GENETICS IN MODERN BIOLOGY — BEADLE 403 



How does tlie model help answer these questions ? The key to the 

 structure of DNA is that its molecules are double in a special way. 

 There are two parallel polynucleotide chains wound around a common 

 axis and bound together through specific hydrogen bonding. 



You can more easily visualize the essential features of DNA if you 

 will imagine a four-unit segment of it pulled out in two dimensions as 

 follows : 



A : T 



T : A 



I ' I 



I I 



C : G 



G : C 



Here the four letters represent the four nucleotides ; and the colons, 

 hydrogen bonds. In fact, you can very nicely represent such a seg- 

 ment with your two hands. Place your forearms vertically before 

 you and parallel. Fold your thumbs against your palms and place 

 homologous finger tips together as though they vv'ere teeth on t"wo 

 combs vertically oriented in a single plane, tooth tip to tooth tip. In 

 this arrangement the two index fingers represent the A : T nucleotide 

 pair and so on. 



Imagine many fingers along your forearms — of four kinds corre- 

 sponding to the nucleotides A, T, C, and G. The four kinds of fingers 

 or nucleotides can be arranged in any order on one arm but must 

 always have the complementary order on the other. T opposite A, A 

 opposite T, G opposite C, C opposite G. Thus if one Imows the 

 sequence of nucleotides in one chain, the sequence in the other can be 

 determined by the simple rule of complementarity. 



This structure suggests that genetic information is contained in the 

 sequence of nucleotides; in other words, DNA is a kind of molecular 

 code written in four symbols. One can think of the code as a sequence 

 of nucleotide pairs or of nucleotides in a single chain, for it is obvious 

 that the double chain and the two single component chains all contain 

 equivalent information. In essence the two complementary chains 

 are analogous to forms of a single message, one written in conventional 

 Morse code, the other in a complementary code in which each dot is 

 changed to a dash and A'ice versa. 



Let us now ask the question : how much information is packed away 

 in the nucleus of a human egg? It is estimated that there are about 

 five billion nucleotide pairs per single cell. How much information 

 does this correspond to in terms of, say, information spelled out in the 



