Biotechnology

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Recombinant DNA technology involves combining DNA from different sources to create new, artificial DNA molecules that can be introduced into host organisms. This is the basis of genetic engineering and genetic modification (GM).

Restriction endonucleases (restriction enzymes) cut DNA at specific recognition sequences. These sequences are 4—8 base pairs long and palindromic (the sequence reads the same on both strands in the 5” to 3’ direction).

Enzyme	Recognition Sequence	Cut Type	Sticky Ends
EcoRI	5’-GAATTC-3’	Staggered	AATT
BamHI	5’-GGATCC-3’	Staggered	GATC
HindIII	5’-AAGCTT-3’	Staggered	AGCT
SmaI	5’-CCCGGG-3’	Blunt	None

Staggered cuts produce sticky ends (single-stranded overhangs) that are complementary and can base-pair with any DNA cut with the same enzyme. Blunt cuts produce flat ends that can be joined to any other blunt-ended DNA.

DNA ligase joins DNA fragments by catalysing the formation of phosphodiester bonds between the $3'$-OH of one fragment and the $5'$-phosphate of another. T4 DNA ligase can join both sticky-end and blunt-end fragments.

Vectors are DNA molecules used to carry foreign DNA into host cells:

Vector Type	Capacity	Advantages	Disadvantages
Plasmid	$< 10\ \mathrm{kb}$	Easy to manipulate; high copy number	Limited capacity
Bacteriophage ($\lambda$)	$< 25\ \mathrm{kb}$	Efficient delivery to E. Coli	More complex handling
BAC (bacterial artificial chromosome)	$100$—$300\ \mathrm{kb}$	Large inserts	Low copy number
YAC (yeast artificial chromosome)	$100$—$2000\ \mathrm{kb}$	Very large inserts; eukaryotic processing	Unstable; low yield

1. Isolation: the target gene is located and cut from donor DNA using a restriction enzyme.
2. Vector preparation: the same restriction enzyme cuts the plasmid vector at a specific site (e.g., within a gene for antibiotic resistance or a reporter gene like lacZ).
3. Ligation: the target gene and the cut plasmid are mixed with DNA ligase. The complementary sticky ends anneal by base pairing, and DNA ligase seals the nicks, producing a recombinant plasmid.
4. Transformation: the recombinant plasmid is introduced into host bacterial cells by:

• Heat shock: cells are chilled ($4\ ^\circ\mathrm{C}$), mixed with plasmid DNA, briefly heated ($42\ ^\circ\mathrm{C}$90 seconds), and placed on ice. The temperature change makes the membrane permeable to DNA.
• Electroporation: an electric field creates temporary pores in the membrane.

5. Selection: cells are grown on agar plates containing an antibiotic. Only cells that have taken up the plasmid (with the antibiotic resistance gene) survive.
6. Identification: further screening identifies cells with the recombinant plasmid (containing the target gene inserted into the lacZ gene, disrupting it). White colonies (non-functional lacZ) indicate successful insertion; blue colonies (functional lacZ) indicate the plasmid without insertion.

• Recombinant human insulin: produced by E. Coli with the human insulin gene. Structurally identical to human insulin, eliminating immune reactions associated with animal-derived insulin.
• Factor VIII: blood clotting factor for treating haemophilia, produced by genetically modified mammalian cells (which perform the necessary post-translational modifications).
• GM crops: herbicide resistance (e.g., BAR gene), pest resistance (Bt toxin gene), nutritional enhancement (Golden Rice with beta-carotene).

PCR amplifies a specific DNA sequence in vitro. For details of the basic mechanism, see Genetics and DNA.

Real-time PCR (qPCR) allows quantification of the initial amount of DNA in a sample by measuring the accumulation of amplified DNA during the PCR cycles, in real time.

Methods of detection:

• SYBR Green: a fluorescent dye that intercalates into double-stranded DNA. Fluorescence increases proportionally to the amount of dsDNA produced. Simple but non-specific (binds to any dsDNA, including primer-dimers).
• TaqMan probes: sequence-specific oligonucleotide probes labelled with a fluorescent reporter at one end and a quencher at the other. When the probe is intact, the quencher absorbs the reporter’s fluorescence. During extension, the probe is cleaved by the $5' \to 3'$ exonuclease activity of Taq polymerase, separating reporter from quencher and allowing fluorescence. Highly specific.

The $C_t$ value (threshold cycle) is the PCR cycle at which the fluorescence exceeds a defined threshold. It is inversely proportional to the logarithm of the initial template quantity:

$C_t \propto -\log(\text{initial template amount})$

A lower $C_t$ value indicates more initial template.

Worked Example. A qPCR assay gives $C_t = 25$ for a sample and $C_t = 28$ for a standard with known concentration of $10^4$ copies. If each $C_t$ difference of 1 represents a doubling:

Number of copies in sample $= 10^4 \times 2^{(28-25)} = 10^4 \times 2^3 = 10^4 \times 8 = 80000$ copies.

RT-PCR is used to amplify RNA. First, reverse transcriptase converts the RNA template into complementary DNA (cDNA). The cDNA is then amplified by standard PCR. This is essential for studying gene expression (mRNA levels), detecting RNA viruses (e.g., SARS-CoV-2, HIV), and analysing transcriptomes.

DNA fragments are separated by size through an agarose gel under an electric field. For details, see Genetics and DNA.

SDS-PAGE (sodium dodecyl sulfate polyacrylamide gel electrophoresis) separates proteins by molecular mass:

1. Proteins are denatured by heating with SDS (which gives them a uniform negative charge) and $\beta$-mercaptoethanol (which breaks disulfide bridges).
2. The denatured proteins are loaded into wells in a polyacrylamide gel.
3. An electric field is applied; proteins migrate towards the positive electrode.
4. Smaller proteins travel faster through the gel matrix.
5. A protein ladder (proteins of known molecular mass) is used to estimate the mass of unknown proteins.

Technique	Target	Probe/Antibody	Purpose
Southern blot	DNA fragments	Radioactive DNA probe	Identify specific DNA sequences
Northern blot	RNA fragments	Radioactive DNA probe	Measure gene expression (mRNA levels)
Western blot	Proteins	Specific antibody	Detect specific proteins

Developed by Frederick Sanger (1977). For details, see Genetics and DNA.

NGS methods enable massively parallel sequencing of millions of DNA fragments simultaneously:

• Illumina sequencing: DNA fragments are attached to a flow cell and amplified by bridge PCR to form clusters. Sequencing-by-synthesis uses fluorescently labelled reversible terminators; each cycle adds one nucleotide and captures an image.
• Pyrosequencing (454): detects the release of pyrophosphate ($\mathrm{PP_i}$) when a nucleotide is incorporated, which is converted to light by a cascade of enzymatic reactions.

NGS has dramatically reduced the cost and time required for genome sequencing (the Human Genome Project took 13 years and cost $2.7 billion; today a human genome can be sequenced in hours for under$1000).

Gene therapy involves introducing functional copies of a gene into a patient’s cells to compensate for a defective gene. It is a potential treatment for genetic disorders caused by single-gene defects (e.g., cystic fibrosis, severe combined immunodeficiency, sickle cell anaemia).

Somatic gene therapy: the functional gene is introduced into the patient’s body cells (not gametes). The changes are not inherited.

• In vivo: the gene is delivered directly to the patient’s body using a viral vector (e.g., adenovirus, lentivirus) or a non-viral method (liposomes, nanoparticles).
• Ex vivo: cells are removed from the patient, the functional gene is introduced in the laboratory (using a viral vector), the modified cells are selected and expanded, and then returned to the patient.

Germ line gene therapy: the functional gene is introduced into gametes or early embryos, producing changes that are inherited by future generations. This is currently illegal in most countries due to ethical concerns and the risk of unintended consequences.

Challenges of gene therapy:

• Delivering the gene to the correct cells and ensuring it is expressed at the right level.
• Immune response to the viral vector.
• The therapeutic effect may be temporary (if the modified cells are not stem cells and do not divide indefinitely).
• Risk of insertional mutagenesis (the viral DNA may insert near an oncogene, activating it).
• Ethical concerns about enhancement (designer babies).

Stem cells are undifferentiated cells with the capacity for self-renewal (mitosis producing identical stem cells) and differentiation into specialised cell types.

Type	Source	Potency	Ethical Issues
Embryonic (ESC)	Inner cell mass of blastocyst (5—7 days)	Totipotent/pluripotent	Significant (destruction of embryo)
Adult	Bone marrow, adipose tissue, brain	Multipotent	Fewer
Induced pluripotent (iPSC)	Adult cells reprogrammed with transcription factors (Oct4, Sox2, Klf4, c-Myc)	Pluripotent	Minimal (no embryos)

Therapeutic cloning (somatic cell nuclear transfer, SCNT): the nucleus from a patient’s somatic cell is transferred into an enucleated egg cell. The resulting embryo is allowed to develop to the blastocyst stage, and embryonic stem cells are harvested. These cells are genetically identical to the patient and can be directed to differentiate into the required cell type for transplantation (e.g., insulin-producing beta cells for Type 1 diabetes, dopaminergic neurons for Parkinson’s disease). The embryo is not implanted into a uterus.

Ethical considerations of stem cell research:

• Embryonic stem cells require the destruction of human embryos (some consider this morally equivalent to taking a life).
• Therapeutic cloning creates embryos that are destroyed after stem cell extraction.
• iPSCs offer an ethically less contentious alternative but are technically more challenging (lower efficiency, risk of incomplete reprogramming).
• The potential for stem cells to be used for enhancement rather than therapy raises ethical concerns about equity and access.

CRISPR-Cas9 (Clustered Regularly Interspaced Short Palindromic Repeats) is a gene editing tool derived from a bacterial immune defence system.

Components:

1. Cas9 protein: an endonuclease enzyme that cuts double-stranded DNA.
2. Guide RNA (gRNA): a 20-nucleotide RNA sequence complementary to the target DNA sequence. The gRNA guides Cas9 to the correct location.
3. PAM sequence: a short DNA sequence (5’-NGG-3’ for Streptococcus pyogenes Cas9) immediately adjacent to the target. Cas9 requires a PAM to bind and cut.

Mechanism:

1. The gRNA is designed to be complementary to the target DNA sequence.
2. The gRNA-Cas9 complex searches the genome for the target sequence adjacent to a PAM.
3. When the gRNA base-pairs with the target DNA and the PAM is recognised, Cas9 cuts both strands of the DNA, creating a double-strand break (DSB).
4. The cell repairs the DSB by one of two mechanisms:

• Non-homologous end joining (NHEJ): the broken ends are joined directly, often introducing small insertions or deletions (indels) that can disrupt the gene (gene knockout).
• Homology-directed repair (HDR): if a donor DNA template with the desired sequence is provided, the cell uses it as a template to repair the break, introducing the desired sequence (gene knock-in or correction).

• Gene knockout: disrupting a gene to study its function (research) or to disable a disease-causing gene (therapy).
• Gene correction: repairing a disease-causing mutation in situ (e.g., sickle cell anaemia, beta-thalassaemia).
• Gene regulation: fusing a catalytically dead Cas9 (dCas9) with transcriptional activators or repressors to up- or down-regulate gene expression without cutting DNA.
• Diagnostics: SHERLOCK and DETECTR use Cas13/Cas12 to detect specific RNA/DNA sequences with high sensitivity, applied to pathogen detection (e.g., SARS-CoV-2).

• Off-target effects: Cas9 may cut at sites with partial homology to the gRNA, causing unintended mutations.
• Mosaicism: in embryos, the edit may not occur in all cells, producing individuals with a mixture of edited and unedited cells.
• Germline editing: edits to germ cells or embryos are heritable. The 2018 case of He Jiankui, who edited the CCR5 gene in human embryos to confer HIV resistance, was widely condemned.
• Enhancement: CRISPR could be used for non-therapeutic enhancement (e.g., increased intelligence, athletic ability), raising concerns about equity and “designer babies.”

Bioinformatics is the application of computational tools to store, analyse, and interpret biological data, particularly DNA and protein sequences.

• Sequence alignment: comparing DNA or protein sequences to identify regions of similarity (homology). Tools: BLAST (Basic Local Alignment Search Tool), ClustalW.
• Genome annotation: identifying genes, regulatory elements, and other functional features within a genome sequence.
• Phylogenetic analysis: constructing evolutionary trees from sequence data.
• Protein structure prediction: using algorithms (e.g., AlphaFold) to predict the 3D structure of proteins from their amino acid sequence.
• Genome-wide association studies (GWAS): scanning genomes of large populations to identify genetic variants associated with diseases or traits.

Database	Content
GenBank	Nucleotide sequences
UniProt	Protein sequences and function
OMIM	Human genes and genetic disorders
Ensembl	Vertebrate genomes
PDB	Protein 3D structures

The human microbiome is the collective genome of all microorganisms (bacteria, archaea, fungi, viruses) that live on and in the human body. The human gut microbiome alone contains approximately $3 \times 10^{13}$ bacterial cells — roughly equal to the number of human cells in the body.

• Digestion: gut bacteria ferment dietary fibre to produce short-chain fatty acids (SCFAs) such as butyrate, propionate, and acetate, which provide energy for colonocytes and have anti-inflammatory effects.
• Immune system development: exposure to gut bacteria in early life is essential for normal immune system maturation. The microbiome helps train the immune system to distinguish harmless commensals from pathogens.
• Vitamin synthesis: gut bacteria produce vitamin K and some B vitamins.
• Protection against pathogens: commensal bacteria compete with pathogens for nutrients and attachment sites, and produce antimicrobial substances (bacteriocins).
• Metabolic health: the composition of the gut microbiome is linked to obesity, type 2 diabetes, and cardiovascular disease.

Dysbiosis (imbalance of the microbiome) is associated with:

• Inflammatory bowel disease (Crohn’s disease, ulcerative colitis).
• Obesity and metabolic syndrome.
• Mental health conditions (via the gut-brain axis).
• Allergies and autoimmune diseases.

Worked Example 1: PCR product yield. A PCR reaction starts with 3 copies of a target sequence. After 32 cycles, how many copies are produced?

$N = 3 \times 2^{32} = 3 \times 4294967296 = 1.29 \times 10^{10}\ \text{copies}$

Worked Example 2: Restriction fragment analysis. A plasmid of $5000\ \mathrm{bp}$ is cut with EcoRI at position $1000\ \mathrm{bp}$ and with BamHI at position $3500\ \mathrm{bp}$. How many fragments are produced and what are their sizes?

EcoRI cuts at $1000\ \mathrm{bp}$Producing fragments of $1000\ \mathrm{bp}$ and $4000\ \mathrm{bp}$. BamHI cuts the $4000\ \mathrm{bp}$ fragment at position $3500\ \mathrm{bp}$Producing fragments of $2500\ \mathrm{bp}$ ($3500 - 1000$) and $1500\ \mathrm{bp}$ ($5000 - 3500$).

Final fragments: $1000\ \mathrm{bp}$, $2500\ \mathrm{bp}$And $1500\ \mathrm{bp}$ (three fragments).

Worked Example 3: Calculating DNA concentration from absorbance. A DNA solution has an absorbance of 0.40 at $260\ \mathrm{nm}$. The relationship between absorbance and double-stranded DNA concentration is: $A_{260} = 1.0$ for $50\ \mu\mathrm{g\ mL^{-1}}$ dsDNA.

Concentration $= \frac{0.40}{1.0} \times 50 = 20\ \mu\mathrm{g\ mL^{-1}}$.

## Practice Problems

Problem 1

Describe the process of producing recombinant human insulin using genetic engineering. Include reference to restriction enzymes, DNA ligase, vectors, and selection methods. (6 marks)

Answer. (1) The human insulin gene is located and cut from human DNA using a restriction enzyme (e.g., EcoRI), producing sticky ends. (2) The same restriction enzyme cuts a plasmid vector at a specific site, often within the lacZ gene. (3) The insulin gene and the cut plasmid are mixed with DNA ligase, which joins them by forming phosphodiester bonds, producing a recombinant plasmid. (4) The recombinant plasmid is introduced into E. Coli host cells by transformation (heat shock or electroporation). (5) Transformed bacteria are selected by growing them on agar plates containing ampicillin. Only bacteria that have taken up the plasmid (which carries an ampicillin resistance gene) survive. (6) To distinguish bacteria with the recombinant plasmid from those with the original plasmid, bacteria are grown on plates containing X-gal and IPTG. Bacteria with the original plasmid produce functional $\beta$-galactosidase (from the intact lacZ gene) and form blue colonies. Bacteria with the recombinant plasmid have the insulin gene inserted into lacZ, disrupting it, and form white colonies. (7) White colonies are selected and grown in large fermenters. Insulin is extracted and purified.

If you get this wrong, revise: Procedure for Producing Recombinant DNA

Problem 2

Explain how CRISPR-Cas9 can be used to correct a disease-causing mutation. In your answer, describe the roles of the guide RNA, Cas9 protein, PAM sequence, and the cell's DNA repair mechanisms. (6 marks)

Answer. CRISPR-Cas9 is a gene editing tool that can precisely cut DNA at a specific location. A guide RNA (gRNA) is designed to be complementary to the target DNA sequence containing the disease-causing mutation. The gRNA-Cas9 complex scans the genome for the target sequence adjacent to a PAM (protospacer adjacent motif, 5’-NGG-3’). When the gRNA base-pairs with the target and the PAM is recognised, Cas9 cuts both strands of the DNA, creating a double-strand break (DSB). To correct the mutation, a donor DNA template is provided that contains the correct (wild-type) sequence flanked by regions homologous to the target. The cell repairs the DSB using homology-directed repair (HDR), using the donor template as a guide to insert the correct sequence. This corrects the mutation. If HDR does not occur, the cell may use non-homologous end joining (NHEJ), which can introduce indels that disrupt the gene further. The efficiency of HDR can be increased by using modified Cas9 (nickase versions that create single-strand breaks, which favour HDR).

If you get this wrong, revise: CRISPR-Cas9

Problem 3

Compare and contrast somatic gene therapy and germ line gene therapy. Discuss the ethical issues associated with each approach. (5 marks)

Answer. Somatic gene therapy involves introducing a functional gene into the patient’s body cells (somatic cells). The changes affect only the treated individual and are not inherited by offspring. It is used to treat conditions such as cystic fibrosis, SCID, and haemophilia. Germ line gene therapy involves modifying the DNA in gametes or early embryos, producing changes that are inherited by all cells of the organism and can be passed to future generations. It could potentially eliminate genetic diseases from a family line permanently. Ethical issues: somatic gene therapy is generally accepted because it is analogous to conventional medical treatment — the patient consents, and the changes are not heritable. Germ line gene therapy is widely considered unethical because: (1) the embryo cannot consent; (2) changes are heritable, affecting future generations who cannot consent; (3) there is a risk of off-target effects with unpredictable long-term consequences for the gene pool; (4) it could be used for enhancement rather than therapy; (5) it raises concerns about “designer babies” and the commodification of human life. Germ line gene therapy is currently illegal in most countries.

If you get this wrong, revise: Gene Therapy

Problem 4

A plasmid has a total length of $4500\ \mathrm{bp}$. It is cut with two restriction enzymes: EcoRI cuts at position $900\ \mathrm{bp}$And HindIII cuts at position $2700\ \mathrm{bp}$. (a) How many fragments are produced? (b) What are their sizes? (c) If the fragments are separated by gel electrophoresis, which fragment will travel furthest from the wells?

Answer. (a) Two cuts produce three fragments.

(b) Fragment 1: from position 0 to 900 = $900\ \mathrm{bp}$ (circular plasmid) or from the start of the linearised plasmid to the first cut. Fragment 2: from position 900 to 2700 = $1800\ \mathrm{bp}$. Fragment 3: from position 2700 to 4500 = $1800\ \mathrm{bp}$.

The three fragments are: $900\ \mathrm{bp}$, $1800\ \mathrm{bp}$And $1800\ \mathrm{bp}$.

(c) The smallest fragment ($900\ \mathrm{bp}$) will travel furthest from the wells because smaller DNA fragments migrate faster through the gel matrix.

If you get this wrong, revise: Calculations in Biotechnology

Problem 5

Explain the advantages and disadvantages of using embryonic stem cells compared to adult stem cells for medical research and therapy. (5 marks)

Answer. Embryonic stem cells (ESCs) are pluripotent — they can differentiate into any cell type in the body, making them more versatile for therapy. Adult stem cells are multipotent — they can only differentiate into a limited range of cell types within their tissue of origin. ESCs can divide indefinitely in culture, providing an unlimited supply, whereas adult stem cells have a limited capacity for self-renewal. However, ESCs have significant disadvantages: they are derived from embryos, which must be destroyed (raising ethical objections about the moral status of the embryo); they carry a risk of teratoma formation (tumours) if transplanted without full differentiation; and they require immunosuppression if the donor and recipient are not genetically matched. Adult stem cells avoid the ethical issues associated with embryo destruction and can sometimes be harvested from the patient’s own body (autologous transplant), eliminating immune rejection. However, adult stem cells are rare, difficult to isolate and expand in culture, and have limited differentiation potential. Induced pluripotent stem cells (iPSCs) offer a compromise: they are pluripotent like ESCs but derived from adult cells, avoiding embryo destruction.

If you get this wrong, revise: Stem Cells

Vitamin A deficiency (VAD) affects approximately 250 million children worldwide, particularly in Southeast Asia and sub-Saharan Africa. VAD causes xerophthalmia (dry eyes), night blindness, and in severe cases, total blindness and increased susceptibility to infections. Rice is a staple food for billions of people but contains no $\beta$-carotene (provitamin A) in the endosperm (the edible white part).

Golden Rice was developed by introducing two genes from other species into rice:

1. psy (phytoene synthase) from daffodil (Narcissus pseudonarcissus) — later replaced by the maize psy gene for higher expression.
2. crtI (phytoene desaturase) from the soil bacterium Erwinia uredovora (now reclassified as Pantoea ananatis).

These genes enable the rice endosperm to produce $\beta$-carotene from geranylgeranyl diphosphate (GGPP), a precursor already present in the endosperm:

$\text{GGPP} \xrightarrow{\text{psy}} \text{phytoene} \xrightarrow{\text{crtI}} \text{lycopene} \xrightarrow{\text{lcy (endogenous)}} \beta\text{-carotene}$

The lycopene cyclase (lcy) step is catalysed by an enzyme already present in rice (endogenous gene).

Golden Rice 1 produced insufficient $\beta$-carotene ($1.6\ \mu\mathrm{g\ g^{-1}}$). Golden Rice 2 replaced the daffodil psy gene with a maize psy gene, which has higher expression in rice endosperm, increasing $\beta$-carotene to $23\ \mu\mathrm{g\ g^{-1}}$ (a 14-fold improvement). This level is sufficient to provide the recommended daily intake of vitamin A from a typical serving of rice.

Argument For	Argument Against
Addresses a major public health problem (VAD blindness)	Concerns about corporate control of food supply (patents on GM crops)
Rice is self-pollinating, so transgenes do not spread to wild relatives	Gene flow to wild rice species could have unintended ecological consequences
Golden Rice seeds are distributed free to subsistence farmers (humanitarian licence)	Some argue that VAD is better addressed by dietary diversification (eating more vegetables) rather than GM technology
Reduces reliance on vitamin A supplements (cost, distribution logistics)	Long-term health effects of consuming GM foods are unknown (though no evidence of harm to date)
Could be combined with other traits (drought resistance, pest resistance)	Public opposition to GM food may prevent adoption

Somatic gene therapy involves inserting a functional copy of a gene into the patient’s body cells (somatic cells). This does not affect the patient’s germ cells, so the change is not inherited by offspring.

Vectors for gene delivery:

Vector	Advantages	Disadvantages
Retrovirus (e.g., lentivirus)	Integrates into host genome, providing permanent expression	Risk of insertional mutagenesis (activating oncogenes); only infects dividing cells
Adenovirus	Does not integrate into genome; high transduction efficiency	Transient expression (lost as cells divide); immune response
Adeno-associated virus (AAV)	Low immunogenicity; long-term expression in non-dividing cells	Small capacity ($< 5\ \mathrm{kb}$); expensive to produce
Liposomes (lipid nanoparticles)	Non-viral; no risk of viral infection; can carry larger DNA	Low transfection efficiency; transient expression
Naked DNA injection	Simple; no vector needed	Very low efficiency

Germline gene therapy involves modifying the DNA in germ cells (sperm, eggs) or early embryos. The changes would be inherited by all cells of the resulting individual and passed to future generations.

Germline gene therapy is currently illegal in most countries due to:

• Ethical concerns about modifying the human germline (“designer babies”).
• Unknown long-term consequences for future generations.
• Risk of off-target effects (unintended mutations).
• Social justice concerns (access limited to the wealthy).

SCID (“bubble boy disease”) is caused by mutations in genes essential for immune cell development (e.g., IL2RG on the X chromosome, causing X-linked SCID). In 2000, gene therapy using a retroviral vector successfully restored immune function in several SCID patients. However, 5 of 20 patients developed leukaemia because the retrovirus inserted near the LMO2 oncogene, activating it. This highlighted the risk of insertional mutagenesis and led to improved vector designs with safer integration profiles.

BLAST is the most widely used bioinformatics tool. It compares a query DNA or protein sequence against a database of known sequences to find similar sequences.

How BLAST works:

1. The query sequence is broken into short “words” ( 3 amino acids for protein BLAST).
2. The database is scanned for exact matches to these words.
3. For each exact match found, BLAST extends the alignment in both directions.
4. High-scoring segment pairs (HSPs) are identified using a substitution matrix (e.g., BLOSUM62 for proteins).
5. Statistical significance is assessed using the E-value (expect value).

E-value interpretation:

• E-value $< 10^{-10}$: highly significant match (almost certainly homologous).
• E-value $< 10^{-5}$: significant match (likely homologous).
• E-value $> 0.01$: not significant (match may be due to chance).

When comparing multiple sequences (e.g., the same gene from different species), a multiple sequence alignment (MSA) is performed using algorithms such as ClustalW or MUSCLE. The aligned sequences are then used to construct a phylogenetic tree.

Tree-building methods:

Method	Principle	Speed	Accuracy
UPGMA (unweighted pair group method with arithmetic mean)	Clusters sequences by pairwise similarity; assumes a molecular clock	Fast	Lower (assumes equal rates)
Neighbour-joining	Minimises total branch length; does not assume a molecular clock	Fast	Moderate
Maximum parsimony	Chooses the tree requiring the fewest evolutionary changes	Slow	Moderate
Maximum likelihood	Chooses the tree with the highest probability given an evolutionary model	Very slow	High
Bayesian inference	Uses probability distributions to estimate the most likely tree	Very slow	Highest

Two species have the following aligned DNA sequences for a 20 bp region:

Species A: ATG-CCT-AGG-TCA-GCT-AGA-TCC Species B: ATG-CCT-AGG-TCA-GCT-AGA-TCC Species C: ATG-CCT-AAG-TCA-GCT-AGA-TCC Species D: ATG-CCT-AAG-TCA-GCA-AGA-TCC

Counting differences:

• A vs B: 0 differences
• A vs C: 1 difference (position 10: G $\to$ A)
• A vs D: 2 differences (position 10: G $\to$ A; position 16: T $\to$ A)
• B vs C: 1 difference
• B vs D: 2 differences
• C vs D: 1 difference

Genetic distance matrix:

	A	B	C	D
A	0	0.00	0.05	0.10
B		0	0.05	0.10
C			0	0.05
D				0

Species A and B are most closely related (identical in this region). Species C is equally distant from A and B. Species D is the most divergent.

Plant tissue culture (micropropagation) involves growing plants from small pieces of plant tissue (explants) on an artificial nutrient medium under sterile conditions. It exploits the property of totipotency: the ability of any plant cell to develop into a complete organism.

1. Stage 0: Selection and preparation. A healthy, disease-free mother plant is selected. Explants (leaf discs, stem sections, meristems) are surface-sterilised using sodium hypochlorite or ethanol.
2. Stage 1: Initiation. Explants are placed on a nutrient medium containing:

• Macronutrients: N, P, K, Ca, Mg, S.
• Micronutrients: Fe, Mn, Zn, Cu, B, Mo.
• Carbon source: sucrose (since the explant cannot photosynthesise).
• Vitamins: thiamine, nicotinic acid.
• Plant growth regulators: auxin (e.g., 2,4-D) and cytokinin (e.g., BAP). The auxin:cytokinin ratio determines the response:
• High auxin : low cytokinin $\to$ root formation.
• Low auxin : high cytokinin $\to$ shoot formation.
• Balanced ratio $\to$ callus formation (undifferentiated cell mass).
• Gelling agent: agar.
• The medium is solidified with agar and sterilised by autoclaving.

3. Stage 2: Multiplication. Callus or shoot tips are subcultured onto fresh medium to produce multiple shoots. Each shoot can be further divided, allowing exponential multiplication.
4. Stage 3: Rooting. Individual shoots are transferred to a rooting medium (high auxin) to induce root formation, producing complete plantlets.
5. Stage 4: Acclimatisation (hardening off). Plantlets are transferred from the sterile, high-humidity culture vessel to soil. This is the most critical stage because the plantlets must transition from heterotrophic growth (using sucrose from the medium) to autotrophic growth (photosynthesis) and develop a functional cuticle to prevent water loss.

• Mass production of ornamental plants (orchids, ferns) and crop plants (bananas, potatoes, strawberries).
• Production of disease-free plants (e.g., virus-free potatoes by culturing meristem tissue, which has low viral load).
• Conservation of endangered plant species.
• Production of secondary metabolites (e.g., shikonin, a dye, from cell cultures of Lithospermum).

Advantages	Disadvantages
Large numbers of identical plants (clones) produced rapidly	Genetic uniformity makes all plants equally susceptible to the same disease
Disease-free plants can be produced	Requires sterile conditions and skilled labour
Year-round production, independent of seasons	Expensive equipment and consumables
Enables conservation of rare species	Somaclonal variation (genetic changes during tissue culture) can reduce quality
Small amount of parent tissue needed	Some species are difficult to culture

Type	Organism	Conditions	Products
Aerobic	Aspergillus niger (fungus)	Aerate, $30$ degrees C, pH 2—3	Citric acid
Anaerobic	Saccharomyces cerevisiae (yeast)	No aeration, $30$ degrees C, pH 4—6	Ethanol, $\mathrm{CO_2}$
Continuous	Escherichia coli (bacterium)	Continuous nutrient supply	Insulin, growth hormone

Feature	Batch Fermentation	Continuous Fermentation
Setup	Fixed volume of medium; closed system	Constant inflow of fresh medium; constant outflow of product
Growth phases	All four phases (lag, log, stationary, death) occur	Cells maintained in exponential (log) phase
Product yield	Varies over time; harvest at end of run	Constant, steady-state yield
Contamination risk	Lower (sealed vessel)	Higher (open system for extended periods)
Cost	Lower initial cost; labour-intensive	Higher initial cost; automated
Best for	Secondary metabolites (e.g., antibiotics, produced in stationary phase)	Primary metabolites (e.g., insulin, produced during exponential growth)

To maximise product yield, the following factors must be optimised:

• Temperature: affects enzyme activity and growth rate. Each organism has an optimum temperature (e.g., $37$ degrees C for E. Coli, $30$ degrees C for S. Cerevisiae). Fermentation generates heat, so cooling systems are needed.
• pH: affects enzyme activity and membrane transport. Buffers or acid/base addition maintain pH.
• Oxygen concentration: aerobic fermentations require continuous aeration (sparging with sterile air). Dissolved oxygen probes monitor $\mathrm{O_2}$ levels.
• Nutrient concentration: limiting nutrients can control growth rate (chemostat). Carbon, nitrogen, and phosphate sources must be optimised.
• Stirring: ensures uniform mixing of nutrients, temperature, and $\mathrm{O_2}$.
• Foam control: fermentation produces foam (from proteins and gas bubbles), which can block air filters and reduce vessel volume. Anti-foaming agents (silicone-based) are added automatically.

After fermentation, the product must be extracted and purified:

1. Filtration or centrifugation: separates cells from the culture medium.
2. Cell disruption (if product is intracellular): sonication, enzymatic lysis, or high-pressure homogenisation.
3. Chromatography: separates the product from other molecules based on size (gel filtration), charge (ion exchange), or affinity (affinity chromatography using a specific antibody or ligand).
4. Ultrafiltration/diafiltration: concentrates and desalts the product.

Principle	Description
Beneficence	Technology should benefit humanity (e.g., curing disease, feeding the hungry)
Non-maleficence	Technology should not cause harm (assessing risks of GM crops, gene therapy)
Autonomy	Individuals have the right to make informed choices (e.g., informed consent for genetic testing)
Justice	Benefits and risks should be distributed fairly (avoiding “genetic divide” between rich and poor)
Precautionary principle	Where there is scientific uncertainty about potential harm, precautionary measures should be taken

Prenatal genetic testing (amniocentesis, chorionic villus sampling) can detect chromosomal abnormalities (e.g., Down syndrome) and single-gene disorders (e.g., cystic fibrosis, sickle cell anaemia) before birth.

Preimplantation genetic diagnosis (PGD) involves testing embryos created by IVF for genetic disorders before implantation. Only unaffected embryos are implanted.

Ethical concerns:

• Should parents be allowed to select embryos based on non-medical traits (sex, eye colour)? This is sometimes called “designer babies” and is illegal in many countries.
• What about disorders with variable penetrance (e.g., BRCA1 mutations increase breast cancer risk but do not guarantee it)?
• The psychological impact of genetic information on individuals and families.
• Genetic discrimination by employers or insurance companies (addressed by legislation such as the Genetic Information Nondiscrimination Act, GINA, in the US).

Environmental concerns:

• Gene flow to wild relatives could create “superweeds” (e.g., herbicide resistance transferred to weedy relatives of oilseed rape).
• Impact on non-target organisms (e.g., Bt toxin from GM maize may affect butterflies — though field evidence is limited).
• Loss of biodiversity if GM monocultures replace diverse traditional varieties.

Socioeconomic concerns:

• Patents on GM seeds mean farmers must buy new seeds each year (cannot save seed), increasing dependence on multinational corporations.
• Smallholder farmers in developing countries may not benefit if the technology is too expensive.

Scientific consensus: the World Health Organization, the American Medical Association, and the Royal Society have all concluded that GM foods currently available are safe to eat and no different in safety from conventional foods. However, each new GM crop must be assessed on a case-by-case basis.

The human gut microbiome consists of approximately $10^{13}$—$10^{14}$ bacteria from over 1000 species, predominantly from the phyla Firmicutes and Bacteroidetes. The microbiome varies between individuals and is influenced by diet, age, medication (especially antibiotics), and environment.

Function	Mechanism
Fermentation of dietary fibre	Bacteria produce short-chain fatty acids (SCFAs): butyrate, propionate, acetate. Butyrate is the primary energy source for colonocytes and has anti-inflammatory properties.
Vitamin synthesis	Bacteria synthesise vitamin K and several B vitamins (B12, folate, riboflavin, biotin).
Immune system development	Bacterial antigens stimulate the development of gut-associated lymphoid tissue (GALT) and regulatory T cells.
Protection against pathogens	Commensal bacteria compete with pathogens for nutrients and adhesion sites; they produce antimicrobial peptides (bacteriocins).
Metabolism of xenobiotics	Bacteria modify drugs and toxins, affecting their absorption, efficacy, and toxicity.

Dysbiosis is a disruption of the normal composition of the microbiome, associated with:

• Inflammatory bowel disease (IBD): reduced diversity; increased Faecalibacterium prausnitzii is protective.
• Obesity: altered Firmicutes:Bacteroidetes ratio (controversial; findings are inconsistent between studies).
• Type 2 diabetes: reduced abundance of butyrate-producing bacteria.
• Mental health: the “gut-brain axis” — the vagus nerve and immune signals communicate between the gut and brain. Dysbiosis has been linked to anxiety, depression, and autism spectrum disorder.
• Clostridioides difficile infection: broad-spectrum antibiotics kill commensal bacteria, allowing C. Difficile to overgrow. Treated with faecal microbiota transplantation (FMT) — transferring stool from a healthy donor to the patient’s colon, which restores a healthy microbiome.

Probiotics are live microorganisms (e.g., Lactobacillus, Bifidobacterium) that, when administered in adequate amounts, confer a health benefit. Evidence supports their use for antibiotic-associated diarrhoea and pouchitis.

Prebiotics are non-digestible food components (e.g., inulin, fructooligosaccharides) that selectively stimulate the growth and activity of beneficial gut bacteria.

Synbiotics combine probiotics and prebiotics.

## 17. Genetically Modified Organisms: Case Studies

Bt cotton has been engineered to express a gene from the bacterium Bacillus thuringiensis that produces Bt toxin (Cry protein). When insect pests (e.g., cotton bollworm) ingest Bt cotton tissue, the toxin is activated in their alkaline gut, binds to specific receptors on the gut epithelial cells, and forms pores, causing cell lysis and death of the insect.

Advantages:

• Reduces the need for chemical insecticide sprays (beneficial for the environment and farmer health).
• Increases yield by reducing pest damage.
• Bt toxin is specific to certain insect groups and is harmless to humans, livestock, and most beneficial insects.

Disadvantages:

• Resistance can evolve in pest populations (refuges of non-Bt cotton are planted to maintain susceptible alleles in the pest population).
• Concerns about gene flow to wild relatives.
• Seeds are more expensive than conventional varieties.

Crops engineered to be resistant to the herbicide glyphosate (Roundup) by expressing a glyphosate-insensitive version of EPSP synthase (the enzyme glyphosate normally inhibits). This allows farmers to spray glyphosate to kill weeds without harming the crop.

Advantages:

• Simplifies weed management (one herbicide kills all weeds).
• Reduces the need for tillage (reducing soil erosion).
• Enables no-till farming.

Disadvantages:

• Over-reliance on a single herbicide has selected for glyphosate-resistant weeds (“superweeds”).
• Glyphosate residues may persist in soil and water.
• Concerns about effects on non-target organisms and human health (controversial; regulatory agencies consider glyphosate safe at approved levels).

AquAdvantage salmon: genetically modified Atlantic salmon that expresses a growth hormone gene from Chinook salmon, regulated by an ocean pout antifreeze protein promoter. The salmon grows to market size in approximately 18 months instead of 36 months. Approved for human consumption by the FDA (USA) in 2015.

Goats producing antithrombin: goats engineered to express human antithrombin (a blood-clotting protein) in their milk. The protein is purified from the milk and used to treat patients with antithrombin deficiency.

Quantitative PCR (real-time PCR) allows the amount of DNA in a sample to be quantified by measuring the increase in fluorescence during the amplification reaction.

Two main methods:

Method	Principle	Advantages
SYBR Green	A fluorescent dye that binds to double-stranded DNA. Fluorescence increases as more dsDNA is produced.	Simple, cheap
TaqMan probes	A fluorescent probe with a reporter dye and a quencher. When the probe binds to the target sequence and is degraded by Taq polymerase’s 5’ exonuclease activity, fluorescence is released.	Highly specific; can distinguish between closely related sequences

The threshold cycle ($C_t$) is the cycle number at which the fluorescence exceeds a set threshold. A lower $C_t$ value indicates a higher starting concentration of the target DNA.

The amount of DNA doubles with each cycle: after $n$ cycles, the amount of DNA $= \text{initial amount} \times 2^n$.

If Sample A has $C_t = 20$ and Sample B has $C_t = 26$The difference is 6 cycles. The starting concentration of DNA in Sample A is $2^6 = 64$ times higher than in Sample B.

RT-PCR (not to be confused with real-time PCR/qPCR) is used to detect RNA. The process:

1. Reverse transcription: reverse transcriptase enzyme converts RNA into complementary DNA (cDNA).
2. PCR: the cDNA is amplified using standard PCR.

This technique is widely used to detect RNA viruses (e.g., SARS-CoV-2) and to measure gene expression (by quantifying mRNA levels).

Good PCR primers must:

Criterion	Optimal Value	Reason
Length	18—25 bases	Long enough for specificity, short enough for efficient binding
Melting temperature ($T_m$)	55—65 degrees C	Both primers should have similar $T_m$
GC content	40—60%	Ensures stable binding without excessive non-specific binding
3’ end	End in G or C (“GC clamp”)	Increases binding stability at the 3’ end where extension begins
Self-complementarity	Avoid	Prevents primer-dimer formation (primers annealing to each other)
Repeated sequences	Avoid	Reduces non-specific binding

Modern DNA fingerprinting uses short tandem repeats (STRs) — short (2—6 base pair) sequences repeated in tandem. The number of repeats at a given locus varies between individuals.

The UK National DNA Database uses 10 standard STR loci. The probability of two unrelated individuals having the same profile at all 10 loci is less than 1 in $10^{13}$ (one in ten trillion).

PCR is essential in forensic DNA analysis because:

• Crime scene samples often contain very small amounts of DNA (trace DNA from skin cells, hair roots, or bodily fluids).
• PCR can amplify the DNA from a single cell to a detectable amount (approximately $10^9$ copies).
• STR loci are short enough to be amplified even from degraded DNA.

A suspect’s DNA profile matches the crime scene sample at 10 STR loci. The frequency of each allele in the population is:

Locus	Genotype	Population Frequency
D3S1358	15, 16	0.08 $\times$ 0.06 = 0.0048
vWA	17, 18	0.10 $\times$ 0.12 = 0.012
FGA	22, 24	0.15 $\times$ 0.04 = 0.006
…	…	…
D18S51	13, 16	0.07 $\times$ 0.09 = 0.0063

The match probability is the product of the frequencies at all 10 loci. If the average frequency per locus is approximately $0.005$Then:

$\text{Match probability} = (0.005)^{10} = 9.77 \times 10^{-24}$.

This means the probability of a random person matching the DNA profile is approximately 1 in $10^{23}$Making it virtually certain that the suspect is the source of the DNA.

Dolly the sheep (1996) was the first mammal cloned from an adult somatic cell:

1. A somatic cell (udder cell) was taken from the donor sheep and cultured.
2. An enucleated egg cell (egg cell with its nucleus removed) was taken from a second sheep.
3. The donor nucleus was inserted into the enucleated egg by electrofusion.
4. The reconstructed egg was stimulated with an electric shock to initiate cell division.
5. The resulting embryo was implanted into a surrogate mother.
6. Dolly was born and was genetically identical to the donor sheep (the udder cell donor).

Therapeutic cloning uses the same SCNT technique but the embryo is not implanted. Instead, embryonic stem cells are harvested from the blastocyst (at approximately 5—7 days) and used to grow tissues or organs for medical treatment. Because the stem cells are genetically identical to the patient, they would not be rejected by the immune system.

Plants can reproduce asexually (vegetatively) by various methods:

Method	Description	Examples
Runners (stolons)	Horizontal stems growing along the ground that produce new roots and shoots at nodes	Strawberry, spider plant
Tubers	Swollen underground stems that can sprout new shoots	Potato
Bulbs	Underground food storage organs with fleshy leaf bases	Onion, daffodil, tulip
Corms	Swollen underground stem bases	Crocus, gladiolus
Cuttings	Pieces of stem or leaf that develop roots when placed in moist conditions	Geranium, sugar cane
Grafting	Joining a scion (shoot) to a rootstock of another plant	Apple, rose, citrus

All of these methods produce clones — genetically identical offspring. This is advantageous because:

• Desirable characteristics are preserved exactly.
• Rapid reproduction without the need for pollination and seed formation.
• Offspring are mature faster than from seed.

However, there is no genetic variation, so all clones are equally susceptible to the same diseases and environmental changes.

Proteomics is the large-scale study of the complete set of proteins (the proteome) produced by a genome, tissue, or cell at a given time under specific conditions.

While the genome is fixed, the proteome is dynamic — it changes with development, disease, and environmental conditions due to alternative splicing, post-translational modifications, and differential gene expression.

Techniques used in proteomics:

Technique	Purpose
2D gel electrophoresis	Separates proteins by charge (first dimension) and by molecular mass (second dimension)
Mass spectrometry	Identifies proteins by measuring the mass-to-charge ratio of peptide fragments
Protein microarrays	Detect protein-protein interactions and measure protein expression levels
X-ray crystallography	Determines the 3D structure of proteins

Metabolomics is the study of the complete set of small molecule metabolites (the metabolome) within a biological sample. Metabolomics provides a snapshot of the cell’s current metabolic state.

Applications:

• Biomarker discovery: identifying metabolites whose concentration changes in disease (e.g., elevated glucose in diabetes, altered amino acid profiles in cancer).
• Drug discovery: screening for metabolites with therapeutic potential.
• Nutritional science: understanding how diet affects metabolism.
• Environmental toxicology: detecting metabolic changes caused by exposure to pollutants.