Polytene Chromosomes Flashcards
What was the purpose of this lab?
to isolate and observe the polytene chromosomes from the salivary glands of D. melanogaster larva
What organism was used in this study? what life stage was it at?
larval Drosophila melanogaster (fruit flies)
Why were D. melanogaster larva used?
they have polytene chromosomes in certain cells
What is a polytene chromosome?
chromosomes that have replicated multiple times and have multiple strands of DNA, making them very thick
How are polytene chromosomes formed?
in larva, the number of cells stays the same, but the amount of protein required is high so some tissues will have cells that enlarge and the chromosomes will replicate many times
many strands of DNA from the multiple rounds of replication make extra large chromosomes = polytene chromosomes
What type of cells are polytene chromosomes found in? give examples in larval D. melanogaster
terminal cells:
ex. salivary gland cells ovary nurse cells fat body cells gut cells follicle cells around oocytes abdominal histoblasts
What is a terminal cell in D. melanogaster?
cells that are differentiated during the larval stage and do not divide again during the fly development
they are removed during the pupal stage
T or F: D. melanogaster is the only organism that has polytene chromosomes
false! the same nuclear organization is found in many taxa in both plants and animals
Describe specifically the process of polytene chromosome formation
- somatic homologous chromosomes pair (ex. in salivary glands)
- DNA is repeatedly replicated
What are two unusual characteristics of the formation of polytene chromosomes?
- somatic chromosomes are forming homologous pairs
2. repeated rounds of replication
Approximately how many strands of DNA are there in each salivary gland chromosome of D. melanogaster after replication?
~1024 (~512 in each homologue)
What makes the X chromosome in male D. melanogaster noticeable?
it has no homologue so it is half as thick as the other chromosomes
What is a chromocenter?
heterochromatin around the centromere of polytene chromosomes is joined together forms a chromocenter
Why is the small Y chromosome of male D. melanogaster not visible?
because it is incorporated into the chromocenter
