Biology (9700) | Genetic technology

🧬 Genetic Technology (9700 A-Level Biology) Study Notes 🧬

Welcome to the exciting world of Genetic Technology! This is one of the most advanced and fascinating chapters in A-Level Biology, focusing on how scientists actively manipulate DNA—the blueprint of life. Don't worry if these techniques seem complex; we will break down the essential tools and processes step-by-step. Understanding this topic is crucial, as genetic engineering affects everything from modern medicine to sustainable agriculture.

19.1 Principles of Genetic Technology

Genetic technology relies on the ability to isolate, manipulate, and transfer genetic material between organisms. This is often called genetic engineering.

Key Definitions

Genetic Engineering:
The deliberate manipulation of genetic material to modify specific characteristics of an organism. This often involves transferring a gene so that it is expressed in the host organism.

Recombinant DNA (rDNA):
A DNA molecule formed by joining genetic material from two different sources (e.g., a human gene inserted into a bacterial plasmid). Think of it as mixing and matching DNA pieces.

Sources of the Desired Gene (LO 3)

Where does the gene we want to transfer come from?

• Extraction from DNA: The gene is cut directly out of the chromosomal DNA of the donor organism using restriction enzymes.
• Synthesised from mRNA: This is often the preferred method for eukaryotic genes. Scientists use the mRNA (which is already spliced and contains only the coding sequences/exons) and an enzyme called reverse transcriptase to create a complementary strand of DNA (called cDNA).
  Advantage: cDNA lacks the non-coding introns that are found in eukaryotic DNA, making it ready for expression in prokaryotic hosts (like bacteria) which cannot splice introns.
• Synthesised chemically: Short sequences of nucleotides can be built up in the laboratory.

Key Takeaway: cDNA is better for insertion into bacteria because it’s a clean copy of the coding sequence without introns.

The Molecular Toolkit: Roles of Essential Enzymes and Vectors (LO 4 & 5)

To perform genetic engineering, we use biological tools, often specific enzymes and DNA molecules:

1. Restriction Endonucleases:
   Role: These are often called "molecular scissors." They recognise specific short sequences of DNA (called recognition sequences or restriction sites, e.g., GAATTC) and cut the DNA molecule at or near that sequence.
   Result: They can produce "sticky ends" (staggered cuts) or "blunt ends" (straight cuts). Sticky ends are crucial because they allow complementary DNA fragments (from the vector and the desired gene) to temporarily base-pair, making the joining process much easier.
2. DNA Ligase:
   Role: This enzyme acts as "molecular glue." It forms phosphodiester bonds to permanently join the sticky ends of the gene and the vector, creating the final recombinant DNA molecule.
3. Plasmids:
   Role: Small, circular pieces of DNA found naturally in bacteria. They are used as vectors (carriers) to transport the desired gene into the host cell. Plasmids used in genetic technology are engineered to contain restriction sites and marker genes.
4. DNA Polymerase & Reverse Transcriptase:
   Role: DNA Polymerase is key for synthesising new DNA strands (e.g., in PCR). Reverse transcriptase creates DNA from an RNA template (see above).
5. Promoter: (LO 5)
   Role: An essential sequence of DNA located upstream of a gene. It is the binding site for RNA polymerase and acts as the "on switch" that controls when transcription (and thus gene expression) occurs.
   Why is it needed? If you transfer a human gene into bacteria, the bacteria might not recognise the human promoter. You must transfer a bacterial promoter along with the gene to ensure the bacterium actually produces the protein.

Confirming Gene Expression (LO 6)

After transferring a gene, how do we know if the process worked? We use marker genes. These genes code for an easily identifiable product, often something fluorescent (like GFP) or, commonly in plasmids, resistance to an antibiotic (e.g., ampicillin resistance).

• If the host cell survives on an antibiotic medium (because it successfully took up the plasmid containing the resistance marker gene) or glows under UV light, we know the transfer was successful and the gene is being expressed.

Did you know? The first recombinant human protein ever manufactured was insulin, produced in genetically modified bacteria in 1978.

Gene Editing (LO 7)

Gene editing is a precise form of genetic engineering involving targeted modifications at specific locations within the genome.

• It involves the insertion, deletion, or replacement of DNA at precise, predetermined sites in the genome.
• Unlike earlier genetic engineering which involved random insertions, gene editing is like using a word processor's "Find and Replace" function for DNA.

Cloning DNA: The Polymerase Chain Reaction (PCR) (LO 8)

The Polymerase Chain Reaction (PCR) is a technique used to rapidly create millions of copies of a specific DNA sequence, often described as molecular photocopying. It is crucial for forensic science and diagnostic tests.

Key Components: DNA template, DNA primers (short synthetic DNA strands), free DNA nucleotides, and Taq polymerase.

Step-by-Step PCR Process:

1. Denaturation (95 °C): The mixture is heated to separate the double-stranded DNA template into two single strands. (Hydrogen bonds break).
2. Annealing (~50-65 °C): The mixture is cooled. The short DNA primers bind (anneal) to the complementary sequences at the beginning of the target DNA fragment.
3. Extension (72 °C): The temperature is raised slightly to the optimum temperature for the enzyme Taq polymerase. This enzyme is unique because it is heat-stable (isolated from a thermophilic bacterium, Thermus aquaticus). It synthesises new complementary strands starting from the primers.

The cycle repeats 20–35 times, doubling the amount of DNA each time.

Separating DNA Fragments: Gel Electrophoresis (LO 9)

This technique is used to separate DNA fragments (e.g., after restriction cutting or PCR amplification) based on their size.

The Process:

1. DNA fragments (which are naturally negatively charged due to the phosphate backbone) are placed in wells in a slab of gel (usually agarose).
2. An electric current is applied across the gel.
3. DNA moves towards the positive electrode (anode).
4. Separation Principle: Smaller fragments pass more easily through the porous gel matrix and travel further/faster than larger fragments.
5. The fragments are stained and visualised, resulting in a pattern of bands unique to the sample.

Analysis Tools: Microarrays and Databases (LO 10 & 11)

Microarrays:
Microarrays (or gene chips) are slides used to study the expression of thousands of genes simultaneously. They are typically used for:

• Comparing Gene Expression: Measuring and comparing the mRNA levels (and thus protein production rates) in two different cell types (e.g., healthy cells vs. cancer cells).
• Analysis: If a gene is 'on' (highly expressed) in a cancer cell compared to a healthy cell, it will glow brightly on the microarray, providing valuable insight into disease mechanisms.

Benefits of Using Databases:
The vast amount of genomic data generated by these technologies needs to be stored and accessed efficiently.

• Databases provide essential information on:
  — Nucleotide sequences of genes and genomes.
  — Amino acid sequences of proteins.
  — Protein structures (allowing scientists to predict function and design drugs).
• This allows scientists globally to compare their findings, trace evolutionary relationships, and identify potential drug targets.

19.2 Genetic Technology Applied to Medicine

Genetic technology has revolutionized medicine, offering new treatments and diagnostic tools.

Recombinant Human Proteins (LO 1)

One major application is producing large quantities of pure human proteins needed for treating diseases.

Advantages of using Recombinant Human Proteins:

• They are identical to the human version, reducing the risk of allergic reactions (unlike proteins extracted from animals).
• They can be produced in large amounts cheaply (e.g., using yeast or bacterial fermenters).

Examples:

• Insulin: Used to treat Type I diabetes. Previously extracted from pigs, now produced recombinantly in bacteria (E. coli).
• Factor VIII: A clotting protein deficient in people with haemophilia. Recombinant Factor VIII is pure and carries no risk of transmitting blood-borne diseases (unlike factor VIII sourced from donated blood).
• Adenosine Deaminase (ADA): A deficiency causes Severe Combined Immunodeficiency (SCID). Recombinant ADA is used in enzyme replacement therapy.

Genetic Screening (LO 2)

Genetic screening involves analysing an individual’s DNA to determine if they possess specific alleles or mutations linked to disease.

Advantages of Screening:

• Early Detection: Allows for preventative action (e.g., increased monitoring or lifestyle changes).
• Informed Decisions: Helps prospective parents assess the risk of passing on a disease.

Examples:

• Breast Cancer (BRCA1 and BRCA2): Screening identifies women who carry the mutations, allowing them to undergo preventative surgery or intensive screening.
• Huntington's Disease: An inherited neurodegenerative condition. Screening can determine if a person will develop the disease, providing time for psychological and life planning.
• Cystic Fibrosis (CF): Screening allows for early diagnosis, often leading to better management and treatment from birth.

Gene Therapy (LO 3)

Gene therapy aims to cure genetic diseases by replacing defective genes or inactivating harmful ones. It uses a modified virus (the vector) to carry the therapeutic gene into the patient's cells.

Examples:

• Severe Combined Immunodeficiency (SCID): Often caused by a defective ADA gene. Gene therapy can correct the defect in bone marrow cells, allowing the immune system to function.
• Inherited Eye Diseases: Genes responsible for certain forms of blindness can be introduced into cells of the retina to restore sight.

Common Mistake to Avoid: Gene therapy treats the *somatic* cells (body cells) of the affected individual, it is *not* passed on to their offspring.

Social and Ethical Considerations in Medicine (LO 4)

The power of genetic technology comes with serious ethical questions:

• Privacy and Discrimination: Who owns your genetic data? Could employers or insurance companies discriminate against you based on your risk profile?
• Safety: Are viral vectors used in gene therapy truly safe? Could inserted genes cause unintended mutations?
• Accessibility: These treatments are often extremely expensive. Is it fair if only the wealthy can afford life-saving gene therapy?
• Designer Babies: Where do we draw the line between treating a disease (therapy) and enhancing traits (e.g., intelligence, height)? This is known as the "slippery slope" argument.

19.3 Genetically Modified Organisms (GMOs) in Agriculture

Genetic engineering is used to improve the quality and productivity of farmed animals and crop plants, helping to meet the global demand for food (LO 1).

Improving Quality and Productivity (LO 1)

Examples in Agriculture:

1. GM Salmon: Modified to produce growth hormone year-round, leading to faster growth rates and reaching market size more quickly, thus improving productivity.
2. Herbicide Resistance (e.g., GM Soybean): These plants contain a gene that makes them resistant to specific broad-spectrum herbicides (weed killers). Farmers can spray the entire field, killing the weeds but leaving the crop unharmed.
3. Insect Resistance (e.g., GM Cotton): These plants are engineered to produce a toxin (often from the bacterium Bacillus thuringiensis, or Bt) that is lethal to certain insect pests (like cotton bollworm) but harmless to humans and most non-target insects. This reduces the need for chemical pesticides.

Ethical and Social Implications of GMOs (LO 2)

The use of GM organisms in food production is widely debated due to concerns over their long-term impact:

• Environmental Risks:
  — Gene Flow: The potential for genes (e.g., herbicide resistance) to transfer to wild relatives via cross-pollination, creating "superweeds" that are difficult to control.
  — Biodiversity Loss: Insect-resistant crops might harm non-target beneficial insects (like monarch butterflies) or reduce the overall genetic diversity of native plant populations.
• Health Concerns:
  — Potential unknown effects of consuming novel proteins produced by GM crops (though current evidence suggests GM foods are safe).
  — Development of allergies or antibiotic resistance (if antibiotic marker genes were used in the development process).
• Social/Economic Concerns:
  — Corporate Control: Most GM seed production is dominated by a few large companies, raising concerns about farmers' dependence on them.
  — Labelling: Debates over whether GM products should be clearly labelled to allow consumers informed choice.