KEY FACTS
  • DNA can be replicated artificially by the polymerase chain reaction. The enzyme DNA polymerase is used to make new double stranded DNA by synthesising a new complementary strand to a pre-existing strand, just as in natural replication.
  • PCR makes it possible to synthesise large numbers of copies of very small samples of DNA.
  • DNA fragments can be separated by gel electrophoresis. A voltage is applied to the gel and the negatively charged DNA fragments move towards the positive electrode. Smaller fragments move faster than large ones.
  • The bands of DNA can be seen if radioactive nucleotides are used in the PCR. The pattern of banding in the gel can be made visible by placing the gel next to a sheet of unexposed photographic film overnight. the radioactive bands cause the film to turn black.
  • Cystic fibrosis is a genetic disorder caused by a mutant allele that produces a defective form of the channel protein, called CFTR. This protein normally transports chloride ions out of the cells.
  • The defective CFTR protein causes chloride ions to build up in the cells. This causes those cells to retain water. In the lungs and intestines this is a particular problem. Water fails to pass into the mucus that lines the airways and gut, causing the mucus to become thick and sticky. In the lungs, this leads to breathing difficulties and the risk of infection; in the gut, the mucus blocks the ducts that carry digestive enzymes.
  • Some severe genetic disorders can be treated by gene therapy. Healthy genes are cloned and then transferred to target cells in the body to take over the function of defective genes that cause the disorder.
  • Two forms of gene therapy are being developed to treat cystic fibrosis. In the first, healthy CFTR genes are inserted into liposomes, which fuse with the cell membranes and take the genes into the cells. In the other, harmless viruses are used to insert the CFTR genes into the cells.