Bizarre Facts in Biology

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Bizarre Facts in Biology

Giant Chromosomes
by David Tapley
In most organisms, the chromosomes are difficult to see. But some organisms have unusually large chromosomes that can be seen even under low power with an ordinary microscope.
What makes these chromosomes so large? It turns out that these giant chromosomes consist of many copies of the chromosome that remain attached side-by-side. For that reason, they are called polytene chromosomes (poly = many; -tene = thread). They form when several bouts of DNA replication occur without the daughter strands separating, and without the cell dividing, a process called endoreplication. The best-studied polytene chromosomes, which are found in the salivary gland cells in larvae of the fruit fly Drosophila melanogaster, typically consist of over 2,000 copies of the DNA. Polytene chromosomes can also be found in other organisms, including beans and mosquitos.
What might be the advantage of having so many copies of the DNA? As you will learn in Chapter 10, the information carried by the DNA must be transcribed into a message carried by RNA before it can be expressed. The cells with polytene chromosomes typically are busy manufacturing massive amounts of gene product. For example, the Drosophila salivary gland cells are producing much saliva, which contains several different proteins, particularly digestive enzymes. Having multiple copies of the chromosomes is one strategy for ensuring rapid synthesis of these gene products, since many RNA messenger molecules can be made simultaneously.
Preparing your own polytene chromosomes is easy to do in the biology lab. Here are some instructions in case you are interested in trying this for yourself.

1. How many bouts of replication do you think it would take to produce a polytene chromosome having at least 4000 individual DNA molecules? Assume that each bout of replication involves all copies of the DNA molecule that are present. Explain your answer. [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

2. Polytene chromosomes are characterized by distinct banding patterns that are characteristic of the particular chromosome being examined. For example, you can identify a Drosophila chromosome number one from any individual by its banding pattern. What do you think this consistent banding pattern tells you about the organization of the individual DNA molecules in that chromosome? Explain your answer. [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

3. Describe briefly how polytene chromosomes are formed. [Hint]
To create paragraphs in your essay response, type at the beginning of the paragraph, and at the end.

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1.	How many bouts of replication do you think it would take to produce a polytene chromosome having at least 4000 individual DNA molecules? Assume that each bout of replication involves all copies of the DNA molecule that are present. Explain your answer. [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



2.	Polytene chromosomes are characterized by distinct banding patterns that are characteristic of the particular chromosome being examined. For example, you can identify a Drosophila chromosome number one from any individual by its banding pattern. What do you think this consistent banding pattern tells you about the organization of the individual DNA molecules in that chromosome? Explain your answer. [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.



3.	Describe briefly how polytene chromosomes are formed. [Hint] To create paragraphs in your essay response, type <p> at the beginning of the paragraph, and </p> at the end.

Chapter 9: DNA: The Molecule of Heredity

Bizarre Facts in Biology