The first stage of mitosis is prophase.
Understanding Prophase
Prophase marks the beginning of the mitotic phase, where the replicated genetic material is separated into two new nuclei. Here's what happens during prophase:
- Chromosomes Condense: The long, thread-like chromosomes begin to coil and condense, becoming shorter and thicker, making them visible under a microscope.
- Sister Chromatids: Each chromosome consists of two identical sister chromatids held together by a centromere.
- Spindle Formation: The mitotic spindle, composed of microtubules, starts to form from the centrosomes, which move to opposite poles of the cell.
- Nuclear Envelope Breakdown: The nuclear membrane surrounding the nucleus begins to break down, allowing the spindle fibers to access the chromosomes.
- Kinetochore Attachment: Spindle fibers attach to the kinetochores (specialized protein structures) located at the centromere of each sister chromatid. As described in the reference, "spindles attached to kinetochores begin to shorten," initiating the pulling of the sister chromatids.
Stages of Mitosis
Mitosis is a continuous process, but it is traditionally divided into distinct stages for easier understanding:
| Stage | Key Events |
|---|---|
| Prophase | Chromosomes condense, mitotic spindle forms, nuclear envelope breaks down, spindles attach to kinetochores. |
| Metaphase | Chromosomes align at the metaphase plate, each sister chromatid facing opposite poles. |
| Anaphase | Sister chromatids separate and move to opposite poles of the cell. |
| Telophase | Chromosomes reach the poles, nuclear envelopes reform, and chromosomes decondense. |
| Cytokinesis | The cell's cytoplasm divides, resulting in two daughter cells with identical genetic material. |
Key takeaway about prophase
Prophase is a crucial phase that sets the stage for the equal distribution of genetic material into two daughter cells. It is characterized by the condensation of chromosomes, formation of the mitotic spindle, breakdown of the nuclear envelope, and the attachment of spindle fibers to kinetochores, which, as mentioned in the reference, ultimately facilitates the separation of sister chromatids.
