How can the genome of a microorganism be improved to make useful products?
Improving wild strains of microorganisms by mutagenesis and selective breeding, recombinant DNA technology and the use of plasmids and artificial chromosomes as vectors, the role of restriction endonucleases, ligase and marker genes, and the need for regulatory sequences and import of useful genes.
An SQA Higher Biology answer on the genetic control of metabolism, covering the improvement of microbial strains by mutagenesis and selective breeding, recombinant DNA technology, plasmid and artificial chromosome vectors, restriction endonucleases, ligase and marker genes.
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What this key area is asking
The SQA wants you to explain how wild strains of microorganisms are improved, describe recombinant DNA technology and the vectors used, explain the roles of restriction endonucleases, ligase and marker genes, and explain why regulatory sequences and selectable features are needed for a useful gene to be expressed.
Improving wild strains
Improved mutants are then selected and grown on, keeping only those with the useful new feature. Selective breeding can also gradually improve a strain over generations by choosing the best individuals to reproduce. These methods rely on natural or induced variation, but they are slower and less precise than directly transferring a chosen gene.
Recombinant DNA technology
A vector carries the gene into the host cell:
- Plasmids are small circular pieces of DNA used as vectors, ideal for shorter genes.
- Artificial chromosomes can carry larger pieces of DNA than a plasmid can.
The host is usually a microorganism such as a bacterium, because microorganisms grow quickly and are easy to manage in large numbers.
The enzymes and markers used
For the inserted gene to be expressed, the regulatory sequences that control its transcription must also be transferred, otherwise the gene may never be switched on. The vector also needs an origin of replication so the host can copy it as it divides, and selectable markers so successfully transformed cells can be identified, for example by their survival on a medium containing an antibiotic.
Examples in context
Example 1. Human insulin from bacteria. Before recombinant DNA technology, insulin for diabetics was extracted from pig and cattle pancreas. Now the human insulin gene, together with its regulatory sequences, is inserted into a bacterial plasmid using restriction endonucleases and ligase. The transformed bacteria, identified using a marker gene, are grown in fermenters and produce large amounts of human insulin, which is purer and avoids the risk of allergic reactions to animal insulin.
Example 2. Improving a penicillin-producing mould by mutagenesis. Early strains of Penicillium produced only small amounts of penicillin. By exposing the mould to ultraviolet light and other mutagens and then selecting the mutants that made the most antibiotic, scientists greatly increased the yield over many rounds. This shows how mutagenesis and selection can improve a wild strain without transferring a gene from another organism.
Try this
Q1. State the role of a restriction endonuclease and of ligase in recombinant DNA technology. [2 marks]
- Cue. Restriction endonuclease cuts DNA to leave sticky ends; ligase joins the gene into the vector.
Q2. Explain why a marker gene is included in a vector. [2 marks]
- Cue. It lets scientists identify and select the host cells that have taken up the vector (for example by antibiotic resistance).
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 20195 marksDescribe how a useful gene is inserted into a bacterial plasmid in recombinant DNA technology, naming the enzymes involved and explaining the role of a marker gene.Show worked answer →
A 5-mark answer needs the cutting, joining, enzymes and the marker gene.
The plasmid (the vector) and the source DNA carrying the useful gene are both cut with the same restriction endonuclease. Using the same enzyme leaves matching sticky ends on the gene and the opened plasmid, so they can pair by complementary base pairing.
The enzyme ligase then seals the gene into the plasmid, forming a recombinant plasmid, which is taken up by the host bacterium.
A marker gene, such as an antibiotic resistance gene, is included so that bacteria which have taken up the plasmid can be identified and selected, for example by growing them on a medium containing the antibiotic.
Markers reward the same restriction endonuclease, matching sticky ends, ligase joining the gene, and the marker gene allowing selection.
SQA Higher 20223 marksExplain why transferring only the coding sequence of a useful gene may fail to produce the desired protein, and describe one way wild strains of microorganisms can be improved without recombinant DNA technology.Show worked answer →
A 3-mark answer needs the regulatory sequence point plus an improvement method.
A gene is only expressed if the regulatory sequences that control its transcription are present. If only the coding sequence is transferred, without these regulatory sequences, the gene may not be switched on, so the protein is not made.
Wild strains can also be improved by mutagenesis: mutagenic agents such as ultraviolet light, other radiation or chemicals are used to raise the mutation rate, and improved mutants are then selected and grown on. Selective breeding is another valid method.
Markers reward the need for regulatory sequences and a correct improvement method such as mutagenesis or selective breeding.
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Sources & how we know this
- SQA Higher Biology Course Specification — SQA (2018)