Polytene chromosomes in Drosophila (2n = 4)
Polytene chromosomes
are produced by endoreplication,
in which chromosomal DNA undergoes
mitotic
replication, but the strands do not separate. Ten rounds
produce 210 = 1,024 DNA
strands, which arranged in parallel show distinctive
banding patterns. Endoreplication occurs in cells of the
larval salivary glands of many species of Diptera. They increase
production of mRNA for
Glue Protein that the
larvae use to anchor themselves to surfaces, including the
walls of culture vials.