Source: Chapters 8–9, University of Central Florida
Tags: DNA replication, DNA polymerase, replication fork, sliding clamp, helicase, primase, SSBs, Okazaki fragments, leading strand, lagging strand, proofreading, processivity, telomerase, origin of replication, replisome, E. coli replication, eukaryotic replication, DnaA, DnaB, DnaC, pre-RC, Cdk, topoisomerase
Difficulty: Advanced Prerequisites: Parts 1–3 of this set, understanding of DNA structure (5' to 3' directionality, phosphodiester bonds, base pairing).
This section covers the full machinery and logic of DNA replication, from the chemistry of nucleotide addition to the coordination of dozens of proteins at the replication fork, to how replication is initiated and terminated in both prokaryotes and eukaryotes. You need to understand directionality (5' to 3' synthesis, 3' to 5' reading), the roles of each enzyme, how leading and lagging strands differ, how errors are caught, and how the ends of linear chromosomes are maintained. This is one of the most detail-heavy sections of the course. If you do not have a solid grasp of DNA polarity, go back and review that first.
DNA polymerase synthesises new DNA in the 5' to 3' direction by adding nucleotides to a pre-existing RNA primer. It reads the template 3' to 5'. The replication fork requires a coordinated team: helicase unwinds DNA, primase lays RNA primers, SSBs stabilise single strands, the sliding clamp boosts processivity, and topoisomerase relieves tension. Leading strand synthesis is continuous; lagging strand synthesis is discontinuous (Okazaki fragments). Proofreading occurs via a 3' to 5' exonuclease domain. Telomerase solves the end-replication problem on linear chromosomes.
DNA polymerase
The enzyme that synthesises new DNA in the 5' to 3' direction. It cannot initiate synthesis on its own and requires an RNA primer. It uses dNTPs (dATP, dGTP, dTTP, dCTP) as substrates. DNA polymerase cannot identify the exact nucleotides that enter its active site; instead, it monitors the geometry of the base pair.
3'-OH
The hydroxyl group on the 3' carbon of the growing strand that acts as the nucleophile in DNA synthesis, attacking the alpha-phosphate of the incoming nucleotide.
Alpha-phosphate
The innermost phosphate of the incoming dNTP. The 3'-OH of the growing strand attacks this phosphate, forming the new phosphodiester bond and releasing pyrophosphate (the beta and gamma phosphates).
Template strand
The strand of DNA that is read by DNA polymerase. It is read in the 3' to 5' direction. The new strand is synthesised in the 5' to 3' direction. (Note: the template strand is read, not synthesised.)
RNA primer
A short RNA sequence laid down by primase that provides the 3'-OH needed for DNA polymerase to begin synthesis. DNA polymerase cannot start a new strand without it.
Primase
The enzyme that synthesises RNA primers. In E. coli, primase is recruited to the leading strand by DnaB (helicase).
Helicase
An enzyme that unwinds double-stranded DNA ahead of the replication fork using ATP-driven conformational changes. It is a hexamer (trimer of dimers or dimer of trimers). The mechanism of movement: conformational changes cause subunits to act like alternating hands pulling along the DNA.
Single-stranded binding proteins (SSBs)
Proteins that stabilise single-stranded DNA after helicase unwinds it. They interact with DNA through electrostatic interactions with the phosphate backbone and base stacking with the bases.
Topoisomerase
Relieves torsional stress ahead of the replication fork. Topoisomerase decreases (not increases) the linking number ahead of the fork. Topo II separates catenated (interlinked) daughter DNA molecules after replication.
Sliding clamp
A ring-shaped protein that encircles DNA and tethers DNA polymerase to the template, dramatically increasing processivity. It is released from DNA when it encounters single-stranded DNA (ssDNA). Loaded by the sliding clamp loader.
Sliding clamp loader
Uses ATP to open the sliding clamp and load it onto the primer-template junction. The tau subunit of the clamp loader interacts with helicase in E. coli.
Processivity
The number of nucleotides a polymerase adds before dissociating from the template. Higher processivity means faster, more efficient replication. The sliding clamp is the key factor that maximises processivity.
Palm domain
The catalytic domain of DNA polymerase. Contains the active site for DNA synthesis, binds two divalent metal cations (Mg²⁺), consists of a beta-sheet, and makes hydrogen bond interactions with the minor groove to monitor base pair geometry.
Thumb domain
Helps grip the DNA and maintain contact with the newly synthesised strand.
90-degree bend
A bend in the template DNA at the active site of DNA polymerase. It ensures only a single template base is present in the active site, reducing misincorporation. It also routes the template past the 3' to 5' exonuclease domain for proofreading.
3' to 5' exonuclease domain
The proofreading domain of DNA polymerase. Removes incorrectly incorporated nucleotides from the 3' end of the growing strand. Not all DNA polymerases have this domain.
Discriminator amino acids
Amino acids in the active site of DNA polymerase that prevent misincorporation of rNTPs (ribonucleotides) into the growing DNA strand.
Azidothymidine (AZT)
A thymine analogue used as an anticancer and antiviral drug. Once incorporated, it lacks a 3'-OH, so it terminates DNA synthesis because no further nucleophilic attack can occur. (It is a chain terminator.)
Okazaki fragments
Short DNA fragments synthesised on the lagging strand. Each requires its own RNA primer, sliding clamp, and is later joined by DNA ligase.
DNA ligase
Joins Okazaki fragments by sealing the phosphodiester backbone after RNA primers have been removed and gaps filled.
RNase H
An enzyme that removes RNA primers from Okazaki fragments. Works in conjunction with DNA polymerase (which fills the resulting gap).
Replisome
The multi-protein complex at the replication fork containing helicase, primase, DNA polymerase, sliding clamp, and clamp loader. Having all enzymes in one complex allows DNA to be synthesised in only the 5' to 3' direction on both strands simultaneously.
Holoenzyme
The complete, functional DNA polymerase complex (core polymerase plus sliding clamp and clamp loader). In the traditional view, a single holoenzyme replicates both the leading and lagging strands.
Replication factory model
A model in which the Watson and Crick strands of dsDNA are replicated simultaneously by the same holoenzyme, with the lagging strand looped through the complex (trombone model).
DnaA
The initiator protein in E. coli that recognises and binds the origin of replication (oriC), melting the DNA to begin replication.
DnaB
Helicase in E. coli. Loaded onto DNA by DnaC. Recruits primase on the leading strand.
DnaC
A helicase loader in E. coli that places DnaB onto single-stranded DNA at the origin.
Pre-replicative complex (pre-RC)
The protein assembly that marks a licensed origin of replication in eukaryotes. Formed during G1 when Cdk levels are low. Activated during S phase when Cdk levels rise.
Cdk (cyclin-dependent kinase)
A kinase whose activity regulates cell cycle progression. Low Cdk levels in G1 allow pre-RC formation. High Cdk levels in S phase activate pre-RCs and prevent new ones from forming, ensuring each origin fires only once.
Telomerase
A reverse transcriptase that extends the ends of linear chromosomes using a built-in RNA template. Expressed in germ cells and certain somatic cells (immune cells), not in all cells. Solves the end-replication problem.
The 3'-OH of the growing strand attacks the alpha-phosphate of the incoming dNTP
This releases pyrophosphate (beta and gamma phosphates), whose subsequent hydrolysis drives the reaction forward
All four dNTPs (dATP, dGTP, dTTP, dCTP) must be available
DNA polymerase synthesises in the 5' to 3' direction; it reads the template in the 3' to 5' direction
DNA polymerase cannot identify exact nucleotides entering the active site; it monitors geometry of the resulting base pair
The palm domain contains the catalytic site, binds two Mg²⁺ ions, consists of a beta-sheet, and monitors the minor groove
The 90-degree bend in the template ensures only one base is presented at a time, reducing errors, and routes the template past the 3' to 5' exonuclease
DNA polymerase senses correct incorporation by monitoring the geometry of the base pair (not by detecting hydrogen bonds directly)
Discriminator amino acids in the active site prevent incorporation of rNTPs into DNA
Not all DNA polymerases can proofread; this is a commonly tested false statement
The 3' to 5' exonuclease domain removes misincorporated nucleotides
In eukaryotic cells, there are at least fifteen different DNA polymerase enzymes
DNA polymerase is not always a single polypeptide chain (some are multi-subunit)
DNA polymerase requires an RNA primer (laid down by primase) to initiate synthesis
It cannot begin synthesis de novo
The sliding clamp increases processivity by tethering polymerase to the DNA
The sliding clamp is released when it contacts ssDNA (not dsDNA, not another polymerase)
The rate of DNA polymerase is maximised by: the rate of binding to the primer-template junction, higher processivity, and having a single active site geometry that accommodates all four dNTPs
Helicase: unwinds DNA; hexamer; moves by ATP-driven conformational changes (alternating-hands mechanism); interacts with primase and the tau subunit of the clamp loader
SSBs: stabilise ssDNA via electrostatic and base-stacking interactions
Topoisomerase: relieves positive supercoiling ahead of the fork
DNA ligase: seals Okazaki fragments; does not act directly at the replication fork in the same way
All of helicase, primase, SSBs, and the sliding clamp loader act at the replication fork; DNA ligase acts behind it
Leading strand: synthesised continuously in the 5' to 3' direction, moving with the fork
Lagging strand: synthesised discontinuously as Okazaki fragments, each requiring a new primer
DNA Pol alpha: primer addition and initiation (synthesises a short RNA-DNA primer)
DNA Pol epsilon: leading strand synthesis
DNA Pol delta: lagging strand synthesis
Lagging strand order: DNA is unwound, DNA is primed, sliding clamp loader adds a sliding clamp, DNA associates with DNA polymerase delta
Each replication bubble has two forks, each with its own set of polymerases
One entire bubble contains 4 polymerases total (considering alpha, delta, and epsilon at both forks): two forks, each with leading and lagging strand polymerases, plus the primase/Pol alpha
Traditional view: a single holoenzyme replicates both leading and lagging strands
Replication factory model: the Watson and Crick strands are replicated simultaneously by the same holoenzyme; the lagging strand loops through the complex (trombone model)
E. coli replication allows continuous replication of the entire genome without holoenzyme detachment (False: the holoenzyme must detach and re-engage on the lagging strand for each Okazaki fragment)
After replication, the RNA primer is removed by RNase H and DNA polymerase
DNA polymerase fills the gap left by primer removal
DNA ligase seals the final nick
Order: DnaA binds oriC and melts DNA, DnaC loads DnaB (helicase), SSBs bind ssDNA, primase primes the leading strand, sliding clamp is loaded, primase primes the lagging strand (last step)
DnaB (helicase) recruits primase on the leading strand
E. coli prevents reinitiation by: SeqA binding to hemimethylated DNA at the origin, blocking DnaA from re-binding until both strands are fully methylated by Dam methylase
Replicator selection occurs during G1
Pre-RCs form when Cdk levels are low (G1)
Pre-RCs are activated when Cdk levels rise (S phase entry)
High Cdk levels also prevent new pre-RC formation, ensuring origins fire only once per cycle
Circular DNA replication produces catenated (interlinked) strands that Topo II must separate
Topo II is involved in separating newly synthesised DNA in both circular and linear replication
Both circular and linear DNA replication can be terminated by the presence of a terminator or dsDNA
Linear chromosomes lose a small amount of DNA at each end with every replication cycle (the end-replication problem)
Telomerase extends chromosome ends using a built-in RNA template
Telomerase synthesises DNA in the conventional 5' to 3' direction (it is a reverse transcriptase, using its RNA template)
Telomerase is expressed in germ cells (egg and sperm cells) and certain somatic cells like immune cells, not in all cells and not in red blood cells
AZT (azidothymidine) is used in HIV treatment and some cancer therapies because it terminates DNA synthesis once incorporated. Understanding replication fidelity is central to cancer genetics, since replication errors that escape proofreading and mismatch repair contribute to tumour formation. Telomerase reactivation is a hallmark of most cancers.
Students often think DNA polymerase reads hydrogen bonds to check for correct base pairing. It monitors the geometry (shape) of the base pair, not the hydrogen bonds directly.
Students frequently say DNA polymerase can start a new strand on its own. It cannot; it always requires an RNA primer.
Students assume all DNA polymerases can proofread. They cannot. Only those with a 3' to 5' exonuclease domain can proofread.
Students confuse which polymerase does what in eukaryotes. Pol alpha = initiation/priming, Pol epsilon = leading strand, Pol delta = lagging strand.
⚠️ The 3'-OH attacks the alpha-phosphate of the incoming nucleotide. This is the single most fundamental reaction in DNA synthesis and is tested frequently.
⚠️ Template is read 3' to 5'; new strand is synthesised 5' to 3'. If a question says the template is "synthesised" in any direction, read carefully: the template is read, not synthesised.
⚠️ DNA polymerase monitors geometry of the base pair, not hydrogen bonds. This is a key distinction.
⚠️ The sliding clamp is released when it contacts ssDNA, not dsDNA.
⚠️ Eukaryotic Pol alpha = priming, Pol epsilon = leading, Pol delta = lagging. Commit this to memory.
⚠️ Telomerase uses a built-in RNA template and synthesises in the 5' to 3' direction.
⚠️ Low Cdk = pre-RC formation (G1). High Cdk = pre-RC activation and prevention of re-licensing (S phase).
⚠️ E. coli prevents reinitiation via SeqA binding hemimethylated DNA.
Fill in the blank: During DNA synthesis, the 3'-OH attacks the ______ of the incoming nucleotide.
True or False: DNA polymerase can initiate synthesis without a primer.
Fill in the blank: In eukaryotes, DNA Pol ______ is responsible for lagging strand synthesis.
True or False: The sliding clamp is released when it contacts double-stranded DNA.
True or False: Telomerase is expressed in all human cells.
Q: What does DNA polymerase require to initiate synthesis in the cell?
A: An RNA primer, which provides the 3'-OH needed for nucleotide addition.
Q: How does DNA polymerase sense that the correct nucleotide has been incorporated?
A: By monitoring the geometry of the base pair, not by detecting hydrogen bonds.
Q: What is the mechanism by which AZT (azidothymidine) targets DNA synthesis?
A: AZT is incorporated in place of thymine but lacks a 3'-OH, so it terminates DNA synthesis because no further nucleophilic attack can occur.
Q: What is the correct order for lagging strand replication in eukaryotes?
A: DNA is unwound, DNA is primed (by Pol alpha), sliding clamp loader adds a sliding clamp, DNA associates with DNA polymerase delta.
Q: How does E. coli prevent reinitiation of DNA synthesis?
A: SeqA binds to hemimethylated DNA at the origin, blocking DnaA from re-binding until both strands are fully methylated by Dam methylase.
Q: How does telomerase extend chromosome ends?
A: It uses a built-in RNA template to synthesise new telomeric DNA in the 5' to 3' direction.
Q: In E. coli replication, which enzyme recruits primase on the leading strand?
A: DnaB (helicase).
Q: How many total polymerases are present in one complete eukaryotic replication bubble?
A: 4 (alpha, delta, and epsilon distributed across both forks).
This material builds directly on the chromatin and nucleosome content from Part 3, since replication requires disassembly and reassembly of nucleosomes as the fork passes. Histone inheritance models (semiconservative vs. asymmetric) are only relevant in the context of replication fork progression. The cell cycle content from Part 2 provides the timing framework: pre-RCs form in G1, fire in S phase, and cannot re-form until the next G1. Telomerase connects to ageing biology and cancer.
DNA replication, DNA polymerase, replication fork, 5' to 3' synthesis, 3' to 5' reading, template strand, RNA primer, primase, helicase, SSBs, single-stranded binding proteins, topoisomerase, sliding clamp, PCNA, processivity, palm domain, thumb domain, 3' to 5' exonuclease, proofreading, discriminator amino acids, Okazaki fragments, leading strand, lagging strand, DNA ligase, RNase H, replisome, holoenzyme, trombone model, replication factory, DnaA, DnaB, DnaC, oriC, pre-RC, Cdk, cyclin-dependent kinase, SeqA, Dam methylase, hemimethylation, Pol alpha, Pol delta, Pol epsilon, telomerase, end-replication problem, AZT, azidothymidine, chain terminator, catenane, Topo II