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CHAPTER 3 



material basis for genes. While it is possible 

 that we may find other kinds of genes, using 

 other criteria or operations, the kind that we 

 have identified by recombination possesses 

 certain specific properties including a very 

 regular mode of transmission (Chapter 2). 

 One regular cellular component transmitted 

 by all cells to daughter cells is the chromo- 

 somes. These have several properties of 

 interest with respect to genes: chromosomes 

 reproduce themselves and are transmitted 

 equally to the daughter cells so that these are 

 identical, in this respect, to each other and to 

 their parent cell. 



We shall make the reasonable assumptions 

 that genes arise only by a process which in- 

 volves the replication of pre-existing genes, 

 and also that diff"erent alleles arise only from 

 each other by mutation, that is, by a gene 

 changing to an alternative form of that gene 

 v/hich in turn is involved in reproducing the 

 alternative form until it mutates. It is 

 found that chromosomes may occasionally 

 become visibly altered in certain ways. In 

 these cases all chromosomes mitotically de- 

 rived from such a modified chromosome have 

 exactly the same alteration. Therefore, both 

 genes and chromosomes are capable of mu- 

 tation and are subsequently involved in 

 replicating their new form. 



Another property of the gene is the reten- 

 tion of its individuality regardless of the 

 nature of its allelic gene. One indirect evi- 

 dence has already been cited for believing 

 this is true also for the chromosomes. This 

 is the independent way that each chromo- 

 some arrives at metaphase. It might be 

 thought, when the chromosomes "disappear" 

 during interphase, that their individuality is 

 lost and even that their contents are dispersed. 

 Evidence that the nuclear material is not dis- 

 persed into the cytoplasm between mitoses al- 

 ready has been presented in the retention of 

 the full amount of chromosomal material 

 within the nucleus during interphase, insofar 

 as revealed by the Feulgen-Rossenbeck 



chromosome-staining procedure. Neverthe- 

 less, it is possible that those components of 

 chromosomes which remain intranuclear 

 may become scrambled during interphase 

 and later resynthesize their proper form 

 during the next prophase. Four lines of 

 evidence can be mentioned bearing on this 

 question. The first three come from studying 

 the appearance of successive mitoses. It is 

 possible to observe the positions of chromo- 

 somes at late anaphase or telophase and 

 again observe the position of the chromo- 

 somes as they enter the next prophase. When 

 this is done it is found that the chromosomes 

 have held the same relative positions, as ex- 

 pected had they retained their integrity dur- 

 ing interphase. Second, since the nucleolar 

 material does not disperse during interphase, 

 those parts of the chromosomes with which 

 the nucleolus is associated probably remain 

 associated during that interval. Third, it 

 sometimes happens that two originally identi- 

 cal homologs are modified by mutation so 

 that one is changed in one respect and the 

 other is changed in a diff'erent respect. The 

 finding that, mitosis after mitosis, these two 

 homologs retain their separate diff'erences 

 is evidence that each homolog, like each allele, 

 has retained its individuality cell generation 

 after cell generation. Finally, there is more 

 direct evidence on the retention of chromo- 

 somal individuality during interphase from 

 cells of the larval salivary glands of certain 

 fruit flies. For these cells have interphase 

 nuclei, and it is possible, because of their 

 giant size, to squash them open and show that 

 they contain the correct number of relatively 

 uncoiled chromosomes linearly identical to 

 the chromosomes seen during mitosis. 



You may already be impressed with the 

 points of similarity between genes and chro- 

 mosomes. However, your attention is called 

 to what is an apparent disparity in behavior 

 between the two. If all nuclei divide by 

 mitosis, then a gamete should contain the 

 same number of chromosomes as the other 



