LECTURES IN BIOLOGICAL SCIENCES 



composition of the solution. They would not be difficult to 

 perform with modern methods. 



While the formation of even a small protein is a thermo- 

 dynamically unlikely event, visualize an ocean containing, ac- 

 cording to Urey (1952) and Madison (1953), 3.6 million cubic 

 miles of organic compounds, existing over a period of 1.5 bil- 

 lion years. An event that is unlikely in a laboratory Hask 

 would occur many times in such a gigantic experiment. 



GROWTH AND DIVISION 



Large protein and/or nucleic acid molecules are not enough. 

 Every grocery store is stocked with sterile cans of these sub- 

 stances that do not suddenly spring to life. What is required 

 is a molecule that can duplicate itself and do something else 

 besides— it must be both autocatalytic and heterocatalytic— at 

 least eventually. How can a molecule duplicate itself? Fortu- 

 nately, we now know a mechanism by which the two chains that 

 make up the deoxyribcnucleic acid (DNA) molecule can dupli- 

 cate themselves. 



This molecule is composed of two intertwining strands that 

 are made of only four fundamental units. Each of these units 

 contains a sugar (deoxyribose), phosphoric acid, and a purine or 

 pyrimidine base, either adenine (A), thymine (T), guanine (G), 

 or cytosine (C). The molecule's only uniqueness seems to lie 

 in the order of these bases. The secret of the bonding of the 

 two strands is in the pairing of the bases next to each other, 

 A to T, G to C. This bonding seems to be very specific. Now 

 if the two strands are separated, we find that, while they differ 

 from each other, each contains sufficient information to dupli- 

 cate its opposite. Using the triphosphorylated derivatives of 

 the four nucleotides and a suitable enzyme, it is now possible 

 to make double-stranded DNA from single-stranded DNA in a 

 test tube. A working system for molecular duplication is there- 

 fore at hand. We do not know if the first proto-organism con- 

 tained DNA. It is not impossible that the original ones did. 

 The instability of deoxyribonucleotides in acid solutions is, in 

 itself, a strong argument in favor of a slightly alkaline primeval 

 ocean. 



