Source: Lecture notes, textbook chapters on DNA replication
Tags: DNA replication, replication fork, helicase, primase, DNA polymerase, leading strand, lagging strand, Okazaki fragments, ligase, proofreading, BIO 311C
Difficulty: Intermediate | Prerequisites: DNA structure and base pairing (see companion notes), basic enzyme function
Once you understand what DNA looks like, the next question is: how does the cell make a copy of it before dividing? This topic covers the molecular machinery of DNA replication, from the origin of replication through to the final sealed strand. You need to know which enzymes do what, why the two strands are synthesised differently, and how the cell catches and fixes copying errors. This material builds directly on DNA structure and base pairing, so review those first if they are not solid.
DNA replication starts at origins of replication, where helicase unwinds the double helix and a team of enzymes builds two new strands. The leading strand is synthesised continuously toward the fork; the lagging strand is synthesised in short Okazaki fragments, stitched together by DNA ligase. DNA polymerase proofreads as it goes, and separate repair mechanisms catch anything it misses.
Origin of replication (ORI)
The specific DNA sequence where replication begins. Prokaryotes have one origin; eukaryotes have many, allowing faster copying of their larger genomes.
Replication bubble
The region where the two parental DNA strands have separated and replication is actively occurring. Each bubble has two replication forks moving in opposite directions.
Replication fork
The Y-shaped region at the edge of a replication bubble where the parental DNA is being unwound and new strands are being synthesised. Think of it as the active construction site.
Helicase
The enzyme that unwinds and separates the two DNA strands by breaking the hydrogen bonds between base pairs. In simple terms, it "unzips" the double helix.
Single-strand binding proteins (SSBs)
Proteins that bind to the separated single strands of DNA to keep them from re-annealing (snapping back together) before they can be copied.
Topoisomerase
An enzyme that relieves the torsional strain (overwinding) ahead of the replication fork by cutting, swivelling, and rejoining the DNA strands. Without it, the unwinding would cause the DNA ahead to coil up too tightly.
Primase
An RNA polymerase that synthesises a short RNA primer (approximately 5 to 10 nucleotides) on the template strand. This primer provides the free 3' OH end that DNA polymerase needs to start adding nucleotides. In simple terms, it lays down the starting block.
RNA primer
A short stretch of RNA nucleotides laid down by primase. DNA polymerase cannot start a new strand from scratch; it can only add to an existing 3' end, so the primer gives it something to build on.
DNA polymerase III (in bacteria)
The main replication enzyme. It reads the template strand and adds complementary DNA nucleotides to the 3' end of the growing strand, synthesising in the 5' to 3' direction. It also proofreads, using 3' to 5' exonuclease activity to remove mismatched bases.
DNA polymerase I (in bacteria)
Removes the RNA primers and replaces them with DNA nucleotides. Think of it as the clean-up crew that swaps out the temporary RNA for permanent DNA.
Leading strand
The new DNA strand that is synthesised continuously in the 5' to 3' direction, moving toward the replication fork. It needs only one primer to get started.
Lagging strand
The new DNA strand that is synthesised discontinuously, in short segments called Okazaki fragments, moving away from the replication fork. It needs a new primer for each fragment.
Okazaki fragments
The short DNA segments (about 1,000 to 2,000 nucleotides in prokaryotes) that make up the lagging strand before they are joined together. Named after Reiji Okazaki.
DNA ligase
The enzyme that seals the gaps between Okazaki fragments by forming phosphodiester bonds, joining them into a continuous strand. Think of it as molecular glue.
Proofreading
The ability of DNA polymerase to detect a mismatched base pair immediately after adding it, remove the incorrect nucleotide using 3' to 5' exonuclease activity, and replace it with the correct one.
Mismatch repair
A post-replication repair system that scans newly made DNA for mismatched bases that proofreading missed, removes the incorrect section, and fills in the correct nucleotides.
Nucleotide excision repair
A repair mechanism in which a nuclease enzyme cuts out a stretch of damaged DNA (caused by chemicals, UV light, or spontaneous changes) and DNA polymerase and ligase fill the gap with correct nucleotides.
Replication begins at specific sequences called origins of replication (ORI).
Replication proceeds bidirectionally from each origin, with two replication forks moving in opposite directions until the entire molecule is copied.
Prokaryotes have a single, circular chromosome with one origin of replication and one replication bubble.
Eukaryotes have much larger, linear chromosomes with multiple origins of replication. Many replication bubbles open simultaneously, which is how eukaryotic cells replicate their massive genomes quickly enough.
The replication fork is a coordinated assembly line. Each enzyme has a specific job:
Helicase unwinds the double helix ahead of the fork by breaking hydrogen bonds between base pairs.
Single-strand binding proteins (SSBs) coat the exposed single strands to prevent them from re-pairing or being degraded.
Topoisomerase works ahead of helicase to relieve the torsional stress caused by unwinding. It does this by cutting, swivelling, and rejoining the DNA backbone.
Primase synthesises short RNA primers (approximately 5 to 10 nucleotides) on the template strand. These primers provide the free 3' OH group that DNA polymerase requires to begin adding nucleotides.
DNA polymerase III (in bacteria) is the primary replication enzyme. It reads the template 3' to 5' and synthesises the new strand 5' to 3', adding complementary nucleotides to the 3' end of the growing chain via a dehydration reaction.
DNA polymerase I (in bacteria) removes RNA primers and replaces them with DNA nucleotides.
DNA ligase seals the remaining nicks (gaps in the sugar-phosphate backbone) to produce a continuous strand.
The energy for adding each nucleotide comes from the incoming nucleoside triphosphate itself: two phosphate groups are cleaved off (released as pyrophosphate), and the energy from that hydrolysis drives the formation of the new phosphodiester bond.
Because DNA polymerase can only synthesise in the 5' to 3' direction, and the two template strands run antiparallel, the two new strands are made differently.
The template for the leading strand runs 3' to 5', so the new strand is built 5' to 3' in the same direction the replication fork is moving.
Only one RNA primer is needed.
DNA polymerase III binds and synthesises continuously toward the fork.
The leading strand always moves toward the fork and away from the origin of replication.
The template for the lagging strand runs 5' to 3', so the new strand must be built in the direction away from the fork.
As the fork opens up, primase repeatedly lays down new RNA primers on the exposed template.
DNA polymerase III synthesises a short stretch of DNA from each primer, producing an Okazaki fragment, then detaches.
The lagging strand always moves away from the fork and toward the origin of replication.
Primase synthesises an RNA primer on the lagging strand template.
DNA polymerase III extends from the primer, synthesising Okazaki fragment 1.
DNA polymerase III detaches when it reaches the previous primer.
A new primer is laid down further along the newly exposed template, and fragment 2 is synthesised.
DNA polymerase I removes each RNA primer and replaces it with DNA nucleotides.
DNA ligase seals the gaps between adjacent Okazaki fragments, forming a continuous strand.
The lagging strand is complete.
Leading strand synthesis: moves toward the fork, away from the ORI.
Lagging strand synthesis: moves away from the fork, toward the ORI.
Whether a strand is "leading" or "lagging" is a matter of perspective relative to the origin. At a given replication bubble, the same physical strand is leading on one side and lagging on the other.
DNA replication is remarkably accurate, but errors still occur. The cell has multiple layers of defence.
As DNA polymerase adds each nucleotide, it checks whether the base pair is correct.
If a mismatch is detected, the polymerase uses its 3' to 5' exonuclease activity to remove the incorrect nucleotide.
It then inserts the correct nucleotide and continues.
Specialised enzymes scan newly replicated DNA for any mismatches that proofreading missed.
The incorrect base is removed and replaced with the correct one.
DNA can be damaged by exposure to harmful chemicals, UV radiation, or spontaneous chemical changes.
A nuclease enzyme recognises the distortion in the helix, cuts out the damaged stretch of nucleotides, and the gap is filled in by DNA polymerase and sealed by ligase.
The error rate after proofreading and repair is very low, but not zero.
Sequence changes that escape repair may become permanent mutations, passed on to the next generation.
These mutations are the raw material of genetic variation, the variation upon which natural selection acts.
Without occasional replication errors, evolution by natural selection could not occur.
Students often think the leading strand does not need a primer. It does. Every new DNA strand, leading or lagging, requires at least one RNA primer to provide the 3' OH that DNA polymerase needs. The difference is that the leading strand needs only one primer, while the lagging strand needs many.
Students frequently confuse the direction of synthesis with the direction of reading. DNA polymerase reads the template 3' to 5' but synthesises the new strand 5' to 3'. These are two descriptions of the same process from different perspectives.
A common error is thinking that if primase is mutated, only the lagging strand would be affected. Both strands require primase. A non-functional primase would halt replication entirely.
Students sometimes think topoisomerase unwinds DNA. It does not. Helicase unwinds the helix. Topoisomerase relieves the strain caused by that unwinding.
⚠️ Know the function of every enzyme at the replication fork: helicase, topoisomerase, SSBs, primase, DNA polymerase III, DNA polymerase I, and ligase. Exam questions frequently describe a scenario ("enzyme X is mutated") and ask you to predict the consequence.
⚠️ Be able to explain why the lagging strand must be synthesised in Okazaki fragments. The answer traces back to the antiparallel structure of DNA and the 5' to 3' constraint on polymerase.
⚠️ The primase mutation question is a known exam favourite: if primase is non-functional, replication cannot begin on either strand, because DNA polymerase cannot start without a primer.
⚠️ Understand the three layers of error correction: proofreading (during replication), mismatch repair (after replication), and nucleotide excision repair (for external damage).
⚠️ Know the difference between prokaryotic and eukaryotic replication origins: one bubble vs many.
True or false: DNA polymerase can start a new strand from scratch without a primer. (False. DNA polymerase can only add nucleotides to an existing 3' OH end, which is provided by an RNA primer laid down by primase.)
Fill in the blank: The enzyme that seals the gaps between Okazaki fragments is ______. (DNA ligase)
True or false: Eukaryotic chromosomes have only one origin of replication. (False. Eukaryotes have multiple origins of replication to allow their larger genomes to be copied quickly.)
Fill in the blank: DNA polymerase proofreads using ______ to ______ exonuclease activity. (3' to 5')
True or false: The leading strand requires multiple RNA primers. (False. The leading strand requires only one primer; the lagging strand requires multiple primers.)
Q: List the enzymes involved at the replication fork and state the function of each.
A: Helicase unwinds the double helix. Topoisomerase relieves torsional strain ahead of the fork. Single-strand binding proteins stabilise the separated strands. Primase synthesises RNA primers. DNA polymerase III synthesises new DNA in the 5' to 3' direction. DNA polymerase I removes RNA primers and replaces them with DNA. DNA ligase seals the gaps between Okazaki fragments.
Q: Explain why the lagging strand must be synthesised as Okazaki fragments.
A: DNA polymerase can only synthesise in the 5' to 3' direction. The lagging strand template runs 5' to 3', so the new strand must be built 3' to 5' overall, which is the direction away from the fork. As the fork opens, new template is exposed behind the polymerase, so it must repeatedly return to the fork, lay a new primer, and synthesise another short fragment. These fragments are later joined by ligase.
Q: If a mutation in the primase gene rendered primase non-functional, what would happen to DNA replication?
A: Replication would not occur on either the leading or the lagging strand. Both strands require an RNA primer to provide the 3' OH end that DNA polymerase needs to begin synthesis. Without primase, no primers are made, and DNA polymerase cannot start.
Q: What is the difference between proofreading and mismatch repair?
A: Proofreading happens during replication: DNA polymerase detects a mismatched nucleotide immediately after inserting it and uses its 3' to 5' exonuclease activity to remove and replace it. Mismatch repair happens after replication: separate enzymes scan the completed daughter strand for mismatches that proofreading missed, excise the incorrect base, and fill in the correct one.
Q: Why is the low but non-zero error rate of DNA replication biologically important?
A: Occasional uncorrected errors (mutations) become permanent changes in the DNA sequence. These mutations are the source of genetic variation in a population, and genetic variation is the raw material for natural selection and evolution.
DNA replication connects backward to DNA structure (the antiparallel double helix determines how replication works) and forward to cell division (mitosis and meiosis both depend on accurate replication). The repair mechanisms covered here also link to cancer biology: when DNA repair systems fail, mutation rates rise, increasing the chance of uncontrolled cell growth. The concept of mutations as a source of genetic variation connects to natural selection and evolution, topics you will encounter later in the course.
DNA replication, origin of replication, ORI, replication bubble, replication fork, helicase, unwind, topoisomerase, single-strand binding proteins, SSB, primase, RNA primer, DNA polymerase I, DNA polymerase III, leading strand, lagging strand, continuous synthesis, discontinuous synthesis, Okazaki fragments, DNA ligase, 5' to 3' direction, 3' to 5' exonuclease, proofreading, mismatch repair, nucleotide excision repair, semi-conservative replication, dehydration reaction, phosphodiester bond, pyrophosphate, antiparallel elongation, prokaryotic replication, eukaryotic replication, mutation, genetic variation, natural selection, BIO 311C, intro biology, University of Texas