terium would be able to produce the substance 

 itself. Then the modified cells would be grown in 

 large quantities, and the hormone would be ex- 

 tracted and purified for medical use. This method 

 would be an economical way to produce large 

 amounts of pharmaceuticals because E. coli is 

 easy to grow, and under appropriate conditions 

 the cells undergo division every 20 minutes. Thus, 

 their number increases logarithmically and one 

 cell can give rise to many billions in a single day. 



Other possible applications of recombinant 

 DNA research include studies to increase plant 

 food production by enhancing the efficiency of 

 photosynthesis, and studies to decrease the fertil- 

 izer requirements of crops by transferring directly 

 to plants the microbial enzyme systems that per- 

 form nitrogen fixation. 



It is recognized by recombinant DNA research- 

 ers that there may be certain dangers inherent in 

 the technique, dangers not only to laboratory 

 workers but to the environment as well. For this 

 reason molecular biologists established in 1974 a 

 self-imposed moratorium against some kinds of 

 experiments until adequate safety standards could 

 be developed. It was recommended by the Na- 

 tional Academy of Sciences that NIH draw up a 

 set of guidelines to govern the conduct of recom- 

 binant DNA experiments. The safety standards 

 NIH has now developed prohibit very dangerous 

 experiments, such as transplanting new drug-re- 

 sistant genes into disease-producing organisms. 

 Furthermore, two kinds of safeguards are pre- 

 scribed by the guidelines for permitted DNA re- 

 combination experiments: Physical containment 

 methods are designed to keep recombinant orga- 

 nisms isolated in the laboratory and to prevent 

 any environmental contamination; biological con- 

 tainment measures are designed to ensure that 

 recombinant organisms are so fragile that they 

 can only survive under special laboratory condi- 

 tions, so that even if environmental contamination 

 occurs, the organisms will die quickly without 

 spreading. 



Role of Membranes 



All cells are surrounded by membranes. Mem- 

 branes are the face a cell shows to the rest of the 

 world and are the site of numerous specific recep- 

 tors for a wide variety of signals by hormones and 

 other molecules. They represent the principal bar- 

 rier to the free movement of materials in and out 

 of the cell. Their selectivity, coupled with their 

 electrical insulating properties, permit the storage 

 of both electrical and chemical energy. Mem- 

 branes also contain the antigens that label the cell 

 and are important in cell-cell interactions and cell 

 recognition in growth, in the development and 



maintenance of organ structure, as well as in 

 immune response. In addition, cell membranes are 

 a major site of certain classes of enzymes, such 

 as those involved in the synthesis of lipids, in- 

 cluding steroids. 



Although membranes are essential to the exist- 

 ence of life, not much was known about them un- 

 til recently. In the past decade, however, there has 

 been a remarkable acceleration in the pace of 

 membrane research and an explosion of advances 

 in the field. Areas of progress include the follow- 

 ing: 



Membrane composition. Although numerous 

 plasma and cytoplasmic proteins have been puri- 

 fied over the past half-century, it has proven 

 much more difficult to purify membrane proteins. 

 Several successful techniques have recently been 

 devised to purify these proteins. Membrane-asso- 

 ciated structural proteins, antigens, receptors, and 

 enzymes have been purified from viruses, bacte- 

 ria, muscle, and red blood cells, and from mam- 

 malian cells grown in tissue culture. A good un- 

 derstanding of the relationship between the type 

 of amino acids in the different segments of a 

 membrane protein and its location in relation to 

 the membrane is being developed. 



Fatty structures known as lipids constitute most 

 of the bulk of membranes. Membrane scientists 

 have been steadily improving the techniques of 

 lipid chemistry. Most importantly, novel new 

 methods have made it possible to label lipids 

 selectively on either the inner or outer half of the 

 membrane. It appears that the two halves have 

 different lipid compositions, and scientists are in- 

 terested in how these differences are established 

 and maintained and why they exist. 



Membrane structure. Until recently it was 

 thought that the membrane consisted of a lipid 

 bilayer with a layer of protein on either side. It is 

 now apparent that there are many proteins that 

 span the membrane as well as others that are both 

 in the aqueous phase outside the membrane and 

 partially buried in the membrane lipid. It appears 

 that most membranes are quite fluid and that the 

 membranes essentially float freely in a sea of lip- 

 id. In some cells special structures exist to limit 

 the free mobility of proteins. A large battery of 

 highly sophisticated techniques has been devel- 

 oped over the past decade to probe the structure 

 of membranes. Advanced spectroscopic tech- 

 niques can determine how freely proteins move, 

 how firmly they are associated with surrounding 

 molecules, and how readily the lipids bend, ro- 

 tate, and flip over to the other layer. Freeze frac- 

 ture electron microscopy enables researchers to 

 visualize the inside of the membrane and to local- 

 ize proteins within the lipid matrix. It also permits 



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HEALTH. EDUCATION AND WELFARE 



