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Samford University -- Department of Biological and Environmental Sciences
Experimental Genetics   Biol 334
The Polytene Chromosomes of Drosophila melanogaster
The salivary glands of Drosophila melanogaster (as well as some other flies of the order Diptera) have exceptionally large chromosomes called polytene chromosomes. They are useful in the study of gene-chromosome relationships and have several unique features:
  1. They are unusually large, being the length of uncondensed (interphase) chromosomes.
  2. They are "polytene" or many stranded due to repeated chromosome replication without cell division (about 1000 strands = 2 x 29)
  3. They show somatic synapsis.
  4. The centromeres of all 8 chromosomes (2n=8) are together in a structure called the chromocenter, therefore they appear to have 5 long arms and one short one (chromosome 1 is the X and is telocentric, chromosome 2 and 3 are metacentric, and chromosome 4 is tiny and is telocentric).
  5. Bands (chromomeres) are visible along their length due to heterochromatinization of inactive regions. These can form "puffs" as gene activation occurs.
To see these chromosomes:
  • Select a large, feeding third instar larva (the largest larvae, usually crawling on the wall of the vial) and, using a dissecting needle, transfer it to a petri dish with a drop of insect ringer's solution. Be sure to select one that is still moving.
  • Using two sharp dissecting needles or pins, dissect the larva under a dissecting microscope as follows:
    • Place one pin between the dark mouth parts at the very anterior end.
    • Place the other pin about 1/2 of the way down the body.
    • Pull the two apart.
  • The two salivary glands should be visible as nearly transparent structures connected to each other at their anterior ends.
  • Carefully blot away excess moisture and debris.
  • Place a drop of aceto-orcein stain on the glands, then carefully blot most of it away. (Take care to not suck up the salivary glands.)
  • Place another drop of stain on the glands and stain for 30 minutes.
  • Do not let the stain evaporate (add more as needed).
  • Add a coverslip, place the slide on a double layer of paper towels, and place another double layer on top.
  • Squash firmly with your thumb pressing straight down (use all your strength and weight).
  • Immediately ring the slide with fingernail polish to prevent evaporation.
  • Search for polytenes on low power and sketch any good ones you find below using maximum magnification.

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Answer the QuestionTritiated uridine (3H-uridine) is a radioactive RNA nucleotide that labels newly synthesized RNA. If Drosophila melanogaster larvae were treated with 3H-uridine, salivary glands stained, squashed, then autoradiograhy performed, where on the polytene chromosome would the radioactive label appear?


Answer the QuestionWhat would a polytene chromosomes look like in a paracentric inversion heterozygote?