MITOSIS 



fourth is that each chromosome owes its characteristic shape to the 

 possession of a constant point of attachment to the spindle, marked 

 by a constriction where the chromosome bends. The main body of 

 the chromosome may lie off the spindle and off the plate : only the 

 point of attachment is fixed on the spindle. The fifth is that, except 

 at these attachment constrictions, the chromosomes are double 

 throughout. They consist of pairs of sister chromatids (Fig. 4). 



The importance of these structures becomes apparent at the next 

 stage. All the chromosomes simultaneously begin to split at those 

 very points of attachment where they have previously remained 

 single. It can then be seen, after proper treatment, that the point 

 of attachment consists of a minute particle, the centromere. It is the 

 centromere which organizes the developing spindle. It is also the 

 centromere which suddenly divides, or explodes, so that its two 

 halves are driven apart along the fibres of the spindle, that is towards 

 the poles. Each half drags behind it one of the two chromatids of 

 its chromosome. Hence the whole body of the chromosomes lying 

 on the plate is divided into two groups of daughter chromosomes, 

 identical in number, shape and size. The separation of these daughter 

 chromosomes is known as anaphase. 



Two daughter nuclei are reformed from the two groups. The 

 chromatids now loosen their packed spirals, and their fme threads 

 are lost once more in the optically homogeneous nucleus. One or 

 more nucleoli reappear in each nucleus. A partition or wall develops 

 across the equator of the spindle and separates the daughter cells. 

 This is telophase. Mitosis is complete. 



This course of action shows us that it is through the chromosomes 

 that the nucleus transmits its character to its daughters, that is from 

 cell to cell. It shows us, too, how a complement of chromosomes 

 inside a nucleus is divided into two daughter complements which 

 are distributed to the two daughter nuclei. The significant fact for 

 genetics is that these two complements are always exactly like one 

 another. It was first noted so long ago as 1879 by Flemming, one of 

 the discoverers of mitosis. This observation, however, leaves us with 

 yet another question. How does each chromosome, single as it is 

 when it disappears in telophase, come to be double when it reappears, 

 consisting of two apparently identical halves, in the following 

 prophase ? The obvious explanation, the one long ago suggested by 



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