Here is a question from the book SAT II Success Biology E/M (where the SAT is the exam taken by the American high school students):

Homologous chromosomes line up in pairs in
(A) metaphase of mitosis.
(B) metaphase I.
(C) metaphase II.
(D) interphase.
(E) prophase of mitosis.

In my opinion, we can definitely eliminate D and E since in both mitosis and meiosis, chromosomes line up during the metaphase.
Then, I would choose the answer choice B because as far as I know, after prophase I -- when the chromosomes are in pairs during crossing-over -- the chromosomes are still in pairs and become separated into the different ends of the cell during anaphase. In mitosis and metaphase II, however, they align along the "equator" of the cell, forming a line of single chromosomes.
However, I am confused here because of my book. Here is its explanation for this question:

The correct answer is (B). In meiosis, the homologous pairs line up as individuals in meiosis I in preparation for final segregation in meiosis II. Meiosis simply needs to ensure the pairs end up in opposite daughter cells in meiosis I. It is in mitosis that the homologous pairs line up as pairs, and interphase, of course, is not a phase where the chromosomes even appear.

You can see that even though the first line indicates that the answer actually is B, the phrase that's in the cursive contradicts that statement.
I suspect there is a mistake either in the bold or the cursive statement. Could you help me here?


1 Answer 1


You are correct. The books explanation is mistaken, and badly written ("the pairs line up as individuals"???)


In metaphase I of meiosis I, the pairs of homologous chromosomes, also known as bivalents or tetrads, line up in a random order along the metaphase plate.


So, during metaphase I, homologue pairs—not individual chromosomes—line up at the metaphase plate for separation.


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