Molecular Genetics

Facts

• Polytene chromosomes are found in salivary glands of Diptera such as Drosophila. Their 2 mm length is much longer than metaphase chromosomes. (Image from http://www.nature.com/hdy/journal/v90/n2/images/6800220f1.jpg)
• The polytene chromosomes consist of 100 or more copies of DNA, arranged side by side. The DNA is said to be endoreduplicated. Endoreduplicated DNA is also found in other specialized tissues such as the suspensor of plant embryos.
• Examination of these chromosomes by Feulgen staining and light microscopy reveals alternating highly and moderately dense regions, called bands and interbands.
  

Interpretations

• The expanded length of polytene chromosomes, relative to metaphase chromosomes, allows a much higher resolution mapping of morphological chromosome features than is possible with G- or Q-banding.
• The high local DNA concentration of aligned DNA sequences make polytene chromosomes ideal targets for in situ hybridization with specific sequence probes.
• The lesser density of interbands suggests that the chromatin in these regions is less condensed and may correspond to euchromatin of normal interphase nuclei. Conversely, the bands probably correspond to the condensed heterochromatin.

Further information

• Polytene chromosomes can be prepared very easily.
• Electron micrographs allow the fine definition of banding patterns.
• Polytene chromosomes can be analysed by staining with antibodies.

This is page 1133 of Molecular Genetics by Ulrich Melcher, © 1997, 1998, 2009