In contrast to the monocentric chromosomes of humans, the nematode C. elegans has holocentric chromosomes, where kinetochores and microtubule attachments occur along the entire length. This structural difference leads to a distinct appearance of chromosomes during which mitotic phase?
- Prophase, where holocentric chromosomes condense into spherical, rather than linear, shapes.
- Metaphase, where holocentric chromosomes align perpendicular to the metaphase plate.
- Telophase, where holocentric chromosomes decondense much more rapidly than monocentric ones.
- Anaphase, where sister chromatids separate and move to the poles as parallel bars. (correct answer)
Explanation: When you encounter questions about chromosome structure and mitotic behavior, focus on how the location of kinetochores affects chromosome movement during cell division. The key difference between monocentric and holocentric chromosomes lies in where spindle fibers attach and how this influences their appearance during mitosis. In monocentric chromosomes (like humans), kinetochores form at a single centromere, creating a point of constriction. During anaphase, sister chromatids separate and are pulled toward opposite poles by their centromeres, creating the classic V-shaped appearance as the arms trail behind. However, holocentric chromosomes have kinetochores distributed along their entire length, meaning spindle fibers attach everywhere rather than at a single point. This structural difference creates a dramatically different anaphase appearance. Since microtubules attach along the entire length of holocentric chromosomes, the separated sister chromatids move toward the poles as rigid, parallel bars rather than V-shaped structures. There's no trailing of chromosome arms because the pulling force is distributed evenly. Option A is incorrect because chromosome condensation in prophase isn't significantly affected by kinetochore distribution—both types condense into linear shapes. Option B is wrong because both chromosome types align at the metaphase plate regardless of kinetochore structure. Option C is incorrect because decondensation speed in telophase isn't determined by kinetochore distribution but by other cellular factors. Remember that kinetochore location directly determines how chromosomes move during anaphase. When you see questions about unusual chromosome structures, always consider how they would affect the mechanics of chromosome separation and movement.