44 



Inside the Living Cell 



that when rephcation occurs, each new chromosome consists one 

 half of old material and one half of newly synthesized material. This 

 is exactly what would be expected if replication occurs according to 

 the Watson-Crick model, i.e. if each dna particle divides into two 

 single threads and each half re-forms a complete double thread (see 

 Fig. 9). 



..I 

 ••I 



■. • ■ 



...I 



i 



>■- 



►~ 



Original DNA, 

 double thread 



The two 

 threads 

 separate 



Each single thread 



acquires a new 



complementary 



thread 



FIG. 9. How the DNA particle is reduplicated 



All that has to be added to this picture to make it true of the whole 

 chromosome is that the dna particles should remain attached to 

 each other in a hnear order, even when replication occurs, so that 

 the replication of all the individual dna particles means the replica- 

 tion of the whole chromosome. (Fig. 10.) 



w$^mM^^m)0^^^m 



A D N A particles joined 

 ^ by protein 



Each single fibre retains 

 its position when the 

 double fibre is split and 

 then attracts its 

 complement. 



FIG. 10. How the whole chromosome might reduplicate itself 



In the whole chromosome the dna threads must be coiled up and 

 there must also be an apparatus which will pull the new particles 

 apart towards the poles of the cell when cell division occurs. It is 

 possible that the histones are involved to some extent as minute 

 muscles in bringing about these processes, but very little is known 



