Ch 1 · 19.1 Principles of genetic technology · AS & A Level Biology 9700
Topic 1 of 3 in Genetic technology — AS & A Level Biology 9700 — 98 lessons.
Introduction
Genetic technology uses a small toolkit of enzymes and techniques to isolate, copy, cut, join and identify DNA. Paper 4 questions reward precise enzyme names and reasons, and often combine techniques into one extended question, such as making a recombinant plasmid and then checking it. (Syllabus section 19.1.)
Key ideas — Recombinant DNA and genetic engineering
Recombinant DNA is DNA made by joining DNA from two different sources, usually from different species (e.g. a human gene inserted into a bacterial plasmid).
🧑🏫 Teacher's explanation
Imagine you are trying to fix a broken recipe in a cookbook. If you find a single ingredient instruction in a French cookbook that would make your English cake taste better, you might cut that specific page out and paste it into your English cookbook. The resulting 'hybrid' cookbook is like recombinant DNA.
📖 Definitions
🔍 Worked example — Easy
A scientist wants to produce human insulin in bacteria. They extract the mRNA for insulin from a human pancreatic cell. Name the enzyme required to convert this mRNA into complementary DNA (cDNA) and explain why this step is necessary for expression in bacteria.
🔍 Worked example — Standard
A researcher uses a restriction endonuclease to cut a plasmid and a target gene. Both are cut with the same enzyme. Describe the nature of the DNA ends produced and explain why DNA ligase is needed to complete the recombinant DNA molecule.
🔍 Worked example — Exam-style
A PCR reaction is used to amplify a specific DNA sequence. If the initial amount of template DNA is 10 copies, calculate the number of DNA molecules present after 5 cycles of amplification.
⚠️ Common errors
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🎯 Exam question
(a) State the role of Taq polymerase in this process. [1] (b) Explain why the temperature must be cycled between high (approx. 95^°C) and low (approx. 55^°C) during the PCR process. [2] [3 marks]
Key ideas — Getting the gene
The gene to be transferred can be obtained in three ways:
🧑🏫 Teacher's explanation
Think of genetic engineering like editing a movie. If you want to take a specific scene (a gene) from one film (the donor organism) and put it into a different film (the host organism), you can't just cut out a whole reel of film because it contains irrelevant scenes (introns) that the new film's editor won't understand. Instead, you can either find the exact strip of film you need, or you can transcribe the 'script' (mRNA) into a new, streamlined version (cDNA) that only contains the essential dialogue, or even just type out a new script from scratch using a computer (chemical synthesis).
📖 Definitions
🔍 Worked example — Easy
A scientist extracts a single-stranded mRNA molecule that is 1,200 nucleotides long. This mRNA is used as a template by reverse transcriptase to create cDNA. Once the cDNA is double-stranded by DNA polymerase, what is the total number of nucleotides in the resulting double-stranded cDNA molecule?
🔍 Worked example — Standard
Compare the use of DNA extracted directly from a eukaryotic donor versus DNA synthesized from mRNA for the purpose of expressing a protein in a bacterial host. Focus your answer on the presence of introns.
🔍 Worked example — Exam-style
A researcher uses restriction endonucleases to cut a plasmid and a target gene. The restriction enzyme produces 'sticky ends'. Explain why this is advantageous for the production of recombinant DNA compared to using an enzyme that produces 'blunt ends'.
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🎯 Exam question
Key ideas — Cutting, joining and carrying the gene
🧑🏫 Teacher's explanation
Think of genetic engineering like editing a movie. If you want to insert a new scene (a gene) into an existing film (the organism's genome), you first need to cut out the specific scene you want using specialized scissors. Then, you need to cut a gap in the film where the new scene will go. To make sure the new scene sticks perfectly, you use a special glue to seal the edges together.
📖 Definitions
🔍 Worked example — Easy
A scientist uses a restriction endonuclease to cut a plasmid and a target gene. The enzyme recognizes the sequence GAATTC. If the plasmid is cut once, how many fragments are produced?
🔍 Worked example — Standard
A researcher wants to clone a human insulin gene into a bacterial plasmid. The gene is originally found as mRNA. Describe the enzyme required to create the double-stranded DNA template and the role of DNA ligase in the final step.
🔍 Worked example — Exam-style
In a PCR reaction, the temperature is cycled through three stages. If the initial concentration of target DNA is C_0 and the number of cycles completed is n, derive the expression for the final amount of DNA C_n, assuming 100\% efficiency.
⚠️ Common errors
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🎯 Exam question
\n(a) Explain why the same restriction endonuclease must be used to cut both the human DNA and the plasmid. [2] (b) Describe the role of DNA ligase in this process. [1] [3 marks]
Key ideas — Promoters and marker genes
🧑🏫 Teacher's explanation
Imagine you are trying to install a new piece of software (the gene) onto a computer (the host cell). Even if you copy the code perfectly, the computer won't run it unless you also include the 'Start' button (the promoter). Without that specific instruction telling the computer's processor where to begin reading the code, the software just sits there, useless.
📖 Definitions
🔍 Worked example — Easy
A scientist is transferring a human insulin gene into a bacterial cell. The human gene is isolated from mRNA using reverse transcriptase. Why must the scientist also ensure a bacterial promoter is present upstream of the insulin gene?
🔍 Worked example — Standard
In a genetic engineering experiment, a researcher uses Green Fluorescent Protein (GFP) as a marker gene. Describe how this marker gene allows for the selection of successful recombinants.
🔍 Worked example — Exam-style
A researcher is using a tissue-specific promoter to control the expression of a human growth hormone gene in a transgenic animal. Explain the advantage of using a tissue-specific promoter over a constitutive (always on) promoter.
⚠️ Common errors
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🎯 Exam question
(a) Explain why the promoter must be included with the human gene. [2] (b) Describe how the scientist can identify the sheep that have successfully incorporated the recombinant DNA. [2] [4 marks]
Key ideas — Gene editing
Gene editing is a form of genetic engineering in which DNA is inserted, deleted or replaced at a specific site in the genome, rather than a new gene being added at a random position. Enzymes are guided to one precise base sequence and cut the DNA there; the cell's own repair system then rejoins it, either disabling a gene or inserting a supplied piece of DNA. The best-known system is CRISPR–Cas9.
🧑🏫 Teacher's explanation
Imagine you are editing a massive digital document containing billions of letters. Traditional genetic engineering is like copying a paragraph from one document and pasting it into a random location in another; you know the information is there, but you don't know exactly where it landed. Gene editing, however, is like using the 'Find and Replace' function. You search for one specific sequence of letters, and the software goes directly to that exact coordinate to delete, change, or add a word.
📖 Definitions
🔍 Worked example — Easy
A scientist wants to insert a human insulin gene into a bacterial plasmid. The gene is extracted from human mRNA using which enzyme, and why is this preferred over extracting it directly from human genomic DNA?
🔍 Worked example — Standard
During PCR, the temperature is cycled through three stages. If the initial denaturation temperature is 95^° C and the annealing temperature is 55^° C, explain the role of Taq polymerase during the extension phase.
🔍 Worked example — Exam-style
A researcher uses gel electrophoresis to separate DNA fragments. Fragment A is 500 bp and Fragment B is 1500 bp. Describe the relative movement of these fragments and explain why they separate.
⚠️ Common errors
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🎯 Exam question
(a) Define the term 'gene editing'. [1] (b) Explain how the scientist can confirm that the target gene has been successfully modified and is being expressed. [2] [3 marks]
Key ideas — The polymerase chain reaction (PCR)
PCR makes millions of copies of a chosen DNA sequence in a test tube. Each cycle has three stages:
🧑🏫 Teacher's explanation
Imagine you have a single page from a massive book and you need a million copies of it to share with a huge crowd. You cannot manually photocopy it that fast, so you use a 'biological photocopier'.
📖 Definitions
🔍 Worked example — Easy
A scientist starts a PCR reaction with a single copy of a target DNA sequence. If the reaction undergoes 5 complete cycles of denaturation, annealing, and extension, how many copies of the target DNA will be present?
🔍 Worked example — Standard
Calculate the total number of DNA copies produced after 30 cycles of PCR, starting from 1 original molecule. Express your answer in scientific notation.
🔍 Worked example — Exam-style
A researcher observes that after 10 cycles, the DNA concentration has increased significantly. If the initial concentration was 5 × 10^-9 g/ml, what is the theoretical concentration after 10 cycles (assuming all DNA is amplified)?
⚠️ Common errors
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🎯 Exam question
(a) Describe the three temperature stages of a single PCR cycle and the purpose of each. [3] (b) Explain why the use of Taq polymerase is essential for the efficiency of the PCR process. [2] [5 marks]
Key ideas — Gel electrophoresis
DNA is negatively charged because of its phosphate groups. Samples are loaded into wells at one end of an agarose gel in a buffer and a voltage is applied, so the fragments move towards the positive electrode (anode). The gel acts as a mesh: shorter fragments move faster and further than longer ones, so the fragments separate by length. A DNA ladder of fragments of known lengths, run alongside, lets you estimate each fragment's size. The DNA is made visible with a stain or a fluorescent or radioactive label. Uses include checking a PCR product or a plasmid insert, and genetic fingerprinting.
🧑🏫 Teacher's explanation
Imagine a dense forest where you are trying to run through the undergrowth. If you are a small child, you can weave quickly between the trees and move very far in a short time. If you are a large adult, you will get caught on branches and move much more slowly.
📖 Definitions
🔍 Worked example — Easy
A student runs a gel electrophoresis and observes a DNA band. Using a DNA ladder, the student determines the fragment is 500 bp long. If the fragment was instead 1000 bp long, describe its relative position on the gel compared to the 500 bp fragment.
🔍 Worked example — Standard
During electrophoresis, a DNA sample is loaded into a well. The voltage is increased from 50V to 100V. Predict and explain the effect on the migration of the DNA fragments.
🔍 Worked example — Exam-style
A researcher uses gel electrophoresis to verify a PCR product. The expected size of the target gene is 450 bp. The gel shows a band at 450 bp and a faint band at 100 bp. Interpret these results.
⚠️ Common errors
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🎯 Exam question
(a) Explain why fragments of different lengths separate during this process. [2] (b) Describe how the presence of a specific DNA fragment can be visualized after the electrophoresis is complete. [2] [4 marks]
Key ideas — Microarrays
A microarray (DNA chip) is a small slide carrying thousands of tiny spots, each containing many copies of a different short, single-stranded DNA probe of known sequence.
🧑🏫 Teacher's explanation
Imagine a massive library where every book has a unique barcode. If you wanted to find out which books are currently being read by students in a school, you wouldn't read every page. Instead, you would take a sample of the students' notes, turn them into a list of 'tags', and run them through a machine that matches those tags against the barcodes in the library. If a barcode lights up, it means that specific book is being used.
📖 Definitions
🔍 Worked example — Easy
A researcher uses a microarray to compare mRNA levels in healthy lung tissue and lung cancer tissue. The mRNA from healthy tissue is labelled with a green fluorescent dye, and the mRNA from cancer tissue is labelled with a red fluorescent dye. If a specific spot on the microarray shows a bright yellow color, interpret the gene expression level for that gene in both tissue types.
🔍 Worked example — Standard
A microarray chip contains 10,000 unique DNA probes. A sample of genomic DNA is fragmented and hybridised to the chip. If 450 spots show fluorescence after washing, calculate the percentage of the genome represented by these fluorescent spots.
🔍 Worked example — Exam-style
Explain the complete process of using a microarray to detect a specific mutation in a patient's genome.
⚠️ Common errors
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🎯 Exam question
(a) Describe how the researcher can use the microarray to identify genes that are 'up-regulated' (more active) in the cancer cells compared to healthy cells. [3]
Key ideas — Databases
Gene sequences, whole genomes, amino acid sequences and protein structures found by scientists worldwide are stored in online databases (e.g. GenBank, UniProt, the Protein Data Bank). Benefits:
🧑🏫 Teacher's explanation
Imagine you are trying to assemble a massive, complex LEGO castle, but you have lost the instruction manual. Instead of building every single piece from scratch, you go online to a global library where every single LEGO instruction manual ever made is stored. You can search for a specific turret design, see how it was used in a different castle from 1995, or find out which specific bricks are needed to make it stable.
📖 Definitions
🔍 Worked example — Easy
A scientist discovers a new gene in a species of deep-sea fish. The nucleotide sequence is partially known. Explain how using a database of known amino acid sequences would help identify the function of this gene.
🔍 Worked example — Standard
A researcher is designing a new drug to inhibit a specific viral enzyme. Describe how a database of protein structures would be utilized in this process.
🔍 Worked example — Exam-style
A scientist is studying the evolutionary relationship between three species: Species A, B, and C. They have the nucleotide sequences for a specific conserved gene. Outline how databases facilitate this study.
⚠️ Common errors
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🎯 Exam question
Worked example
Question. A student wishes to produce genetically modified bacteria that synthesise human insulin. (a) Explain why the gene is made from mRNA using reverse transcriptase, rather than cut from human chromosomes. [2] (b) The gene is amplified by PCR for 20 cycles, starting from a single copy. Calculate the number of copies at the end, assuming 100% efficiency. [2] (c) Explain how a fluorescent marker gene allows the student to identify bacteria that have taken up the recombinant plasmid. [2]
🔍 Worked example — Easy
A researcher uses reverse transcriptase to synthesize cDNA from a mature mRNA molecule. If the original mRNA sequence was 1,500 nucleotides long, how many nucleotides will the resulting cDNA contain? (Assume no loss of information during synthesis)
🔍 Worked example — Standard
A scientist performs a Polymerase Chain Reaction (PCR) to amplify a specific DNA sequence. If the reaction starts with 5 copies of the target sequence and undergoes 10 complete cycles of amplification with 100% efficiency, calculate the total number of copies produced.
🔍 Worked example — Exam-style
A student is investigating the expression of a human insulin gene in bacteria. The student must choose between using genomic DNA or mature mRNA as a template for creating the recombinant DNA. Explain why using mature mRNA is necessary for successful expression in a prokaryotic host. [2 marks]
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