Mehmet Ali Özsatıcı
20000302
Drosophila Salivary Gland Chromosomes
Aim: To study the polytene chromosomes through the model organism of
Drosophila melanogaster.
Introduction: The idea of genetic material and hereditary system has always been
an area of research for scientists. The mechanisms of how living beings pass on their
traits to their off-springs or what material does the job has always been questioned.
However, humans being a macro living being, and this biological phenomenon being
a micro matter has made things burdensome, even with the help of technology such
as microscopes. Even though it has been challenging, a situation called ‘polytene
chromosomes’ made it a little bit more untroubled for humans to study genetics. With
its enormous size Polytene chromosome is a special situation where DNA replicates
when there is no cell division, causing chromosome to have more chromatids and
appearing bigger in size – easier to study under microscope, along with evolutionary
advantages of its own (Zhimulev and Koryakov, 2009).
Drosophila melanogaster (fruit fly) is a model organism (meaning it is a non-human
species that is studied to observe certain biological event) for polytene chromosomes.
D. melanogaster has many advantages to be a model organism, They are easy to
catch with fruits, rapid to breed with culture (Reeve and Black, 2001), easy to
distinguish for the separation of the sexes, as known as dimorphism, such as tail
structure or size differences (Setoguchi et al., 2014). Their genome map is sequenced
and known, make it easier to study (Adams, 2000).
The life cycle of D. melanogaster is studied under 4 subgroups: embryo, larva, pupa
and adult. The main focus of this experiment is the third instar larva in the larva stage
because salivary gland produces polytene chromosomes in that stage for the
production of glue-like substance for organism to stick to ground, giving an evolutionary
edge.
Materials and Methods: After the catching and reproduction of laboratory
reproduction of D. melanogaster, experiment was started with putting a drop of %50
acetic acid on the slide, using a Pasteur pipette. A third instar larva D. melanogaster
was transferred onto the slide for dissection and releasing of the salivary glands. Pins
were used to pull its head from tail for further dissection. After removing the rest of the
body besides the salivary glands, %2 aceto-orcein stain was used to stain the sample
for better observation. After two minutes of hold up, the slide was covered with cover
slip and pressed with the help of a napkin, squashing the salivary glands to release the
polytene chromosomes. Lastly, the final sample was observed under a light
microscope.
Results:
Figure
1:
Polytene
chromosomes
under
https://www.sciencephoto.com/media/313051/view)
microscope
(Citation:
Discussions: As the results show, polytene chromosomes have two distinguished
sites. Sites where appears thick (dark bands) is where there are more DNA than RNA,
being tightly packed. However, there are sites where appears loose (interbands),
where there are more RNA than DNA (Sumner, 2008). These interbands are uncoiled
for mRNA transcription with RNA polymerase (Zhimulev et al., 2004). This situation
looks like a bubble under the microscope, as known as puffs or Balbiani rings named
after the founder of the polytene chromosomes Édouard-Gérard Balbiani in 1881. In
D. melanogaster, these Balbiani rings are used for the massive production of
mucoprotein to stick the organism to the ground for preparation of pupa stage.
In the experiment, there could be improvements such as adding other variants to
the experiment for the sake of comparison. Such as using different acids for dissection
stage to see if it causes any changes, what is the best acid to use etc. or causing
different changes in the key elements of the habitat for the fruit flies (coldness, overheating, restriction to food or different pheromone signaling) to observe if it changes in
the production of the mucoprotein from the polytene chromosomes in the offspring.
In conclusion, this biological phenomenon is both a blessing for the fruit flies with
usage of glue, and a blessing for the humans with the understanding of genetics and
nature. One situation, two species, giving them both an evolutionary edge, helping
them survive…
Reference list:
-
Adams, M.D. (2000). The Genome Sequence of Drosophila melanogaster.
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2021].
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Fernández-Moreno, M.A., Farr, C.L., Kaguni, L.S. and Garesse, R. (2007).
Drosophila melanogaster as a Model System to Study Mitochondrial Biology.
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https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4876951/ [Accessed 12 May
2021].
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Reeve, E.C.R. and Black, I. (2001). Encyclopedia of Genetics. [online] Google
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