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 of
endoreplication produces 210 = 1,024 DNA
strands, which when arranged alongside of each other
produce distinctive banding patterns. Endoreplication occurs
in cells of the larval salivary glands of many species of Diptera, and increases
production of mRNA for
Glue Protein that the
larvae use to anchor themselves to the walls of (for example)
culture vials.