Question

How do spindle fibers form and how do chromosomes separate in animal cells?

Short answer

Answer: Spindle fibers form during the prometaphase stage of mitosis by extending from centrosomes located at opposite poles of the cell. Composed of microtubules, the spindle fibers attach to the chromosomes at protein structures called kinetochores. During the anaphase stage, spindle fibers shorten, pulling apart sister chromatids and moving them to opposite poles of the cell, ensuring equal distribution of genetic material between the two daughter cells.

Step-by-step solution

1. Overview of Mitosis

Mitosis is a cellular process that ensures the equal distribution of genetic material between two daughter cells. It consists of five stages: prophase, prometaphase, metaphase, anaphase and telophase.

2. Formation of Spindle Fibers (Prometaphase)

During prometaphase, spindle fibers begin to form. The nuclear envelope breaks down, allowing spindle fibers to extend from centrosomes (microtubule organizing centers) located at opposite poles of the cell. The spindle fibers are made of microtubules, which are long protein structures that can grow and shrink. As the microtubules extend, they eventually overlap to form a bipolar structure called the mitotic spindle.

3. Chromosome Condensation

In prophase, the genetic material in the cell (DNA) becomes densely packed, resulting in the formation of structures called chromosomes. Each chromosome is composed of two sister chromatids, which are identical copies of DNA connected at a point called the centromere.

4. Attachment of Spindle Fibers to Chromosomes

During prometaphase, spindle fibers form connections with the chromosomes. Specifically, some spindle fibers attach to protein structures called kinetochores, which are located at the centromeres of the chromosomes. These interactions are critical for the proper alignment and separation of chromosomes.

5. Alignment of Chromosomes (Metaphase)

During metaphase, the chromosomes, connected to the spindle fibers, become aligned at the center of the cell, known as the metaphase plate. The attachment of spindle fibers to the kinetochores ensures equal tension on each sister chromatid, preventing premature separation.

6. Separation of Chromosomes (Anaphase)

In anaphase, the sister chromatids are pulled apart as the spindle fibers shorten, moving the separated chromatids to opposite poles of the cell. The process is driven by the depolymerization of microtubules. Once the sister chromatids reach the cell poles, they are considered individual chromosomes.

7. Completion of Mitosis (Telophase and Cytokinesis)

During telophase, a nuclear envelope re-forms around the separated chromosomes at each pole, creating two separate nuclei. In animal cells, the process of cytokinesis (cell division) occurs through the formation of a cleavage furrow, leading to the physical separation of the two daughter cells, each with an identical set of chromosomes.