The rearing of D. melanogaster under specific conditions and the experimental extraction of the salivary gland chromosome permits the cytological investigation of the structure, formation and functioning of the polytene (giant) chromosomes and genes during the larval development. The hypothesis being tested is that endomitosis occurs in larval stage development of D. melanogaster which causes one to predict that the polytene chromosomes are formed due to the endomitotic process. Also the formation of polytene chromosomes may be rapid. Endomitosis is the process of chromosome duplication without cell division, therefore the salivary gland cells only enlarge. Materials and Methods
The preparation and extraction procedure followed was according to the Biology Department, 2013 lab manual. However, a prepared slide was used as the extraction was not successful. Results
Figure 1 shows a D. melanogaster polytene chromosome with two visible chromosomal tips in the mass, at the particular magnification, which attach to a large band-free point. There are irregularly spaced bands with different band widths present, but no visible puffs. There is also no nucleus visible. The fruit fly chromosomes are much larger in comparison to the human chromosomes which are stick-like structures that are not continuous. Discussion
The polytene chromosomes in the salivary gland cells of the larval D. melanogaster are giant formations of multiple chromatids lying in a parallel register attached at the chromocentre (Hartwell et al., 2011). The polytene’s arise due to endomitosis (Biology Department, 2013). The lack of mitosis causes successive doubling in the original permanently synapsed homologous chromosome which results in irregular band widths and further produce puffs (Alberts et al., 1994). Puffs are non-banded highly active RNA transcription areas along the length of the chromosome (Demakov et al., 2004).
Alberts, B., D. Bray, Lewis, J., M. Raff, Roberts, K. and J.D. Watson. 1994. Molecular Biology of the Cell 3rd Ed. NY: Garland Sciences.
Biology Department. 2013. Laboratory Manual for Introductory Genetics. Ottawa: Carleton University.
Demakov S, Gortchakov A, Schwartz Y, Schwartz Y, Semeshin V, Campuzano S, Modolell J, Zhimulev I.. 2004. Molecular and genetic organization of Drosophila melanogaster polytene chromosomes: evidence for two types of interband regions. Genetica 122: 311–324.
Hartwell, L.H., L. Hood, M.L. Goldberg, A.E. Reynolds, and L.M. Silver. 2011. Genetics: From Genes to Genomes 4th Ed. NY: McGraw-Hill Companies.