Which of the following depicts a chromosome that started as

A B C D -- E F G H [where -- represents the centromere], but has undergone a pericentric inversion?

Question

Which of the following depicts a chromosome that started as

A B C D -- E F G H [where  -- represents the centromere], but has undergone a pericentric inversion?

BlackTom AI · Accepted Answer

Restating the question and options: The problem asks which depiction shows a chromosome that started as A B C D -- E F G H, where the double dash marks the centromere between C and D, and has undergone a pericentric inversion. The answer choices are:
- A B C -- E F G H
- A B C D -- E F E F G H
- A B C E -- D F G H
- A B C D -- E G F H

Now, analyzing each option in turn:

Option 1: A B C -- E F G H
- This sequence places the centromere between C and E, effectively removing D and reordering around the centromere without showing a contiguous inverted segment that includes the centromere. A pericentric inversion must involve a reversal of a segment that spans the centromere itself, which this option does not illustrate. Therefore this option misrepresents the consequence of a pericentric inversion.

Option 2: A B C D -- E F E F G H
- This option contains a duplicated and scrambled sequence (E F repeated, awkward placement around the centromere) that is not a plausible result of any single inversion event. Since inversions rearrange existing blocks rather than duplicating sections, this option is biologically inconsistent with how inversions work. It can be ruled out for that reason.

Option 3: A B C E -- D F G H
- Here, the sequence shows that the segment on the left of the centromere (A B C) remains, but immediately after, E appears on the left side of the centromere, followed by the centromere, then D and the remaining right-arm genes F G H. This pattern is consistent with a pericentric inversion that reverses a region spanning across the centromere, swapping order of blocks on both arms (the left-arm block C and the right-arm block D…H become rearranged so that E moves adjacent to C on the left, and D moves adjacent to the centromere on the right). In essence, a segment including C through E (encompassing the centromere) has flipped, producing the observed arrangement with E preceding the centromere-side block and D moving to the opposite side. This aligns with the characteristic outcome of a pericentric inversion.

Option 4: A B C D -- E G F H
- This option shows only a rearrangement after the centromere that swaps F and G relative order on the right arm (E G F H), without altering the left arm beyond C. Inversions that do not include the centromere would be paracentric; even then, the depicted change does not reflect a reversal across the centromere across both arms. Since the centromere’s position relative to the inverted region is not satisfied here, this does not depict a pericentric inversion.

In summary, Option 3 presents a rearrangement that involves segments on both sides of the centromere, consistent with a pericentric inversion, while the other options fail to capture the cross-arm reversal or introduce implausible changes. The reasoning above highlights why each alternative does or does not fit the definition and consequences of a pericentric inversion.

Which of the following depicts a chromosome that started as A B C D -- E F G H [where -- represents the centromere], but has undergone a pericentric inversion?单项选择题

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