Microtubules often produce movement through their interaction with motor proteins. But in some cases, microtubules move cell components when the length of the microtubule changes. Through a series of experiments, researchers determined that microtubules grow and shorten as tubulin proteins are added or removed from their ends. Other experiments showed that microtubules make up the spindle apparatus that 'pulls' chromosomes toward opposite ends (poles) of a dividing cell. The figures below describe a clever experiment done in 1987 to determine whether a spindle microtubule shortens (depolymerizes) at the end holding a chromosome or at the pole end of a dividing cell. Experimenters labeled the microtubules of a dividing cell from a pig kidney with a yellow fluorescent dye. As shown on the left half of the diagram below, they then marked a region halfway along the microtubules by using a laser to eliminate the fluorescence from that region. They did not mark the other side of the spindle (right side of the figure).

Verified step by step guidance

1. Examine the experimental setup: The researchers labeled the microtubules with a yellow fluorescent dye and then used a laser to eliminate fluorescence in a specific region halfway along the microtubules. This mark allows observation of changes in microtubule length during cell division.
2. Understand the hypothesis: The experiment aims to determine whether microtubules shorten (depolymerize) at the chromosome end or at the pole end during cell division.
3. Interpret the diagram: The left side of the figure shows marked microtubules, while the right side remains unmarked. The marked region serves as a reference point to track changes in microtubule length as chromosomes are pulled toward the poles.
4. Consider the mechanism of microtubule dynamics: Microtubules are dynamic structures composed of tubulin subunits. They grow by polymerization (adding tubulin subunits) and shorten by depolymerization (removing tubulin subunits). This dynamic instability is crucial for their function in processes like chromosome movement.
5. Analyze the expected results: If microtubules depolymerize at the chromosome end, the mark will remain stationary while the chromosome moves closer to the pole. If depolymerization occurs at the pole end, the mark will move closer to the pole along with the chromosome. Observing the movement of the mark relative to the chromosome and pole will reveal the site of depolymerization.

Key Concepts

Microtubule Dynamics

Microtubules are dynamic structures composed of tubulin proteins that can rapidly grow and shrink. This process, known as dynamic instability, is crucial for various cellular functions, including cell division. The addition and removal of tubulin subunits at the microtubule ends allow them to change length, which is essential for their role in the spindle apparatus during mitosis.

Spindle Apparatus

The spindle apparatus is a structure formed by microtubules that segregates chromosomes during cell division. It consists of spindle fibers that extend from the centrosomes at opposite poles of the cell and attach to the kinetochores of chromosomes. This apparatus ensures that each daughter cell receives an equal and accurate distribution of chromosomes.

Chromosome Movement and Depolymerization

During cell division, chromosomes are pulled toward opposite poles by the spindle apparatus. The process involves the depolymerization of microtubules, which can occur at the end attached to the chromosome or at the pole end. Understanding where this depolymerization occurs is crucial for elucidating the mechanisms of chromosome movement and the overall dynamics of cell division.