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How do the tools of molecular biology let us copy, cut, join and analyse DNA, and what are these techniques used for?

Describe the principles of recombinant DNA technology, PCR, gel electrophoresis and DNA sequencing and outline their applications

A focused answer to the H2 Biology Molecular Genetics outcome on DNA technology. Restriction enzymes and ligase in recombinant DNA, the polymerase chain reaction, gel electrophoresis, DNA sequencing, and applications from insulin production to genetic profiling.

Generated by Claude Opus 4.89 min answer

Reviewed by: AI editorial process; not yet individually human-reviewed

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  1. What this dot point is asking
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  3. Examples in context
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What this dot point is asking

SEAB wants you to describe the core tools of molecular biology (restriction enzymes and ligase for recombinant DNA, the polymerase chain reaction, gel electrophoresis, and DNA sequencing) and to outline their applications. These techniques all build on the structure of DNA and the principles of replication.

The answer

Recombinant DNA technology

A useful gene is cut from one source and joined into a vector (often a plasmid) for transfer into a host cell.

  • Restriction enzymes cut DNA at specific recognition sequences, often leaving short single-stranded sticky ends.
  • Cutting the gene and the vector with the same enzyme gives complementary sticky ends that base-pair.
  • DNA ligase joins them by forming phosphodiester bonds, producing recombinant DNA.
  • The recombinant vector is taken up by a host (transformation), which then expresses the gene.

The polymerase chain reaction (PCR)

PCR copies a target sequence in a tube through repeated cycles of three temperature steps: denaturation (about 95 degrees, strands separate), annealing (about 55 degrees, primers bind), and extension (about 72 degrees, a heat-stable DNA polymerase builds new strands). Each cycle doubles the target, so many cycles give millions of copies.

Gel electrophoresis

DNA fragments are loaded into a gel and an electric field is applied. Because DNA is negatively charged, fragments move toward the positive electrode, and smaller fragments move faster and further. This separates fragments by size, producing a pattern of bands.

DNA sequencing

Sequencing reads the exact base order of a DNA sample. Modern methods are fast and cheap enough to sequence whole genomes, underpinning genomics.

Examples in context

Example 1. Human insulin from bacteria. The human insulin gene is inserted into a plasmid and transferred into bacteria, which then produce human insulin in culture. This recombinant insulin replaced animal-sourced insulin and is a landmark application of recombinant DNA technology in medicine.

Example 2. Genetic profiling. PCR amplifies highly variable regions of DNA and gel electrophoresis separates the fragments, giving a banding pattern unique to an individual. This is used in forensic investigation and in establishing biological relationships.

Try this

Q1. State the role of a restriction enzyme in recombinant DNA technology. [1 mark]

  • Cue. It cuts DNA at a specific recognition sequence, often leaving sticky ends.

Q2. Explain why DNA fragments separate by size during gel electrophoresis. [2 marks]

  • Cue. DNA is negatively charged and moves toward the positive electrode; smaller fragments move through the gel more easily and so travel further than larger ones.

Q3. State the purpose of the primers used in PCR. [1 mark]

  • Cue. They bind by complementary base pairing to the ends of the target sequence, providing a starting point for DNA polymerase to extend.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original6 marksDescribe how recombinant DNA technology can be used to produce a human protein, such as insulin, in bacteria.
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Examiners want the ordered steps with the named enzymes and vector.

First, the human gene for insulin is obtained, for example by isolating it from human DNA or making it from the mRNA. A restriction enzyme cuts the DNA at a specific recognition sequence, often leaving short single-stranded sticky ends.

The same restriction enzyme cuts a plasmid (the vector), producing complementary sticky ends. The human gene and the cut plasmid are mixed, the sticky ends base-pair, and DNA ligase joins them by forming phosphodiester bonds, producing recombinant DNA.

The recombinant plasmid is taken up by bacteria (transformation). The transformed bacteria are identified and cultured. As they divide and express the gene, they transcribe and translate it to produce human insulin, which is harvested and purified.

Markers reward the use of a restriction enzyme to cut both the gene and the vector with matching sticky ends, the role of ligase in joining them, transformation of bacteria, and culturing to express and harvest the protein.

Original4 marksExplain how the polymerase chain reaction is used to make many copies of a specific DNA sequence, referring to the role of temperature changes.
Show worked answer →

The answer should describe the three temperature stages and their purpose.

The polymerase chain reaction (PCR) amplifies a target DNA sequence through repeated cycles of three temperature steps. In denaturation, the mixture is heated to about 95 degrees Celsius to break the hydrogen bonds and separate the two DNA strands.

In annealing, the mixture is cooled to about 55 degrees so that short primers bind by complementary base pairing to the ends of the target sequence on each strand.

In extension, the mixture is warmed to about 72 degrees, the optimum for a heat-stable DNA polymerase, which extends the primers and synthesises new complementary strands.

Each cycle doubles the amount of target DNA, so repeating the cycle many times produces millions of copies. Markers reward the three named steps with their approximate temperatures, the role of primers, the heat-stable polymerase, and the doubling per cycle.

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