Chapter 19 Looking at Polytene Chromosomes

Abstract

Publisher Summary One goal of genome projects is construction of contig maps that allows any DNA or RNA sequence from an organism to be mapped directly to a specific site on its genome. For Drosophila , contig maps come ready-made in the form of polytene chromosomes. The resolution of mapping on Drosophila polytene chromosomes is comparable to the resolution of mapping on an arrayed DNA clone library. On the polytene chromosome, the resolution depends on the morphology of the regions of interest. This chapter focuses on methods and problems associated with the use of polytene chromosomes. The most useful polytene chromosomes are found in the salivary gland of the late third instar larvae. The number of chromatids in a polytene nucleus is affected by the growth conditions of the larvae. It is also affected by the genotype of Drosophila melanogaster and genetics can sometimes be used to advantage for studying the chromosomes. Polytene chromosomes are larger than metaphase chromosomes both because of the multiple chromatids and because of lower compaction. In region 87E1-2, the packing ratio is 180, whereas in region 87D5-10 the ratio is 23. The variations in packing reflect the chromatid structure of a given region and because of the precise alignment of chromatids, the packing differences form bands and interbands that extend across the width of the chromosomes.