Polytene chromosomes are specific interphase chromosomes consisting of thousands of
deoxyribonucleic acid (DNA) strands. For this reason they are very large and display a
characteristic band–interband morphology. Polyteny arises in tissues, organs and at
developmental stages when there is need for the rapid development of an organ at an
unaltered high level of function. Organs containing cells with polytene chromosomes
are, as a rule, involved in intense secretory functions accomplished during a short time
against a background of rapid growth. Chromosome rearrangements and in situ
hybridization on polytene chromosomes allow genes to be mapped to a resolution of a
few tens of kilobases. Polytene chromosomes allow a specific narrow region to be
dissected out with a micromanipulator and a library of DNA clones to be derived from
the region