MCAT Question A Day - 5/15/13 - Answer!
Synapsis occurs between two homologous chromosomes during Prophase I of meiosis. During synapsis, one of two identical chromatids on one homologous chromosome can exchange genetic material with one of two identical chromatids on the other homologous chromosome. What will result from such an exchange?
A. The two “genetically identical” chromatids on one of the homologous chromosomes will no longer be identical.
B. The two “genetically identical” chromatids on both of the homologous chromosomes will no longer be identical.
C. All four chromatids will be genetically different.
D. B and C
The correct answer is (D). When a part of a paternal chromatid “crosses over” with the equivalent part of the maternal chromatid, all four chromatids will be different. There will be an “original” paternal chromatid, a second paternal chromatid that now has a part of the maternal chromatid, an “original” maternal chromatid, and a second maternal chromatid that now has a part of the paternal chromatid.
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