EXERCISE IV 



MITOSIS (KARYOKINESIS) 

 Also Called 



INDIRECT CELL DIVISION 



A. INTRODUCTION 



In this type of cell division characteristic changes occur in the nucleus, 

 the cytoplasm, and the centrosome. The most important changes take 

 place in the chromatin, the deeply staining portion of the nucleus. The 

 nuclear membrane disappears, and the chromatin, which was arranged 

 in a net-like reticulum, gradually rearranges itself into fine coiled threads 

 (fine spireme) which shorten and thicken into coarser, more loosely 

 coiled threads (coarse spireme). From the coarser threads are developed, 

 by further shortening and thickening, definite bodies called chromosomes. 

 The number of chromosomes is different for different species, and is 

 constant for any given species. 



In the cytoplasm a spindle-shaped figure composed of thread-like 

 structures is formed. At the ends of the spindle, where the threads 

 converge, are two deeply staining bodies, the centrosomes, from which 

 other threads radiate in all directions. These latter radiating threads 

 are the astral rays. Upon the middle of this spindle the chromosomes 

 take their place in a flattened group, the equatorial plate. Each chromo- 

 some splits longitudinally and equally and the two halves go to opposite 

 ends of the spindle. Thus two new groups of chromosomes are formed, 

 each of the same number as was present in the group from which they 

 came. A cell membrane forms between the groups of chromosomes, 

 dividing the cell into two cells. The chromosomes of each group now 

 undergo a series of changes approximately the reverse of those in the 

 early stages of division; that is, they become diffuse again, spinning 

 out into a fine retieulum, thereby forming two new nuclei like the original 

 one. 



For convenience the process of mitosis may be divided into four 

 intergrading stages: (1) the prophases, that is, the early stages up to and 

 including the equatorial plate; (2) the metaphase, in which the chromo- 

 somes divide longitudinally; (3) the anaphases, in which the half chro- 

 mosomes are distributed to opposite ends of the spindle; and (4) the 

 telophases which include the division of the body of the mother cell and 

 the reconstruction of the daughter nuclei. 



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