QUEST FOR LIFE BEYOND THE EARTH — SAGAN 299 



recent ideas on the origin of life on Earth some 4 billion years ago, and 

 then continue with a discussion of the physical environments of the 

 moon and planets, and finally, a look at the more direct evidence for 

 life beyond the Earth. 



Wlien life began depends upon the definition of life, and, curiously 

 enough, there is no definition acceptable to all biologists. Yet, the 

 many characteristic features of living systems — their complex and 

 highly structured forms, their growth, metabolism, and reproduction — 

 are all ultimately attributable to evolution by natural selection. And 

 evolution occurs in plants and animals because of the interaction of 

 the environment with the hereditary material, a kind of molecular 

 blueprint which controls metabolism, produces a replica of itself for 

 the next generation to follow, and, through the centuries, gradually 

 changes, or mutates, occasioning new forms of life. The key mole- 

 cules of the hereditary material are the nucleic acids, ribonucleic acid 

 (ENA) and deoxyribonucleic acid (DNA) . Thus, the problem of the 

 origin of life seems to be connected with the problem of the origin 

 of nucleic acids. 



The structure and function of DNA have been elucidated chiefly by 

 James D. Watson, of Harvard, and Francis H. C. Crick, of Cambridge 

 University. It is a long molecule, comprising two molecular strands 

 wound about each other in a coil, or helix. During cell division, the 

 strands separate, and each synthesizes a copy of the other, yielding two 

 molecules of DNA where originally there was one. The building 

 blocks for this synthesis are called nucleoside phosphates, and much of 

 the activity of the cell is devoted to constructing these building blocks 

 from yet simpler molecules, and joining them together to form nucleic 

 acids. The nucleoside phosphates are each composed of a sugar, a 

 base, and one, two, or three phosphates. A given nucleic acid molecule 

 is generally composed of four kinds of nucleoside phosphates. Their 

 sequencing along the chain is a kind of four-letter code that deter- 

 mines which proteins a cell will make. Proteins, in turn, are long 

 chains of amino acids, and recent evidence indicates that three nucle- 

 oside phosphates are required to specify each amino acid in a protein. 

 The transcription sequence is this : DNA makes KNA ; several kinds of 

 RNA make proteins — in particular, enzymes ; and enzymes govern the 



Figure 1. — Schematic illustration of a short section of the Watson-Crick model of DNA. 

 The two helical strands can be seen running vertically, in opposite directions, on the 

 right and left sides of the figure. As the detailed inset shows, the strands are 

 connected by pairs of bases, chosen from the four bases adenine (A), cytosine (C) , 

 guanine (G), and thymine (T). The strands themselves are made of sugars (S) and 

 phosphates (P). A combination of a base and a sugar (e.g., A-S) is called a nucleoside; 

 a combination of a base, a sugar, and a phosphate (e.g., A-S-P) is known as a nucleoside 

 phosphate. Thus, the DNA molecule can be considered to be constructed of a linear 

 sequence of nucleoside phosphates. The sequence of bases (e.g.. along the left 

 strand of the inset TCAG) specifies the genetic code. 



