Question

Which of the following is not an event of anaphase of mitosis? a. The nuclear envelope breaks apart. b. Sister chromatids separate. c. Kinetochore microtubules shorten, pulling the chromosomes to the poles. d. Polar microtubules push against each other, moving the poles farther apart. e. All of the above occur during anaphase.

Short answer

Option (a) - The nuclear envelope breaks apart, is not an event of anaphase of mitosis.

Step-by-step solution

Identify the Stages of Mitosis

Mitosis is divided into five phases: prophase, prometaphase, metaphase, anaphase and telophase, each characterized by certain processes. Both prophase and prometaphase occur prior to anaphase, while metaphase precedes it and telophase happens afterwards.

Relate Each Option to Mitosis Phase

In option (a), the nuclear envelope breaks apart during prometaphase, not anaphase. Sister chromatids separate at option (b) during anaphase, the same thing happens for option (c) where kinetochore microtubules shorten, pulling the chromosomes to the poles. In option (d), polar microtubules push against each other, moving the poles further apart - it's a phenomenon happening during anaphase. Option (e) suggests that all listed phenomena occur during the anaphase, which we have determined to be incorrect.

Identify the Incorrect Option

Upon correlating each option with its representative process during mitosis, it is clear that option (a) referring to the nuclear envelope breaking apart, does not happen during anaphase, but during prometaphase. Thus, option (a) is not an event of Anaphase of Mitosis.

Key concepts

Stages of Mitosis

Understanding mitosis is essential for comprehending how our cells duplicate their genetic material and divide. Mitosis is a multi-stage process that ensures the equal distribution of chromosomes into two new nuclei - a fundamental mechanism in cell replication. The stages of mitosis are prophase, prometaphase, metaphase, anaphase, and telophase.

During prophase, the chromatin condenses into chromosomes, and the mitotic spindle begins to form. Prometaphase sees the breakdown of the nuclear envelope and the binding of spindle fibers to chromosomes. Metaphase aligns chromosomes at the cell's equator. Anaphase, the phase in focus, is characterized by the separation of sister chromatids. Finally, in telophase, the chromosomes decondense, and two new nuclear envelopes begin to form around the separate sets of chromosomes in each of the new cells. Each stage plays a critical role in the division of cells and ensures that each new cell contains the correct genetic information.

Sister Chromatids Separation

The moment that sister chromatids separate marks the essence of anaphase. Sister chromatids, which were once duplicated and held together to form an 'X' shape—known as chromosomes—are pulled apart by spindle fibers. This separation effectively halves the genetic material, setting the stage for two new cells. Before anaphase, the sister chromatids are identical, joined at a central region called the centromere.

When anaphase begins, enzymes cleave the cohesin proteins holding the sister chromatids together. The freed chromatids, now individual chromosomes, are rapidly dragged towards opposite poles of the cell. This pivotal moment guarantees that each daughter cell will inherit an identical set of chromosomes, preserving the fidelity of cellular reproduction and function.

Kinetochore Microtubules

Kinetochore microtubules are integral during mitosis, especially in anaphase. Each chromatid has a specialized region known as the kinetochore, where certain microtubules attach. These kinetochore microtubules are responsible for the movement of chromosomes during cell division.

The spindle fibers shorten as motor proteins 'walk' along the microtubules, pulling the chromatids apart. This is like reeling in a fish on a fishing rod—a gradual decrease in length that brings chromosomes closer to the cell poles. Shortening occurs through a process called depolymerization, where microtubule subunits are removed from their plus ends, facilitating the movement of chromatids toward the centrosomes that anchor the spindle poles.

Polar Microtubules

Polar microtubules, also known as non-kinetochore microtubules, are not involved in the pulling forces on chromosomes. Instead, their main function is to push the spindle poles apart during anaphase and later in cytokinesis, when the cell divides into two. They interdigitate at the spindle midzone and push against one another, extending the cell along its division axis.

This elongation is essential for providing enough space for chromosome segregation and ensuring that the two new nuclei are sufficiently far apart before the cell splits in two during cytokinesis. Without polar microtubules, proper cell division and spacing of the new daughter cells could be compromised, potentially leading to cellular dysfunction.