Course: PCB 3023, Cell Biology (University of Florida)
Difficulty: Intermediate Prerequisites: Chapter 5 (DNA and chromosome structure). You need to understand antiparallel strands, complementary base pairing, and the 5'/3' directionality of DNA before this material will make sense.
DNA replication is semiconservative: each new double helix contains one old strand and one new strand. Replication proceeds bidirectionally from origins, with a leading strand synthesised continuously and a lagging strand built in short Okazaki fragments. DNA polymerase, primase, helicase, and ligase work together at the replication fork. After replication, mismatch repair catches copying errors, and separate pathways repair chemical damage like depurination, deamination, and thymine dimers. A single base change (as in sickle cell anaemia) illustrates how mutations link DNA sequence to disease.
Semiconservative replication
The model of DNA replication in which each daughter molecule contains one original (parental) strand and one newly synthesised strand. Think of it as splitting a zip and building a new half onto each side.
Template strand
The parental strand that DNA polymerase reads to synthesise the new complementary strand. The new strand's sequence is dictated entirely by the template.
Replication origin
A specific DNA sequence where replication begins. Prokaryotes typically have one origin; eukaryotes have many (thousands), allowing their much larger genomes to be copied in a reasonable time. Origins tend to be AT-rich, because A-T base pairs have only two hydrogen bonds and are easier to separate.
Replication fork
The Y-shaped region where the two parental strands are being unwound and new strands are being synthesised. Replication is bidirectional, so two forks move outward from each origin.
Leading strand
The new strand synthesised continuously in the 5' to 3' direction towards the replication fork. It needs only one RNA primer to get started.
Lagging strand
The new strand synthesised discontinuously, in short Okazaki fragments, away from the replication fork. Each fragment requires its own RNA primer. The fragments are later joined.
Okazaki fragments
Short stretches of newly synthesised DNA on the lagging strand (roughly 100 to 200 nucleotides in eukaryotes, 1,000 to 2,000 in prokaryotes). They are later processed and ligated into a continuous strand.
DNA polymerase
The enzyme that synthesises new DNA by adding nucleotides to the 3' end of a growing strand, using the template strand and complementary base pairing. It has two key catalytic activities: 5' to 3' polymerisation and 3' to 5' exonuclease (proofreading).
Primer / DNA primase
A primer is a short stretch of RNA synthesised by the enzyme DNA primase (an RNA polymerase). DNA polymerase cannot start a chain from scratch; it can only add nucleotides to an existing 3'-OH group, so the primer provides that starting point.
Helicase
An enzyme that unwinds the double helix at the replication fork, separating the two parental strands using energy from ATP hydrolysis.
Single-strand binding proteins (SSBs)
Proteins that coat exposed single-stranded DNA after helicase unwinds it, preventing the strands from re-annealing or forming secondary structures.
DNA ligase
The enzyme that seals the nick (phosphodiester bond) between adjacent Okazaki fragments after the RNA primers have been replaced with DNA. Requires energy input (ATP or NAD+).
DNA mismatch
An incorrectly paired base (e.g. G-T instead of G-C) that was incorporated during replication and escaped polymerase proofreading.
DNA damage
A chemical alteration to the DNA structure caused by environmental agents or spontaneous reactions (e.g. depurination, deamination, UV-induced thymine dimers). Distinct from a mismatch: damage changes the chemistry of the base, not just the pairing.
Mutation
A permanent change in the DNA sequence. Mismatches and unrepaired damage both become mutations once the cell divides and the altered sequence is passed on.
Depurination
Spontaneous loss of a purine base (A or G) from the DNA backbone, leaving an abasic site. If unrepaired, it can lead to a random base being inserted opposite the gap during the next replication.
Deamination
Spontaneous loss of an amino group from a base. The most common case: cytosine deaminates to uracil. If unrepaired, the next round of replication reads the uracil as thymine, producing a C-G to T-A transition mutation.
Thymine dimer
A covalent linkage between two adjacent thymine bases on the same strand, caused by UV light. Distorts the helix and blocks replication if not repaired.
Mismatch repair
A post-replication repair system that detects and corrects mismatched bases. It identifies the newly synthesised strand (which contains the error) and replaces the incorrect nucleotide. Reduces the replication error rate by roughly 100- to 1,000-fold.
Semiconservative model: Each parental strand serves as a template. After replication, each daughter molecule has one old strand and one new strand. This was demonstrated by the Meselson-Stahl experiment.
Key elements of replication:
The template strand is read 3' to 5' by DNA polymerase.
Complementary base pairing ensures accuracy: A pairs with T, G pairs with C on the new strand.
New strands are always synthesised in the 5' to 3' direction.
Because the strands are antiparallel, the two new strands at a replication fork are synthesised differently (one continuously, one discontinuously).
Replication origins:
Prokaryotes: typically a single origin of replication.
Eukaryotes: thousands of origins, needed because their genomes are much larger.
Origins are generally AT-rich. A-T base pairs have only 2 hydrogen bonds (vs 3 for G-C), so AT-rich regions are easier to melt (separate) to initiate replication.
Bidirectional replication: Two replication forks form at each origin and move in opposite directions, creating a replication bubble.
DNA polymerase has two catalytic activities (performed by separate domains):
5' to 3' polymerase activity: Adds nucleotides complementary to the template, extending the new strand at its 3' end.
3' to 5' exonuclease activity (proofreading): Removes incorrectly paired nucleotides from the 3' end of the growing strand. This gives the enzyme a built-in error-correction mechanism.
Error rate: DNA polymerase has an error rate of roughly 1 in 10^7 nucleotides (after proofreading). Mismatch repair further reduces this to roughly 1 in 10^9.
Energy source: The energy for chain elongation comes from hydrolysis of the incoming nucleoside triphosphate (dNTP). Cleavage of the high-energy phosphoanhydride bond (releasing pyrophosphate) drives the reaction forward.
Why nucleotides are added to the 3' end, not the 5' end:
If a nucleotide were added to the 5' end, it would carry the triphosphate. If proofreading then removed that nucleotide (because it was wrong), the triphosphate energy source would be lost with it, and no energy would remain to attach the next correct nucleotide.
With 3' addition, the triphosphate is always on the incoming nucleotide, so the growing chain always has a free 3'-OH ready to accept the next addition, even after proofreading removes an error.
Primers:
DNA polymerase cannot start a new chain from scratch. It requires a pre-existing 3'-OH group.
DNA primase (an RNA polymerase) synthesises a short RNA primer (~10 nucleotides) complementary to the template.
DNA polymerase then extends from the primer's 3' end.
Leading vs lagging strand (the two differences in one sentence):
The leading strand is synthesised continuously with a single primer, while the lagging strand is synthesised discontinuously as Okazaki fragments, each requiring its own primer.
Both strands are made 5' to 3', but the leading strand grows toward the fork and the lagging strand grows away from it.
Three catalytic activities needed to join Okazaki fragments:
RNA primer removal and replacement with DNA: A nuclease (or the 5' to 3' exonuclease activity of DNA polymerase I in prokaryotes) removes the RNA primer. DNA polymerase fills the resulting gap with DNA. This step requires energy (dNTP hydrolysis).
Nick sealing by DNA ligase: Once the gap is filled, a nick remains in the sugar-phosphate backbone. DNA ligase seals it by forming a phosphodiester bond. This step also requires energy (ATP or NAD+).
In total, two of the three reactions (polymerase gap-filling and ligation) require energy input.
RNA primer replacement is required for both strands: The leading strand has one primer that must be replaced; the lagging strand has many (one per Okazaki fragment).
Helicase:
Unwinds the double helix ahead of the replication fork.
Uses ATP hydrolysis to break the hydrogen bonds between base pairs.
Single-strand binding proteins (SSBs):
Bind cooperatively to exposed single-stranded DNA.
Prevent re-annealing and protect the single-stranded DNA from nuclease degradation and secondary structure formation.
Coordination at the fork:
The replication machinery is organised as a large multi-protein complex (the replisome).
Components are physically linked, so the leading and lagging strand polymerases, helicase, and primase work in a coordinated manner. This increases efficiency and speed.
The lagging-strand template loops back so that both polymerases can move in the same direction as the fork.
Distinguish the three terms:
A DNA mismatch is a wrong base pairing introduced during replication (e.g. G paired with T). The chemistry of the bases is normal; they are simply paired incorrectly.
DNA damage is a chemical alteration to the structure of DNA (e.g. a lost base, a modified base, a crosslink). This is caused by environmental agents or spontaneous chemical reactions, not by the replication machinery.
A mutation is the permanent, heritable change in DNA sequence that results when a mismatch or damage goes unrepaired through the next round of replication.
Types of DNA damage:
Depurination: Spontaneous loss of a purine (A or G), leaving an abasic (AP) site. If unrepaired, replication may insert any base opposite the gap, often causing a mutation.
Deamination: Loss of an amino group. Cytosine deaminates to uracil. If unrepaired, the next replication reads uracil as thymine, producing a C:G to T:A transition.
Thymine dimer formation: UV light causes adjacent thymines on the same strand to form a covalent bond. This distorts the helix and stalls replication.
Mismatch repair:
Occurs shortly after replication, while the newly synthesised strand can still be distinguished from the template.
The repair system identifies and corrects the new strand (not the template).
Reduces the error rate of replication by roughly 100- to 1,000-fold (from ~10^-7 to ~10^-9).
Sickle cell anaemia illustrates the full chain from a single DNA mutation to a clinical phenotype.
The mutation: A single base change in the beta-globin gene (GAG → GTG at the sixth codon) substitutes valine for glutamic acid.
The protein effect: Valine is hydrophobic; glutamic acid is charged. The hydrophobic patch on the surface of the altered haemoglobin (HbS) causes haemoglobin molecules to stick together under low-oxygen conditions.
The cellular effect: Aggregated HbS forms long fibres inside red blood cells, distorting them into a rigid sickle shape.
The clinical effect: Sickle-shaped cells block capillaries, causing pain crises, organ damage, and anaemia (because sickled cells are fragile and break apart).
This is a classic exam example of how a point mutation leads to an altered protein, an altered cell, and a disease phenotype.
DNA replication fidelity is why cancer research focuses heavily on mismatch repair genes. Defects in mismatch repair (e.g. in Lynch syndrome) dramatically increase the mutation rate and the risk of colorectal and other cancers.
Understanding thymine dimer repair is the basis for why dermatologists warn about UV exposure. Your cells have repair pathways to fix UV damage, but if the damage rate exceeds the repair capacity, mutations accumulate and skin cancer risk increases.
"DNA polymerase can start a new strand from scratch." It cannot. It always requires a primer with a free 3'-OH group. Only primase can start de novo.
"The leading strand does not need a primer." It does. It needs one primer at the origin. The lagging strand needs many (one per Okazaki fragment), but "one" is not "none."
"Mismatch repair fixes the template strand." The opposite. Mismatch repair identifies and corrects the newly synthesised strand, because the error is in the new copy, not the original.
"DNA damage and DNA mismatch are the same thing." They are not. A mismatch is a normal base incorrectly paired during replication. Damage is a chemical change to the base or backbone caused by an external agent or spontaneous reaction. Both can lead to mutations if unrepaired, but they arise differently and are fixed by different pathways.
⚠️ Be able to state the two differences between leading and lagging strand synthesis in one sentence. The review sheet asks for exactly this.
⚠️ Know the two catalytic activities of DNA polymerase (5' to 3' polymerisation, 3' to 5' exonuclease proofreading) and be able to explain why nucleotides are added to the 3' end.
⚠️ Know the three enzymatic steps for joining Okazaki fragments (primer removal, gap filling, ligation) and which two require energy.
⚠️ RNA primer replacement is needed for both leading and lagging strands. This is a common trick question.
⚠️ Be able to draw a replication bubble with labelled 5'/3' polarity on all four strands, Okazaki fragments numbered in the order they were made, and two forks moving outward from the origin.
⚠️ Distinguish clearly between DNA mismatch, DNA damage, and mutation. Know the three types of damage (depurination, deamination, thymine dimers) and what mutation each causes if unrepaired.
⚠️ Know the sickle cell chain: single base substitution → amino acid change (Glu → Val) → hydrophobic aggregation → sickle shape → blocked capillaries.
True or False: In semiconservative replication, both strands of the daughter molecule are newly synthesised.
Fill in the blank: DNA polymerase adds nucleotides to the ___ end of the growing strand.
True or False: The lagging strand is synthesised as a single continuous piece.
Fill in the blank: DNA primase is a type of ___ polymerase.
True or False: Mismatch repair corrects errors on the template (parental) strand.
Answers: 1. False (one old strand, one new strand). 2. 3' end. 3. False (synthesised discontinuously as Okazaki fragments). 4. RNA polymerase. 5. False (it corrects the newly synthesised strand).
Q: Explain the semiconservative model of replication and the role of the template strand.
A: Each parental strand acts as a template for a new complementary strand. After replication, each daughter molecule has one parental strand and one new strand. The template strand is read 3' to 5', and the new strand is built 5' to 3' by complementary base pairing.
Q: State the two differences between leading and lagging strand synthesis in one sentence.
A: The leading strand is synthesised continuously with one primer, while the lagging strand is synthesised discontinuously as Okazaki fragments, each requiring its own RNA primer.
Q: What are the two catalytic activities of DNA polymerase, and why are nucleotides added to the 3' end?
A: DNA polymerase has 5' to 3' polymerase activity (adding nucleotides) and 3' to 5' exonuclease activity (proofreading). Nucleotides are added to the 3' end because the energy for the bond comes from the incoming nucleoside triphosphate. If nucleotides were added to the 5' end, removing an error by proofreading would also remove the triphosphate needed for the next addition.
Q: Name the three enzymatic steps required to join Okazaki fragments and state which require energy.
A: (1) Removal of RNA primers and replacement with DNA by nuclease and DNA polymerase (requires dNTP energy). (2) Sealing of the nick by DNA ligase (requires ATP/NAD+ energy). The third activity is the nuclease removal itself, which is often grouped with step 1. Two of the three (gap-filling polymerisation and ligation) require energy input.
Q: Is RNA primer replacement required for leading strands, lagging strands, or both?
A: Both. The leading strand has one primer that must be replaced; the lagging strand has many.
Q: Describe depurination, deamination, and thymine dimer formation, and state the mutation each can cause.
A: Depurination is the spontaneous loss of a purine base, leaving an abasic site; any base may be inserted opposite during replication. Deamination is the loss of an amino group (e.g. cytosine becomes uracil), leading to a C:G to T:A transition if unrepaired. Thymine dimers are covalent bonds between adjacent thymines caused by UV light, which stall replication and may cause insertions, deletions, or base substitutions if bypassed incorrectly.
Q: What is mismatch repair, which strand does it correct, and by how much does it reduce the error rate?
A: Mismatch repair is a post-replication system that detects incorrectly paired bases and replaces them on the newly synthesised strand (not the template). It reduces the error rate by roughly 100- to 1,000-fold.
Q: Trace the molecular basis of sickle cell anaemia from DNA sequence to sickle-shaped cells.
A: A single base change (GAG → GTG) in the beta-globin gene substitutes valine for glutamic acid at position 6. The hydrophobic valine causes deoxygenated haemoglobin molecules (HbS) to aggregate into fibres. These fibres distort red blood cells into a rigid sickle shape, which blocks capillaries and causes the symptoms of sickle cell disease.
This material connects back to Chapter 5 (Chromosome Structure), because the replication fork must disassemble nucleosomes ahead of it and reassemble them behind it. Histone modification and chromatin remodelling are part of the replication process.
The concept of mutations and DNA repair connects forward to topics in gene regulation and cancer biology. Defective repair pathways (e.g. mismatch repair deficiency in Lynch syndrome, or nucleotide excision repair deficiency in xeroderma pigmentosum) are studied in later chapters on cell signalling and disease.
Sickle cell anaemia is a bridge to protein structure and function, because the mutation changes one amino acid and thereby alters the physical behaviour of the entire haemoglobin molecule.
DNA replication, semiconservative, template strand, replication origin, replication fork, leading strand, lagging strand, Okazaki fragments, DNA polymerase, 5' to 3' polymerisation, 3' to 5' exonuclease, proofreading, primer, DNA primase, RNA primer, helicase, single-strand binding proteins, SSB, DNA ligase, replisome, replication bubble, bidirectional replication, AT-rich origins, nucleoside triphosphate, dNTP, pyrophosphate, DNA mismatch, DNA damage, mutation, depurination, abasic site, deamination, cytosine to uracil, thymine dimer, UV damage, mismatch repair, error rate, sickle cell anaemia, sickle cell disease, HbS, beta-globin, point mutation, Glu to Val, PCB 3023, cell biology, UF