94 CELL HEREDITY 



microscope, in terms of length, position of the centromere, arrangement 

 along the chromosome of small thickenings called chromomeres, and of 

 larger ones called knohs, of various constrictions at particular locations, 

 and of differential intensity of staining with basic dyes (which react 

 with nucleic acids). 



Very detailed observations of chromosome morphology are possible 

 with the remarkable many-stranded chromosomes found in some insect 

 larvae, the best known being the salivary gland chromosomes of Droso- 

 phila lar\ae (Figures 3.7 and Plate VIII). These chromosomes are about 

 100 times the length of metaphase chromosomes of adult Drosophila tis- 

 sues, and contain about 1024 times as much DNA as do the correspond- 

 ing chromosomes of adult diploid cells. The salivary gland chromo- 

 somes are polytene, each consisting of a parallel array of 1024 strands, 

 with each strand corresponding to one chromosome of the adult cell. 

 The homologous chromosomes are synapsed and show the same precision 

 of point-for-point pairing found in meiotic pachytene. As seen in 

 Figure 4.2, each salivary chromosome has a distinctive pattern of deeply 

 staining bands separated by faintly staining regions. These patterns 

 have provided the basis for detailed cytological maps which have been 

 correlated with genetic maps derived from recombination data (Figure 

 3.7). 



Another extraordinary type of chromosome is that found in the unfer- 

 tilized eggs (oocytes) of many amphibians and other vertebrates. These 

 so-called lampbrush chromosomes attain lengths of over a millimeter 

 during meiotic prophase. Electron microscope studies of their fine 

 structure will be discussed in Chapter 7. No cytogenetic correlations 

 have been made with this material because, in general, the species with 

 good lampbrush chromosomes have not been well suited to genetic 

 analysis. 



Cytogenetic correlations in Drosophila have been established primarily 

 with salivary gland chromosomes, and in other organisms, primarily with 

 the chromosomes of meiotic stages. In maize, for example, as shown 

 in Plate Vb, the morphological individuality of each of the ten chromo- 

 somes is readily distinguishable in the prophase stages of meiosis. 



Associated with the chromosomes is a body (sometimes more than one) 

 called the nucleolus, which consists mainly of protein and RNA, and 

 which breaks down in every mitotic cycle and reappears at telophase. 

 The reorganization of the nucleolus has been shown to be under the 

 control of a special chromosomal region, called the nucleolus organizer. 

 McClintock studied a maize strain in which this region was split by 

 X-rays and one part was translocated to another chromosome. She 

 found that each part independently organized some nucleolar material. 



