How is DNA structured to store genetic information, and how is it copied accurately before cell division?
The structure of DNA as an antiparallel double helix of nucleotides, the requirements and process of DNA replication by DNA polymerase, and the amplification of DNA by the polymerase chain reaction (PCR).
An SQA Higher Human Biology answer on DNA structure and replication, covering nucleotides, the antiparallel double helix, complementary base pairing, the role of DNA polymerase, primers and ligase in replication, leading and lagging strands, and PCR amplification.
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What this dot point is asking
The SQA wants you to describe DNA as an antiparallel double helix of nucleotides, explain how DNA polymerase copies each strand using primers and free nucleotides, describe why the leading and lagging strands are built differently, and describe how PCR amplifies DNA in the laboratory.
The structure of DNA
Nucleotides join through their sugars and phosphates to form a strong sugar-phosphate backbone, with the bases projecting sideways. The two strands of DNA are held together by hydrogen bonds between paired bases, following complementary base pairing: adenine always pairs with thymine, and guanine always pairs with cytosine. The two strands are antiparallel, running in opposite directions, described by the carbon numbering of the sugar as 3 prime to 5 prime on one strand and 5 prime to 3 prime on the other. This antiparallel arrangement matters because the enzymes that copy DNA can only move in one direction along a strand.
DNA replication
Before a cell divides by mitosis, it must copy its DNA exactly. Replication requires:
The double helix first unwinds and the hydrogen bonds break, so each original strand acts as a template. DNA polymerase adds free nucleotides to the exposed template, pairing them by the complementary base rule. Because polymerase can only add a nucleotide to the 3 prime end of the growing strand, it builds only in the 5 prime to 3 prime direction.
Since the template strands are antiparallel, this has a key consequence:
- The leading strand is built continuously towards the point where the helix is unwinding.
- The lagging strand is built away from the unwinding point in short fragments, which the enzyme ligase later joins into one continuous strand.
Each new molecule has one original and one new strand, so replication is semi-conservative and produces two identical DNA molecules.
The polymerase chain reaction (PCR)
PCR repeats a three-step temperature cycle:
- Denaturation. Heating to about 92 to 98 degrees Celsius separates the two strands by breaking the hydrogen bonds.
- Annealing. Cooling to about 50 to 65 degrees Celsius lets short primers bind to the target sequences at the ends of the region to be copied.
- Extension. Warming to about 70 to 80 degrees Celsius, the optimum for a heat-tolerant DNA polymerase, allows the enzyme to add free nucleotides and build new complementary strands from the primers.
Because each cycle copies both strands, the amount of target DNA roughly doubles every cycle. PCR is used in forensic science (amplifying DNA from tiny crime-scene samples), in diagnosis (detecting the DNA of a virus or bacterium), and in genetic testing.
Examples in context
Example 1. Forensic DNA profiling. A crime scene may yield only a few cells. PCR amplifies the DNA from those cells into millions of copies, enough to produce a DNA profile that can be compared with a suspect or a database. The reliability of profiling depends on accurate replication during PCR.
Example 2. Diagnosing infection. During an outbreak, PCR amplifies any viral DNA (or DNA copied from viral RNA) present in a patient sample. Detecting the amplified target confirms infection far earlier than waiting for the body to produce antibodies, which is why PCR became central to rapid disease testing.
Try this
Q1. State the base that pairs with cytosine in DNA. [1 mark]
- Cue. Guanine.
Q2. Explain why the lagging strand is synthesised in fragments. [1 mark]
- Cue. DNA polymerase only builds in the 5 prime to 3 prime direction, so on the antiparallel template it must work away from the replication point in short pieces.
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 molecules and conditions required for DNA replication, and explain why one strand is built continuously while the other is built in fragments.Show worked answer →
A 4-mark answer needs the requirements of replication and an explanation of leading versus lagging strands.
Replication requires a template DNA strand, primers, a supply of the four free DNA nucleotides, the enzyme DNA polymerase, and ATP for energy. The double helix first unwinds and the strands separate so each acts as a template.
DNA polymerase can only add nucleotides to the 3 prime end of a growing strand, so it works only in the 5 prime to 3 prime direction. Because the two template strands are antiparallel, one new strand (the leading strand) is built continuously towards the unwinding point, while the other (the lagging strand) is built away from it in short fragments. The enzyme ligase then joins these fragments.
Award (1) template and primers, (2) free nucleotides, DNA polymerase and ATP, (3) the 3 prime to 5 prime / one-direction rule, and (4) the leading and lagging strand consequence.
SQA Higher 20223 marksDescribe the three stages that are repeated in each cycle of the polymerase chain reaction (PCR) and state one use of PCR.Show worked answer →
This is a 3-mark question on the PCR cycle plus a use.
Each PCR cycle has three temperature steps. First the DNA is heated to about 92 to 98 degrees Celsius to separate (denature) the strands. Second it is cooled to about 50 to 65 degrees Celsius so that short primers anneal to the target sequences. Third it is warmed to about 70 to 80 degrees Celsius, the optimum for a heat-tolerant DNA polymerase, which extends new strands from the primers.
Repeating the cycle doubles the amount of target DNA each time, amplifying tiny samples. A valid use is amplifying DNA for forensic analysis, diagnosis of infection, or genetic testing. Markers reward the three correctly ordered steps and a sensible use.
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