BIOLOGICAL ORDER 



is essentially the formation, by each of the strands, of a comple- 

 mentary strand. The new base, hypoxanthine, will bind cytosine, 

 which in turn will bind guanine. Thus, as a consequence of a de- 

 amination by nitrous acid, the original base pair, adenine/thymine, 

 is in one of the daughter helices, replaced by the pair guanine/ 

 cytosine. A gene mutation, in fact a so-called "point mutation," is 

 nothing more. 



The "mutated" protein. We have learned that nucleic acid con- 

 trols protein synthesis. What is the result of a gene mutation in the 

 protein? What is the difference between the proteins corresponding, 

 respectively, to a given wild-type gene and to this gene once it has 

 undergone a mutation? Hemoglobin, the protein of the red blood 

 cells responsible for the transfer of oxygen, has been the subject 

 of extensive studies. The normal human red cell is a disc: it looks 

 round when it lies on its flat surface. In a hereditary blood disease 

 of man, the red cells are reduced in number and exhibit the form 

 of a sickle, hence the name of the disease: sickle-cell anemia. The 

 sickle hemoglobin is different from the normal hemoglobin in a 

 number of properties (Table I). 



But the differences have been traced back to one amino acid. 

 By mild digestion, the hemoglobin can be cut into small peptides. 

 As discovered by Vernon Ingram, one of these peptides is different 

 in the normal and mutated proteins. In the sickle hemoglobin S, the 



Table I. Amino Acids Sequence in Normal and Abnormal 

 Hemoglobins. 



Peptide fragments of normal hemoglobin (A), sickle-cell hemoglobin (S), 

 and hemoglobin (C), associated with the so-called "hemoglobin C disease." 



[28] 



