In simple terms
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Principles of genetic technology
Cambridge 9700 Paper 4 — Principles of genetic technology (19.1). A-Level Notes diagram-backed lesson with premium structure and live visuals.
- 1
1. Isolation of the Gene of Interest: The desired gene is identified and isolated from the source organism's DNA. This can be done using restriction enzymes to cut it out from the genome, or by using reverse transcriptase to create a complementary DNA (cDNA) copy from an mRNA template. Using mRNA is advantageous as it has already had the non-coding introns removed.
- 2
2. Insertion into a Vector: The isolated gene is inserted into a vector (e.g., a plasmid). The same restriction enzyme is used to cut both the gene and the plasmid, creating complementary 'sticky ends'. The gene anneals to the plasmid, and DNA ligase is used to form permanent phosphodiester bonds, creating a recombinant plasmid.
- 3
3. Transformation: The recombinant vector is introduced into a host organism (usually bacteria like E. coli or yeast). This process is called transformation. Methods to increase the permeability of the host cell membrane, such as heat shock or electroporation, are often used.
- 4
4. Identification and Selection: Not all host cells will successfully take up the vector. Marker genes (e.g., for antibiotic resistance or fluorescence) included in the vector are used to identify and select the transformed cells. For example, if the plasmid carries a gene for ampicillin resistance, only transformed bacteria will grow on a medium containing ampicillin.
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 19.1.1
Define the term recombinant DNA
- 19.1.2
Explain that genetic engineering is the deliberate manipulation of genetic material to modify specific characteristics of an organism and that this may involve transferring a gene into an organism so that the gene is expressed
- 19.1.3
Explain that genes to be transferred into an organism may be: • extracted from the DNA of a donor organism • synthesised from the mRNA of a donor organism • synthesised chemically from nucleotides
- 19.1.4
Explain the roles of restriction endonucleases, DNA ligase, plasmids, DNA polymerase and reverse transcriptase in the transfer of a gene into an organism
- 19.1.5
Explain why a promoter may have to be transferred into an organism as well as the desired gene
- 19.1.6
Explain how gene expression may be confirmed by the use of marker genes coding for fluorescent products
- 19.1.7
Explain that gene editing is a form of genetic engineering involving the insertion, deletion or replacement of DNA at specific sites in the genome
- 19.1.8
Describe and explain the steps involved in the polymerase chain reaction (PCR) to clone and amplify DNA, including the role of Taq polymerase
- 19.1.9
Describe and explain how gel electrophoresis is used to separate DNA fragments of different lengths
- 19.1.10
Outline how microarrays are used in the analysis of genomes and in detecting mRNA in studies of gene expression
- 19.1.11
Outline the benefits of using databases that provide information about nucleotide sequences of genes and genomes, and amino acid sequences of proteins and protein structures
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Full topic notes
Formal explanation with the rigour you need for the exam.
Recombinant DNA Technology: The Core Principles
Recombinant DNA (rDNA) technology refers to the process of joining DNA molecules from two different species and inserting them into a host organism. This process aims to produce new genetic combinations that are of significant value to science, medicine, agriculture, and industry. The fundamental idea is to take a gene of interest and insert it into a 'vector' (often a plasmid or virus), which then carries it into a host cell to be replicated and expressed.
The Process of Creating a Recombinant Organism
1. Isolation of the Gene of Interest: The desired gene is identified and isolated from the source organism's DNA. This can be done using restriction enzymes to cut it out from the genome, or by using reverse transcriptase to create a complementary DNA (cDNA) copy from an mRNA template. Using mRNA is advantageous as it has already had the non-coding introns removed.
2. Insertion into a Vector: The isolated gene is inserted into a vector (e.g., a plasmid). The same restriction enzyme is used to cut both the gene and the plasmid, creating complementary 'sticky ends'. The gene anneals to the plasmid, and DNA ligase is used to form permanent phosphodiester bonds, creating a recombinant plasmid.
3. Transformation: The recombinant vector is introduced into a host organism (usually bacteria like E. coli or yeast). This process is called transformation. Methods to increase the permeability of the host cell membrane, such as heat shock or electroporation, are often used.
4. Identification and Selection: Not all host cells will successfully take up the vector. Marker genes (e.g., for antibiotic resistance or fluorescence) included in the vector are used to identify and select the transformed cells. For example, if the plasmid carries a gene for ampicillin resistance, only transformed bacteria will grow on a medium containing ampicillin.
5. Expression and Harvesting: The transformed host cells are cultured in large quantities (e.g., in a fermenter). Under the right conditions, they will express the inserted gene, producing the desired protein (e.g., human insulin). The protein is then extracted, purified, and harvested.
The Genetic Engineer's Toolkit
Enzymes: Molecular Scissors and Glue
Enzymes are the precise molecular tools and workhorses of genetic technology, each with a specific and crucial function:
Vectors and Markers: Transport and Identification
To successfully transfer and express genes, we need delivery vehicles (vectors) and methods to find the successfully modified cells (markers).
Vectors: These are DNA molecules that can carry foreign genetic material into a host cell where it can be replicated and/or expressed.
- Plasmids: Small, circular, double-stranded DNA molecules found naturally in bacteria. They replicate independently of the bacterial chromosome and often carry genes for antibiotic resistance. Key features for effective cloning include an origin of replication, a multiple cloning site (MCS) containing recognition sites for various restriction enzymes, and one or more selectable marker genes.
- Viruses: Bacteriophages (viruses that infect bacteria) or other viruses (like retroviruses or adenoviruses) can be engineered to deliver genes. They are highly efficient at injecting their genetic material into host cells naturally.
Markers: Genes incorporated into vectors that allow scientists to distinguish transformed cells (those that have successfully taken up the vector) from untransformed cells.
- Antibiotic Resistance Genes: If a plasmid contains a gene for resistance to a specific antibiotic (e.g., ampicillin), only host cells that have successfully taken up the plasmid will survive when grown on a selective medium containing that antibiotic.
- Fluorescent Markers: Genes encoding naturally fluorescent proteins (e.g., Green Fluorescent Protein, GFP) can be used. The GFP gene is often linked to the desired gene under the same promoter, ensuring that cells expressing the desired gene also fluoresce. Transformed cells will visibly glow under UV light, allowing for easy identification.
- Enzyme Markers: Genes coding for enzymes that produce a visible colour change (e.g., β-galactosidase, used in blue-white screening) are also employed to identify successful transformations.
Restriction Endonucleases: These are molecular 'scissors' that recognise specific, short DNA sequences (recognition sites, often palindromic) and cleave the phosphodiester backbone of the DNA at or near these sites. They produce either 'blunt ends' (straight cuts) or 'sticky ends' (overhanging single-stranded sequences). Sticky ends are particularly valuable as they can anneal (base-pair) with complementary sticky ends from other DNA fragments, facilitating the joining of different DNA pieces.
DNA Ligase: This enzyme acts as the molecular 'glue'. It catalyses the formation of phosphodiester bonds between the sugar-phosphate backbones of adjacent nucleotides, effectively joining DNA fragments together. It is indispensable for inserting the desired gene into the vector.
Reverse Transcriptase: This unique enzyme synthesises a complementary DNA (cDNA) strand from an mRNA template. This is exceptionally useful for isolating eukaryotic genes, as mRNA templates lack non-coding introns, making it easier to express the gene correctly in prokaryotic hosts which cannot splice introns.
Polymerase Chain Reaction (PCR)
The Polymerase Chain Reaction (PCR) is an incredibly powerful and widely used technique employed to amplify (make many copies of) specific DNA sequences exponentially in a test tube. It's often likened to a biological photocopying machine for DNA, capable of creating millions or billions of copies from just a tiny starting sample.
Principle: PCR relies on repeated cycles of heating and cooling to denature DNA, anneal primers, and extend new DNA strands using a heat-stable DNA polymerase. Each cycle effectively doubles the amount of target DNA.
Applications: PCR is indispensable in numerous fields, including forensics (for DNA fingerprinting and identification), medical diagnostics (detecting pathogens like viruses or bacteria, or identifying genetic diseases), gene cloning, evolutionary studies, and paternity testing.
Denaturation (94-96°C): The double-stranded DNA template is heated to break the hydrogen bonds between complementary base pairs, separating it into two single strands.
Annealing (50-65°C): The temperature is lowered, allowing primers (short, synthetic DNA sequences, typically 18-24 nucleotides long, complementary to the target DNA region) to bind to the single-stranded DNA templates.
Extension (72°C): The temperature is raised slightly to the optimal working temperature for Taq polymerase (a heat-stable DNA polymerase). Taq polymerase synthesises new DNA strands by adding nucleotides complementary to the template, starting from the bound primers.
These three steps constitute one cycle. The entire process is repeated for 20-40 cycles, leading to exponential amplification of the target DNA sequence, generating billions of copies in just a few hours.
Gel Electrophoresis: Separating Molecules by Size
Gel electrophoresis is a standard laboratory technique used to separate macromolecules like DNA, RNA, and proteins based on their size and electrical charge. It is an essential tool for analysing the results of PCR, restriction digests, and for DNA fingerprinting.
Principle:
- A gel matrix, typically made of agarose (for DNA) or polyacrylamide (for proteins), acts as a molecular sieve.
- Samples are loaded into wells at one end of the gel.
- An electric current is applied across the gel. Since the phosphate backbone gives DNA and RNA a uniform negative charge, they migrate towards the positive electrode (anode).
- Shorter molecules travel more easily and quickly through the pores of the gel matrix, moving further than longer molecules in a given amount of time. This separates the fragments by size.
- A DNA ladder, containing fragments of known sizes, is run alongside the samples to allow for size estimation of the unknown fragments.
Separating Proteins: Protein separation is more complex as their charge depends on the R-groups of their amino acids and the pH of the buffer. Typically, proteins are treated with a detergent like SDS (sodium dodecyl sulfate) to give them a uniform negative charge, allowing them to be separated primarily by mass. The pH is kept constant using a buffer solution.
Visualisation: After separation, the molecules are invisible. DNA is typically stained with a fluorescent dye (like ethidium bromide or SYBR Green) that binds to it and glows under UV light, revealing bands corresponding to different fragment sizes.
Applications: Key uses include DNA fingerprinting (analysing variable number tandem repeats, VNTRs), checking the success of a PCR reaction or restriction digest, and separating different protein variants (e.g., haemoglobin A vs. haemoglobin S).
Gene Sequencing
Gene sequencing is the process of determining the precise order of nucleotides (adenine, guanine, cytosine, and thymine) within a DNA molecule. Knowing the sequence of a gene or an entire genome is fundamental to understanding its function, identifying mutations, and designing effective genetic technologies.
- Sanger Sequencing (Chain Termination Method): This 'first-generation' sequencing method involves DNA replication using dideoxynucleotides (ddNTPs), which lack a 3'-hydroxyl group and thus terminate DNA synthesis when incorporated. Fragments of varying lengths are generated, and their order is determined to read the sequence. While still used for smaller, targeted projects, it has largely been superseded by newer technologies.
- Next-Generation Sequencing (NGS): This refers to a suite of high-throughput technologies that allow for rapid, cost-effective, and massively parallel sequencing of entire genomes or large numbers of genes simultaneously. These methods typically involve sequencing millions of short DNA fragments in parallel and then computationally assembling the data to reconstruct the full sequence. NGS has revolutionised genomics, enabling projects like the Human Genome Project and vast applications in personalized medicine.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
Explain how sticky ends are produced by restriction enzymes and why they are important in forming recombinant DNA.
- 1
Restriction endonucleases recognise specific, short, palindromic nucleotide sequences (known as recognition sites) on a double-stranded DNA molecule.
A scientist starts a PCR reaction with 15 molecules of a target double-stranded DNA sequence. Assuming 100% efficiency, calculate the number of copies of the target sequence after 25 cycles.
- 1
The number of DNA molecules produced by PCR increases exponentially. The formula to calculate the number of copies (N) after a certain number of cycles (n) is:
How it all connects
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Glossary
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Revision flashcards
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What is the primary goal of recombinant DNA technology?
To join DNA molecules from two different species and insert them into a host organism to create new genetic combinations, often to produce a specific protein or confer a new trait.
Key takeaways
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- ✓
1. Isolation of the Gene of Interest: The desired gene is identified and isolated from the source organism's DNA. This can be done using restriction enzymes to cut it out from the genome, or by using reverse transcriptase to create a complementary DNA (cDNA) copy from an mRNA template. Using mRNA is advantageous as it has already had the non-coding introns removed.
- ✓
2. Insertion into a Vector: The isolated gene is inserted into a vector (e.g., a plasmid). The same restriction enzyme is used to cut both the gene and the plasmid, creating complementary 'sticky ends'. The gene anneals to the plasmid, and DNA ligase is used to form permanent phosphodiester bonds, creating a recombinant plasmid.
- ✓
3. Transformation: The recombinant vector is introduced into a host organism (usually bacteria like E. coli or yeast). This process is called transformation. Methods to increase the permeability of the host cell membrane, such as heat shock or electroporation, are often used.
- ✓
4. Identification and Selection: Not all host cells will successfully take up the vector. Marker genes (e.g., for antibiotic resistance or fluorescence) included in the vector are used to identify and select the transformed cells. For example, if the plasmid carries a gene for ampicillin resistance, only transformed bacteria will grow on a medium containing ampicillin.
- ✓
5. Expression and Harvesting: The transformed host cells are cultured in large quantities (e.g., in a fermenter). Under the right conditions, they will express the inserted gene, producing the desired protein (e.g., human insulin). The protein is then extracted, purified, and harvested.
Practice — then mark it
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9700/42 · Q3(c)(ii)
Another method being investigated to treat LCA10 is to use a gene editing tool known as the CRISPR/Cas9 system. The CRISPR/Cas9 system uses a short length of RNA called guide RNA. Guide RNA is complementary to the target DNA and is linked to a nuclease enzyme called Cas9. Cas9 breaks phosphodiester bonds in DNA. The cell repair mechanisms repair the cut in DNA after the modification has taken place. • A vector delivers Cas9 and two specific guide RNAs to the photoreceptor cells. • They act on the section of DNA which contains the mutation. • Exon X is no longer added to the mRNA. Explain how this method used to treat LCA10 is an example of gene editing.
9700/42 · Q5(d)
Recombinant human insulin analogues are insulin proteins that have slightly altered amino acid sequences compared with recombinant human insulin. These analogues can be more effective than human insulin. Synthetic genes coding for insulin analogues have been developed. The bacterium Escherichia coli can be used as a host for a synthetic gene for the large-scale manufacture of an analogue. When scientists have determined the changes that are needed to produce an insulin analogue, they can obtain a synthetic gene coding for the analogue by making changes to a length of DNA using genetic engineering. Suggest how scientists genetically engineer a synthetic gene coding for the insulin analogue and explain how the changes they make allow the correct analogue to be produced.
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