Polytene Chromosomes

Question 1

What was the purpose of this lab?

Answer 1

to isolate and observe the polytene chromosomes from the salivary glands of D. melanogaster larva

Question 2

What organism was used in this study? what life stage was it at?

Answer 2

larval Drosophila melanogaster (fruit flies)

Question 3

Why were D. melanogaster larva used?

Answer 3

they have polytene chromosomes in certain cells

Question 4

What is a polytene chromosome?

Answer 4

chromosomes that have replicated multiple times and have multiple strands of DNA, making them very thick

Question 5

How are polytene chromosomes formed?

Answer 5

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

Question 6

What type of cells are polytene chromosomes found in? give examples in larval D. melanogaster

Answer 6

terminal cells:

ex. 
salivary gland cells 
ovary nurse cells
fat body cells
gut cells 
follicle cells around oocytes
abdominal histoblasts

Question 7

What is a terminal cell in D. melanogaster?

Answer 7

cells that are differentiated during the larval stage and do not divide again during the fly development

they are removed during the pupal stage

Question 8

T or F: D. melanogaster is the only organism that has polytene chromosomes

Answer 8

false! the same nuclear organization is found in many taxa in both plants and animals

Question 9

Describe specifically the process of polytene chromosome formation

Answer 9

1. somatic homologous chromosomes pair (ex. in salivary glands)
2. DNA is repeatedly replicated

Question 10

What are two unusual characteristics of the formation of polytene chromosomes?

Answer 10

1. somatic chromosomes are forming homologous pairs

2. repeated rounds of replication

Question 11

Approximately how many strands of DNA are there in each salivary gland chromosome of D. melanogaster after replication?

Answer 11

~1024 (~512 in each homologue)

Question 12

What makes the X chromosome in male D. melanogaster noticeable?

Answer 12

it has no homologue so it is half as thick as the other chromosomes

Question 13

What is a chromocenter?

Answer 13

heterochromatin around the centromere of polytene chromosomes is joined together forms a chromocenter

Question 14

Why is the small Y chromosome of male D. melanogaster not visible?

Answer 14

because it is incorporated into the chromocenter