Which of the following events occurs during metaphase?
A. Spindle fibers shorten.
B. A cell begins to divide in two.
C. Nuclear membrane breaks down.
D. Chromosomes line up at the equator.
The Correct Answer and Explanation is:
The correct answer is D. Chromosomes line up at the equator.
Explanation:
Metaphase is a critical stage of cell division in both mitosis and meiosis, specifically occurring after prophase and before anaphase. During this phase, several key events take place, with the primary focus being the alignment of chromosomes at the cell’s equatorial plane, known as the metaphase plate. This arrangement is crucial for the accurate distribution of genetic material to the daughter cells.
In preparation for metaphase, during prophase, the chromatin condenses into distinct chromosomes, each consisting of two sister chromatids held together at a region called the centromere. The nuclear membrane disintegrates, allowing the spindle apparatus to interact with the chromosomes. The spindle fibers, which are made of microtubules, extend from the centrosomes (the cell’s organizing centers for microtubules) and attach to the kinetochores—protein complexes located at the centromeres of the chromosomes.
Once all chromosomes are properly attached to the spindle fibers and aligned at the metaphase plate, the cell is ready to transition to anaphase. This alignment ensures that, when the spindle fibers begin to shorten in anaphase, the sister chromatids can be pulled apart and segregated to opposite poles of the cell. If chromosomes do not align correctly during metaphase, it can lead to errors in chromosome distribution, resulting in aneuploidy, a condition where daughter cells have an abnormal number of chromosomes.
Options A, B, and C describe events that either occur in different phases of the cell cycle or are not characteristic of metaphase. For instance, spindle fibers shorten during anaphase (option A), a cell begins to divide in two during cytokinesis (option B), and the breakdown of the nuclear membrane occurs during prophase (option C). Thus, the primary event that defines metaphase is the alignment of chromosomes at the equator, ensuring proper separation during subsequent stages of cell division.