PsychDr2B
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A medical student is studying a liver biopsy taken from a regenerating liver following a partial
hepatectomy. The student sees a dividing cell in which the chromosomes are aligned on a plate. Which of
the following best describes the character of the chromosomes in the plate?
A. 23 chromosomes with 2N DNA total and with each chromosome having one chromatid
B. 23 chromosomes with 4N DNA total and with each chromosome having two chromatids
C. 46 chromosomes with 2N DNA total and with each chromosome having one chromatid
D. 46 chromosomes with 4N DNA total and each chromosome having two chromatids
E. 92 chromosomes with 2N DNA total and with each chromosome having two chromatids
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| kalsam
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answer. D
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| PsychDr2B
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The correct answer is D.
The cell division is mitotic rather than meiotic. The cell is in metaphase,
as evidenced by alignment of chromosomes on the metaphase plate.
Doubling of the DNA occurs much earlier, in prometaphase (late
prophase), so the cell has "4N" DNA (double the normal 2N diploid DNA).
The chromosomes at the metaphase plate consist of 2 chromatids each,
but the total number of chromosomes is still the normal 46.
23 chromosomes with 2N DNA (choice A) is wrong because stages with 23 chromosomes only occur in meiosis.
23 chromosomes with 4N DNA (choice B) is wrong because stages with 23 chromosomes only occur in meiosis. Additionally, there is never a point in either meiosis or mitosis when 23 chromosomes contain 4N DNA.
46 chromosomes with 2N DNA, each chromosome having one chromatid (choice C), is wrong because the chromosomes have two chromatids and the DNA is therefore doubled (producing 4N DNA total) in metaphase. The cell normally has 46 chromosomes with 2N DNA only after mitosis is complete.
92 chromosomes (choice E) is wrong because in anaphase and telophase, at which points there are 92 chromosomes (in two cells which are not yet cleaved), there is a total of 4N DNA and each of the newly split chromosomes contains one chromatid.
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| Ritalin
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Thank you very much for sharing it with us, PsychDr2B. It was an excellent and thoughtful question!
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