Giant Chromosomes

Giant chromosomes are of two types, polytene chromosomes (salivary gland chromosomes) and lampbrush chromosomes.
 

Polytene Chromosomes (Salivary Gland Chromosomes)

Normally chromosomes are not visible during interphase. Polytene chromosomes are exceptions. They were first observed by Balbiani in the salivary glands of the midge Chironomous, and are hence called salivary gland chromosomes. Also, all the chromosomes of a particular type in the individual have an equal number of bands with similar distribution. Topographical maps of the bands and interbands have been made, parallel to genetic maps.






Also mutations are associated with certain bands (deletions). This led to the concept that the bands are the sites of genes (DNA), and that the interbands are relatively inert linker regions. If this were so there would be only about 5,000 genes in Drosophila. This number is far too low. It is now believed that the interbands also contain many genes. Recent discoveries suggest that the DNA extends continuously through the bands and interbands.

Unit Chromatids. In the salivary gland chromosomes of Drosophila, DNA is concentrated in the bands, while the interbands have very little DNA. However, the fact that the interbands do eontain DNA has been established by quantitative staining with Feulgen stain, by fluorescent to form nucleoprotein fibers (100-500A).

DuPraw and Rae (1966) have shown that these fibers are comparable to the 100-500A nucleoprotein fibers that are found in typical interphasic nuclei. They have postulated that the fibers correspond to unit chromatids (chromonemata). The polytene chromosome is formed by about 1,000-4,000 unit chromatids. Each unit chromatid consists of a single DNA molecule, several centimeters long, associated with protein. The fibers are continuous from one end of the chromosome to the other. They are tightly folded in the bands and relatively extended in the interbands.