Chapter 8 : Genetic Engineering

8.1 Genetic engineers

Genetic engineers are scientists who manipulate genes. They may alter a gene, turn a gene on or off, or move a gene from one organism to another. Most genetic engineers work in colleges or universities, for government agencies or for private companies.

Genetic engineering has many potential applications, for example:
  1. Improving the nutritional value, or disease resistance of crop plants.
  2. Making farm animals that grow faster, or produce more milk than normal.
  3. Working to cure genetic diseases, such as sickle cell anemia or cystic fibrosis.
  4. Curing degenerative diseases such as diabetes and Alzheimer's.
  5. Cloning endangered species.

8.2 Protein synthesis

Protein synthesis involves using the instructions in a gene (DNA) to build a particular protein. DNA contains the instructions for making all the proteins in the body ( roughly 30,000 in humans ). Each gene carries instructions for one protein.

To produce a protein, the cell goes through 2 steps :
1) Transcription : the code on the DNA is copied to mRNA. Fig 8.4. This is done by the enzyme RNA polymerase. The 2006 Nobel prize for Chemistry was given to the scientist who worked out the details of transcription.

2) Translation : the mRNA code is read by ribosomes in the cytoplasm, which join amino acids together. Fig 8.7

The code on DNA and mRNA is a triplet code : 3 bases code for 1 amino acid
eg GGA codes for Glycine. Table 8.1. One code on mRNA is the "start" code : AUG. The ribosome always starts reading the mRNA at this point. Several codes on mRNA signal "stop" eg UGA.

Transfer RNAs ( tRNA) bring each amino acid to the ribosome. Fig 8.6. Each tRNA has an anticodon which matches the 3 base codon on mRNA. The ribosome moves along the mRNA strand, adding amino acids one at a time. Fig 8.7. Transcription & Translation

Mutations (changes in the DNA code ) can cause a different protein to be made. Fig 8.8. Most mutations are harmful eg cancer, but very occasionally they produce a better protein than the original, which is important in evolution.

Genes can be switched on or off, for example repressor proteins switch a gene off by preventing RNA polymerase from binding to DNA. Activators are proteins that switch genes on, by helping the RNA polymerase to bind. Small pieces of "interfering RNA" could be used medically to switch off particular genes, for example to cure cancer or genetic diseases.

8.3 Producing Recombinant Proteins

To produce a protein such as Bovine Growth Hormone (BGH, which improves milk yield in cows), you first remove the gene from a cow's DNA using restriction enzymes. Restriction enzymes are found naturally in bacteria, and they cut DNA at specific sites (where there is a particular code of bases). In other words they act like molecular scissors, cutting out a particular gene from the DNA. Fig 8.12.

The BGH gene from the cow is now put into a plasmid: a small circle of DNA from a bacterium. The altered (or recombinant) plasmid is then put back into the bacterium. Bacteria can copy plasmids very quickly, and the recombinant bacteria will now produce lots of Bovine Growth Hormone (rBGH).

Any new food (like milk produced from cows that are injected with rBGH) must be approved by the Food and Drug Administration, FDA, before it is sold in stores. The FDA approved this type of milk in 1993. At the moment food like this does not have to be specially labelled. However some companies put on their own label saying "Contains no GMOs" (Genetically Modified Organisms) or "This milk is hormone free" (meaning the cows have not been injected with rBGA to increase their milk production. The milk does still contain natural hormones from the cow!). Organic foods are increasingly common in supermarkets, but some critics say that some of the "organic" labels are a marketing ploy.

8.4 Genetically modified food.

Foods have been genetically modified since 1994. Generally the genetic modification is to improve the nutritional value, shelf life or yield of the crop. Roughly 50% of corn and other crops grown in US are GM.

Genes can be added to plants using a plasmid Fig 8.15, or by shooting DNA into the plant cells with a gene gun (Fig 8.16). This can be used to improve crops, for example by adding a gene for Vitamin A to rice. Golden rice. Rice contains very little vitamin A and 500,000 children, mainly in Asia, go blind every year due to a lack of vitamin A.

Pharmaceutical companies are also genetically modifying crops so that the plants make important medicines, this is sometimes abbreviated as "pharming".

Potential problems with GM crops:
  1. GM crops that contain the natural insecticide Bt may kill beneficial insects eg butterflies. Fig 8.19.
  2. GM crops contain new proteins, so may produce allergic reactions in some people. Corn.
  3. GM crops that have a gene for Bt (which kills insects) could cause the evolution of Bt resistant insects.
  4. It is possible that genes from GM crops will be transfered (by pollen) to wild plants, or to nearby crops.

8.5 Human Genetic Engineering.

Gene sequencing means working out the exact order of bases in DNA. Sequencing. The Human genome project is working on human genes. The human genome has 3 billion base pairs. The human genome project was completed in 2003. The new phase involves producing a detailed map. CBS news. The human genome project will be useful in predicting and treating disease. Gene bank

Gene therapy
Genes can be inserted into cells using retroviruses. Potentially this can cure genetic diseases. The main problem is getting human cells to accept and use new DNA. Gene therapy first worked in 2000. Fig 8.21. Gene therapy. In addition to treating genetic diseases, gene therapy can also be used to boost the immune system, for example to treat cancer.

Cloning
Cloning produces an exact genetic copy of an individual. Some plants and animals clone themselves naturally: eg grass plugs, banana plants, Hydra.

In artificial cloning, the nucleus of the cell to be cloned is inserted into the cytoplasm of another cell. Fig 8.22. Cloning. First clone using this method was a sheep: Dolly in 1997. Cloning Will a clone live as long as a normal animal? Dolly dies young

Cloning would be useful for:

A) copying domestic animals with useful genes.

B) cloning could help increase the population of an endangered species, or possibly revive extinct species. In 2001 a rare wild mouflon sheep was cloned.

C) copying genetically modified (GM) animals. Cloned chicks Cloned pigs

D) human cloned cells could be used to cure diseases. Therapeutic cloning means taking the DNA from the cells of a patient and using stem cells to grow new tissue. Therapeutic cloning

No government supports reproductive cloning (producing a new baby that is the clone of an adult), but several countries are doing research on therapeutic cloning, where cells are copied and then put back into the same patient. This could help cure many diseases, such as Alzheimer's and Parkinson's. Cloning debate

Stem cells
Stem cells are cells that divide rapidly to produce new cells. They are important in growth and repairing damaged areas.
Adult stem cells, from bone marrow for example, can form a limited number of types of cells. Bone marrow. Stem cells from umbilical cords can form a larger variety of cells. Cord cells

Stem cells from embryos can form into any cell in the body. Originally, harvesting the stem cells killed the embryo, but a new technique allows the embryo to survive. New Scientist. Stem cells could be useful in curing several diseases including diabetes and Alzheimer's disease.

Last edited November 2009, by David Byres, dbyres@fscj.edu