase in eukaryotic cells.
Polymerase ε (eukaryotes)
The main leading-strand polymerase in eukaryotic cells.
Polymerase γ (eukaryotes)
Replicates mitochondrial DNA (mtDNA). It operates independently of the nuclear replication machinery.
Watson and Crick proposed that replication is semiconservative: each daughter molecule keeps one old strand and builds one new one.
Meselson and Stahl confirmed this in 1958 by growing bacteria in heavy nitrogen (15N), then shifting them to light nitrogen (14N) and tracking DNA density over generations.
The alternative models (conservative and dispersive) were ruled out by the density patterns observed.
Replication begins at a single origin of replication (oriC in E. coli).
Initiator proteins (DnaA in bacteria) recognise and bind the origin, causing local unwinding of the double helix.
Helicase loads onto the single-stranded DNA and continues unwinding.
Single-stranded DNA-binding proteins (SSBs) stabilise the exposed strands.
Replication proceeds bidirectionally, creating two replication forks that travel in opposite directions around the circular chromosome.
Leading strand: synthesised continuously in the 5'-to-3' direction by DNA Polymerase III, following the fork.
Lagging strand: synthesised discontinuously as Okazaki fragments (roughly 1,000 to 2,000 nucleotides each).
Primase lays down a short RNA primer for each fragment.
DNA Polymerase III extends from the primer.
Once the next primer is reached, the fragment is complete.
DNA Polymerase III also has 3'-to-5' exonuclease activity, allowing it to proofread as it goes.
Replication ends when the two forks meet on the opposite side of the circular chromosome.
DNA Polymerase I removes the RNA primers and fills the gaps with DNA.
DNA ligase seals the remaining nicks, producing two continuous daughter molecules.
Eukaryotic chromosomes are linear and far longer than bacterial ones, so replication uses multiple origins of replication (10,000 to 100,000 in human cells).
Each stretch of DNA replicated from a single origin is called a replicon.
Licensing controls ensure each replicon fires only once per cell cycle, preventing over-replication.
Synthesis is semidiscontinuous, just as in bacteria: one leading strand (continuous) and one lagging strand (Okazaki fragments, shorter at roughly 100 to 200 nucleotides).
Polymerase α (Pol α): associated with primase; lays down the RNA primer plus a short DNA extension at each origin and each Okazaki fragment.
Polymerase ε (Pol ε): takes over on the leading strand and synthesises continuously.
Polymerase δ (Pol δ): takes over on the lagging strand and extends each Okazaki fragment.
Polymerase γ (Pol γ): replicates mitochondrial DNA independently of the nuclear machinery.
Pol ε and Pol δ both carry 3'-to-5' exonuclease activity for proofreading.
Combined with post-replication mismatch repair (covered in the DNA Repair notes), the overall error rate drops to roughly one mistake per billion base pairs copied.
"The lagging strand is synthesised 3'-to-5'." It is not. All DNA polymerases synthesise in the 5'-to-3' direction. The lagging strand simply has to be made in short fragments because its template runs in the opposite orientation relative to the fork's movement.
"Primase makes DNA primers." Primase makes RNA primers. They are later removed and replaced with DNA by Polymerase I (bacteria) or other repair polymerases (eukaryotes).
"Helicase breaks covalent bonds to unwind DNA." Helicase breaks hydrogen bonds between complementary bases. The covalent backbone of each strand stays intact.
"Eukaryotic and bacterial replication use the same polymerases." They do not. Bacteria rely on Pol I and Pol III; eukaryotes use Pol α, δ, ε for nuclear DNA and Pol γ for mitochondrial DNA. The names and roles differ.
⚠️ The Meselson-Stahl experiment is a classic exam question. Know the logic: after one round of replication in light nitrogen, all DNA was intermediate density (one heavy strand, one light strand), ruling out conservative replication. After two rounds, half was intermediate and half was light, ruling out dispersive replication.
⚠️ You will almost certainly be asked to distinguish leading and lagging strand synthesis. The key: both are synthesised 5'-to-3', but only the lagging strand requires repeated priming and produces Okazaki fragments.
⚠️ Know which enzyme does what. A common exam format: "Which enzyme seals nicks between Okazaki fragments?" (ligase), "Which enzyme removes RNA primers in bacteria?" (Pol I), "Which enzyme is the main replicative polymerase in bacteria?" (Pol III).
⚠️ Eukaryotic replication origins and licensing are frequently tested. The concept that each replicon fires only once per S phase prevents the genome from being copied twice, which would be catastrophic.
Understanding replication machinery is the basis for many laboratory techniques. PCR (polymerase chain reaction), the workhorse of molecular biology and forensic science, is essentially controlled DNA replication in a test tube using a heat-stable polymerase. Antiviral drugs such as acyclovir work by mimicking nucleotides and jamming viral DNA polymerases, halting replication of the virus without affecting the host's polymerases as severely.
True or false: In semiconservative replication, both strands of the daughter molecule are newly synthesised. (False: one strand is parental, one is new.)
Fill in the blank: The short RNA sequences laid down by primase are called ______. (RNA primers.)
True or false: The leading strand is synthesised as Okazaki fragments. (False: that is the lagging strand.)
Fill in the blank: In eukaryotes, the leading strand polymerase is ______. (Polymerase ε.)
True or false: Bacteria have multiple origins of replication on their chromosome. (False: bacteria typically have a single origin, oriC.)
Q: Describe the Meselson-Stahl experiment and explain how its results supported the semiconservative model of DNA replication.
A: Meselson and Stahl grew E. coli in medium containing heavy nitrogen (15N) until all DNA was fully labelled. They then transferred the bacteria to light nitrogen (14N) medium and sampled DNA after each generation. After one generation, all DNA banded at an intermediate density, consistent with each molecule having one heavy and one light strand. After two generations, half the DNA was intermediate and half was light. This pattern matched only the semiconservative prediction and ruled out conservative (which would have produced one heavy and one light band after generation one) and dispersive (which would never produce a purely light band) models.
Q: Why is the lagging strand synthesised discontinuously while the leading strand is synthesised continuously?
A: All DNA polymerases add nucleotides only in the 5'-to-3' direction. The leading strand template runs 3'-to-5' toward the fork, so synthesis can follow the fork continuously. The lagging strand template runs 5'-to-3' toward the fork, so the polymerase must work away from the fork. As the fork opens up new template, the polymerase has to jump back and start a new fragment (an Okazaki fragment), each time requiring a fresh RNA primer from primase.
Q: Name the eukaryotic DNA polymerases involved in nuclear replication and state the role of each.
A: Polymerase α (with primase) initiates synthesis by laying down an RNA primer and a short DNA extension. Polymerase ε takes over on the leading strand for continuous synthesis. Polymerase δ takes over on the lagging strand, extending each Okazaki fragment. (Polymerase γ handles mitochondrial DNA separately.)
Q: What role does DNA ligase play in replication, and on which strand is its activity most critical?
A: DNA ligase seals the phosphodiester bond between adjacent Okazaki fragments after the RNA primers have been removed and replaced with DNA. Its activity is most critical on the lagging strand, where many such nicks exist. The leading strand, being synthesised continuously, has far fewer nicks to seal.
Q: Explain why eukaryotic cells require multiple origins of replication.
A: Eukaryotic genomes are vastly larger than bacterial genomes (billions of base pairs versus a few million). If replication started from a single origin, it would take far too long to copy the entire genome before cell division. Multiple origins (10,000 to 100,000 in humans) allow many replicons to fire simultaneously, completing replication within the time frame of S phase.
This material connects directly to DNA repair mechanisms (see companion notes). Errors that escape proofreading during replication are caught by mismatch repair, and the replication fork itself can stall at sites of DNA damage, triggering repair pathways.
Replication also links to the cell cycle: S phase is when DNA synthesis occurs, and checkpoints ensure replication is complete before the cell enters mitosis. Understanding replication is a prerequisite for topics in gene expression, mutation, and cancer biology.
DNA replication, semiconservative replication, Meselson-Stahl experiment, origin of replication, oriC, replication fork, leading strand, lagging strand, Okazaki fragments, DNA polymerase III, DNA polymerase I, primase, DNA ligase, helicase, SSB proteins, single-stranded binding proteins, replicon, bidirectional replication, 5'-to-3' synthesis, proofreading, 3'-to-5' exonuclease, Pol alpha, Pol delta, Pol epsilon, Pol gamma, mitochondrial DNA replication, S phase, cell cycle, PCR, polymerase chain reaction