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Once a gene is inside a plasmid, how do you get bacteria to take it up and make millions of copies?

Describe how host cells are transformed with recombinant DNA and how transformed cells are identified and grown

A focused answer to the O-Level outcome on transformation and cloning. Getting recombinant plasmids into bacteria, using marker genes to select them, and growing them to express the gene.

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
  2. The answer
  3. Examples in context
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What this dot point is asking

This outcome asks you to describe how a recombinant plasmid is put into host bacteria (transformation), how the bacteria that took it up are identified (selection using marker genes), and how they are then grown to produce the gene's protein. It is the stage that turns a gene-in-a-plasmid into a working production system.

The answer

Transformation

Transformation is the process by which bacteria take up the recombinant plasmid from their surroundings.

  • The plasmids are mixed with bacteria under conditions that encourage uptake (for example a brief heat shock or chemical treatment).
  • Only some of the bacteria take up a plasmid, so the next step is to find which ones did.

Identifying transformed cells with marker genes

Because not every bacterium is transformed, plasmids carry a marker gene that makes the successful ones detectable:

  • A common marker is an antibiotic-resistance gene.
  • The bacteria are grown on a medium containing the antibiotic.
  • Only bacteria that took up the plasmid have the resistance gene, so only they survive and grow; the rest die.

This way, the colonies that grow are the transformed bacteria.

Cloning and expression

The selected transformed bacteria are then cultured in large numbers, for example in a bioreactor with nutrients at a suitable temperature and pH.

  • As the bacteria divide, they copy the plasmid, so the gene is cloned (multiplied) along with the cells.
  • The bacteria express the gene, transcribing and translating it to make the protein.
  • The protein is then harvested and purified.

So one transformed cell becomes billions of identical cells, all making the desired product.

Examples in context

Example 1. Insulin production line. Recombinant plasmids carrying the insulin gene are taken up by bacteria, the transformed cells are selected on an antibiotic medium, and a single colony is grown into a huge culture that pumps out insulin. Transformation and selection are the gateway to large-scale production.

Example 2. Why colonies are pure. Because each colony on the plate grows from one transformed cell dividing repeatedly, every cell in it is genetically identical, a clone. Picking one colony therefore gives a pure, reliable starting culture for production.

Try this

Q1. Define transformation as used in genetic engineering. [1 mark]

  • Cue. The process by which bacteria take up foreign DNA, such as a recombinant plasmid, from their surroundings.

Q2. Explain how an antibiotic-resistance marker gene is used to identify transformed bacteria. [2 marks]

  • Cue. The bacteria are grown on a medium containing the antibiotic; only those that took up the plasmid have the resistance gene and survive, so they can be identified.

Q3. State what happens to the gene as the transformed bacteria multiply in a bioreactor. [2 marks]

  • Cue. The plasmid is copied as the cells divide, so the gene is cloned, and the bacteria express it to produce the protein.

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 the steps by which bacteria are transformed with a recombinant plasmid and the transformed bacteria are then grown to produce a useful protein.
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Examiners want transformation, selection and culturing in order.

First, the recombinant plasmids (each carrying the gene) are mixed with bacteria under conditions that make the bacteria take up the plasmids, a process called transformation. Only some bacteria take up a plasmid.

Next, the bacteria are grown on a medium that allows the transformed bacteria to be identified, often using a marker gene on the plasmid such as antibiotic resistance: only bacteria that took up the plasmid survive on a medium containing the antibiotic.

The selected transformed bacteria are then cultured in large numbers, for example in a bioreactor with nutrients at a suitable temperature and pH. As they divide, they copy the plasmid and express the gene, producing the protein, which is then harvested and purified.

What markers reward: mixing plasmids with bacteria for uptake (transformation), using a marker gene to select transformed bacteria, and culturing them at scale so they multiply, express the gene and make the protein for harvesting.

Original4 marksExplain the purpose of a marker gene, such as an antibiotic-resistance gene, on a plasmid used in genetic engineering.
Show worked answer →

The answer should explain how the marker lets scientists identify successful transformation.

Not all bacteria take up the plasmid during transformation, so scientists need a way to tell which ones did. A marker gene on the plasmid, such as a gene for antibiotic resistance, provides this.

If the bacteria are grown on a medium containing the antibiotic, only those that took up the plasmid (and so have the resistance gene) survive and grow. Bacteria without the plasmid die. This allows the successfully transformed bacteria to be identified and selected.

What markers reward: the point that only some bacteria take up the plasmid, that the marker gene (for example antibiotic resistance) lets transformed bacteria survive a selective medium while others die, so the transformed cells can be identified and selected.

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