20 



A TEXTBOOK OF THEORETICAL BOTANY 



-Kinetochore 



chromatin reticulum in the metaboHc nucleus into a number of thin, separate 

 threads, the chromonemata, which do not stain very deeply. Each of these 

 chromonemata is double. The two together represent one chromosome, 

 and each individual thread or half-chromosome is also called a chromatid, 

 for convenience in tracing it through the subsequent changes. 



Each chromatid shows a double series of spiral codings or gyres. 

 Firstly, the two members of a pair are loosely twisted round each other 

 (relational coiling), these coils being relic coils, left over from the previous 

 mitosis. As prophase advances the relic coils disappear and a new series 



of minor gyres develops, each chromatid coiling 

 y^flj^ independently. 



^^^^ The two chromonemata of each pair are linked 



together at one point, where there is an unstained 

 spot or granule, known as the kinetochore or 

 centromere, usually situated not far from one end. 

 Its function will appear later. The portions of the 

 chromosome on each side of the kinetochore are 

 called the arms. 



As prophase draws to a close three things happen. 

 Firstly, the chromonemata begin to uncoil, so that 

 the gyres are fewer and larger. Secondly, a densely 

 stainable matrix begins to accrete around each 

 chromatid, forming a double, thickened chromosome 

 (Fig. 6). Thirdly, each chromonema splits length- 

 ways, so that each chromatid now contains, within 

 the matrix, two chromonemata which are relationally 

 twisted together as well as coiled. The entire 

 chromosome at this stage therefore consists of two 

 thickened chromatids, conjoined at the kinetochore, 

 and in each chromatid there are two chromonemata. 

 The chromosomes shorten and become thicker and 

 denser, and the coils of the chromonemata become 

 closer. In this condition they enter the metaphase 



(Fig- 7)- 



The nuclear membrane now disappears, so that 



the chromosomes are no longer separated from the 



cytoplasm, and the nucleolus also disappears. The chromosomes arrange 



themselves across the middle plane of the cell, forming the equatorial plate. 



If the chromosomes are short they may lie horizontally on the plate, but if 



they are long it may be only the portions containing the kinetochore which 



actually lie on the plate, the arms pointing in different directions. 



The kinetochores then divide, so that the chromatids separate and become 



independent chromosomes. Simultaneously two sets of fine kinoplasmic 



fibrils appear, one on each side of the nucleus, converging towards the two 



opposite poles of the cell. These are the spindle fibres, and together they 



make up a double-ended cone, the achromatic spindle, so called because 



-Chromonema 

 thread 



Matrix 



Fig. 6. — Diagram of the 

 structure of a half 

 metaphasic chromo- 

 some, z'.e., a chromatid. 

 {After Schroder.) 



