Coordination, response and gene technology
Nerve impulses and synapses
At rest (about −70 mV), the Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in; the membrane is more permeable to K⁺, which leaks out. A stimulus opens voltage-gated Na⁺ channels; Na⁺ rushes in: depolarisation to about +40 mV. Na⁺ channels close and K⁺ channels open: repolarisation, overshooting (hyperpolarisation). The refractory period ensures one-way, discrete impulses. Action potentials are all-or-nothing; stimulus strength is coded by frequency. Myelin (Schwann cells) insulates the axon so impulses jump between nodes of Ranvier (saltatory conduction), much faster. Speed also rises with axon diameter and temperature. At a cholinergic synapse, Ca²⁺ enters the presynaptic knob, vesicles release acetylcholine by exocytosis; ACh diffuses and binds receptors, opening Na⁺ channels; acetylcholinesterase breaks it down. Synapses allow summation, one-way transmission and inhibition.
≈ 55 minRecombinant DNA and PCR
To make recombinant DNA: obtain the gene by restriction enzymes (cut at specific palindromic sites, leaving sticky ends), by reverse transcriptase (mRNA → cDNA, without introns) or by synthesis. Cut a plasmid vector with the same restriction enzyme, so sticky ends are complementary; DNA ligase joins them. Add a promoter so the gene is transcribed. Transform host cells (heat shock with Ca²⁺, electroporation). Identify transformed cells with marker genes (antibiotic resistance, GFP fluorescence). PCR copies DNA: denature at 95 °C, anneal primers at ~55 °C, extend at 72 °C with Taq polymerase; each cycle doubles the DNA (). Gel electrophoresis separates DNA fragments by size: DNA (negative phosphates) moves to the anode; smaller fragments travel further.
≈ 50 minGenetic engineering applications
Recombinant human proteins made by GM bacteria, yeast or animals: insulin, factor VIII (haemophilia), adenosine deaminase; purer and more plentiful than animal sources. Gene therapy adds a working allele to treat genetic disease (e.g. SCID, cystic fibrosis) using viral vectors or liposomes; somatic therapy affects only the patient, germ-line would be inherited (banned in humans). Genetic screening detects alleles (e.g. BRCA1, CF) with DNA probes; genetic counselling helps people decide. PGD screens IVF embryos. GM crops: Bt maize (insect resistance), herbicide-tolerant soya, Golden Rice (β-carotene for vitamin A). DNA profiling uses variable repeat sequences, PCR and electrophoresis for forensics and paternity. Ethical issues: safety, gene flow to wild plants, monopolies, consent, designer babies.
≈ 45 minGene expression and stem cells
Every cell has the same genes, but different cells express different ones. Stem cells are undifferentiated: totipotent (any cell, including placenta; early embryo), pluripotent (any body cell; embryonic stem cells), multipotent (a range, e.g. bone marrow → blood cells), unipotent (one type). Induced pluripotent stem (iPS) cells are made by adding transcription factor genes to adult cells. Transcription factors bind promoters to start or stop transcription; oestrogen enters cells and binds a receptor, releasing a transcription factor. RNA interference (siRNA) destroys specific mRNA, blocking translation. In prokaryotes the lac operon switches on lactose-digesting genes only when lactose is present. Epigenetics: heritable changes without changing base sequence: DNA methylation (silences genes) and histone acetylation (opens chromatin, activates). Cancer: mutated oncogenes (overactive) or tumour suppressor genes (inactive), sometimes by abnormal methylation.
≈ 50 min