How is DNA replicated accurately before a cell divides?
The replication of DNA by DNA polymerase, requiring a template, primers, free nucleotides and ATP, the formation of leading and lagging strands, and the use of the polymerase chain reaction (PCR) to amplify DNA.
An SQA Higher Biology answer on DNA replication, covering the role of DNA polymerase, primers and free nucleotides, leading and lagging strands, the requirements for replication, and how PCR amplifies DNA in the laboratory.
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What this key area is asking
The SQA wants you to describe how DNA is copied before cell division, name the molecules required for replication, explain the difference between the leading and lagging strands, and describe how the polymerase chain reaction (PCR) amplifies a target piece of DNA.
What replication needs
The double-stranded DNA first unwinds and the hydrogen bonds between the base pairs break, exposing two single strands that each act as a template. Because each new molecule keeps one original strand and gains one newly made strand, replication is described as semi-conservative. Replication happens before a cell divides so that each daughter cell receives a complete and identical copy of the genome.
The role of primers and DNA polymerase
DNA polymerase adds free DNA nucleotides to the new strand, but it can only do so in the 5 prime to 3 prime direction, adding each new nucleotide to the 3 prime end of the growing strand. Complementary base pairing (A with T, G with C) ensures the new strand matches the template exactly. The energy to join each nucleotide comes from ATP. DNA polymerase also has a proofreading role, removing wrongly paired nucleotides, which keeps replication highly accurate.
Leading and lagging strands
Because the two template strands are antiparallel and DNA polymerase only works 5 prime to 3 prime, the two new strands are built differently:
- The leading strand is synthesised continuously towards the unwinding fork, needing only one primer.
- The lagging strand runs the opposite way, so it cannot be made in one piece. It is synthesised in short fragments, each needing its own primer, and the fragments are afterwards joined together by the enzyme ligase.
This neatly explains why the two strands of one molecule, copied at the same fork, are still made in different ways: the directional rule of the enzyme, combined with the antiparallel template, forces it.
The polymerase chain reaction (PCR)
Each PCR cycle has three temperature stages:
- Heating to about 90 to 98 degrees Celsius separates the two DNA strands by breaking the hydrogen bonds.
- Cooling to about 50 to 65 degrees Celsius lets primers bind to the target sequences. The primers are chosen to flank the region of interest, so only that region is copied.
- Heating to about 70 to 80 degrees Celsius allows a heat-tolerant DNA polymerase (such as Taq polymerase, taken from a bacterium that lives in hot springs) to replicate the region between the primers.
Repeating the cycle doubles the number of target molecules each time, so the number of copies after cycles is the starting number multiplied by . This gives millions of copies from a tiny sample. PCR is used in forensics (matching DNA from a crime scene), in the diagnosis of disease (detecting pathogen DNA, as in COVID-19 testing) and in research.
Examples in context
Example 1. Forensic DNA profiling. A single hair or a trace of blood from a crime scene contains far too little DNA to analyse directly. Forensic scientists use PCR to amplify specific regions of the genome millions of times, so that enough DNA is available to produce a profile. The same primers and heat-tolerant polymerase used in the laboratory rely on exactly the replication rules learned here: strand separation by heat, primer binding, and copying in the 5 prime to 3 prime direction.
Example 2. COVID-19 testing. The standard PCR test for SARS-CoV-2 uses primers designed to bind to a unique region of the virus's genetic material. If the virus is present, the polymerase amplifies that region through repeated cycles until it can be detected. Because each cycle doubles the target, even a small amount of virus produces a clear positive result, which is why PCR became the gold-standard diagnostic test during the pandemic.
Try this
Q1. List the molecules required for DNA replication. [2 marks]
- Cue. Template DNA, primers, free DNA nucleotides, DNA polymerase, and ATP.
Q2. Explain why the lagging strand is made in fragments. [2 marks]
- Cue. DNA polymerase only works 5 prime to 3 prime, but the template runs the opposite way, so synthesis happens in pieces joined by ligase.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher 20184 marksDescribe the roles of primers, DNA polymerase and ligase in the replication of DNA.Show worked answer →
A 4-mark describe answer needs the role of each named molecule plus the directional rule.
A primer is a short sequence of nucleotides that binds to the 3 prime end of a template strand, giving DNA polymerase a starting point because the enzyme cannot begin a new strand from nothing.
DNA polymerase adds complementary free DNA nucleotides to the 3 prime end of the growing strand, so it can only build a strand in the 5 prime to 3 prime direction.
Ligase joins the fragments of the lagging strand together into one continuous strand.
Markers reward (1) primer provides a starting point, (2) polymerase adds nucleotides 5 prime to 3 prime, (3) the 3 prime addition point and (4) ligase joins fragments.
SQA Higher 20225 marksA PCR reaction begins with 500 copies of a target DNA sequence. Calculate the number of copies after 8 complete cycles, and explain why PCR doubles the target each cycle.Show worked answer →
This is a doubling calculation. Each PCR cycle copies every target molecule once, so the number doubles each cycle.
Step 1. After each cycle the number of copies is multiplied by 2, so after 8 cycles the starting number is multiplied by .
Step 2. Work out the factor: .
Step 3. Multiply by the starting number: copies.
The doubling happens because heating separates the two strands, primers bind to each separated strand, and a heat-tolerant DNA polymerase then replicates the region between the primers on both strands, turning one molecule into two.
Markers reward the correct method (), the value 128000, and a clear reason for the doubling.
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Sources & how we know this
- SQA Higher Biology Course Specification — SQA (2018)