thymine and guanine-cytosine. Since this molecule is quite stable and 

 fairly rigid, the only way in which one can imagine it carrying genetic 

 information is by virtue of the order of the base pairs. For example, the 

 order may be something like A-T, T-A, T-A, G-C, C-G or any com- 

 bination thereof. The question of replication of this molecule is a com- 

 plex one. Presumably each of the two component strands replicates a 

 complementary one to give a new double helix which is identical with 

 the original but the mechanism involved is still in the realm of specu- 

 lation. 



Lipids 



Lipids are also important constituents of cell organelles, especially of 

 the cell membrane, mitochondria, microsomes, chromosomes, and nucle- 

 olus, and possibly of the nuclear membrane. When involved in structure, 

 they are normally conjugated with protein. Certain of the lipids appear 

 to play essential roles in such processes as phosphorylation, methylation, 

 and protein denaturation. The lipids are also directly involved in oxi- 

 dative metabolism, such as the fatty acid derivative acetyl coA (acetyl 

 coenzyme A) which forms a central point in the pathways of oxidation 

 of carbohydrate and fatty acids (Figure 3-9). There are a number of 

 lipid derivatives the better known of which are the vitamins D, E, and 

 K, and the sex hormones corticosterone and 11-dehydrocorticosterone. 



Lipids include the fats and oils of plant and animal origin, and re- 

 lated substances such as lecithin and cholesterol. The fats are mixtures 

 of triglycerides, each of which contains a molecule of the glycerol asso- 

 ciated with three molecules of fatty acids. Glycerol is a straight-chain 

 carbon compound and is represented by the formula: 



H 



I 

 H— C— OH 



1 

 H— C— OH 



I 

 H— C— OH 



I 

 H 



The fatty acid molecule consists of a long backbone chain of carbon 

 atoms, with one of the terminal carbons forming a carboxyl group such 

 as in stearic acid 



CHo(CHo)i,;COOH 



The formation of a simple triglyceride, such as the fat tristearin, involves 

 the combination of one molecule of glycerol with three molecules of 

 stearic acid by means of ester linkages and may be illustrated as follows: 



20 / CHAPTER 2 



