Question

Describe the events that characterize each stage of mitosis.

Short answer

Question: Describe the main events that occur during each stage of mitosis. Answer: In prophase, chromatin condenses into chromosomes, nucleolus disappears, and spindle fibers start to extend from centrosomes. During prometaphase, nuclear envelope breaks down, and spindle fibers attach to the centromeres of chromosomes. In metaphase, chromosomes align along the metaphase plate with centromeres connected to spindle fibers. Anaphase involves the separation of sister chromatids, which are pulled towards opposite poles of the cell. Finally, in telophase, chromosomes decondense, nuclear envelope reappears, and spindle fibers dissolve, followed by cytokinesis, which results in two identical daughter cells.

Step-by-step solution

1. Prophase

During prophase, the first stage of mitosis, the chromatin inside the cell nucleus condenses and becomes visible as chromosomes. The nucleolus disappears, and the centrosomes, which are specialized organelles, move to opposite ends of the cell, forming the spindle apparatus. Microtubules start to extend from the centrosomes, preparing to attach to the chromosomes.

2. Prometaphase

In the prometaphase, the nuclear envelope breaks down, and the spindle fibers (microtubules) grow longer. The chromosomes become even more condensed and move towards the equator of the cell. Each chromosome consists of two sister chromatids joined by the centromere, which is the region where the spindle fibers attach.

4. Metaphase

The third stage, metaphase, is characterized by the alignment of the chromosomes along the cell's equatorial plane, which is also called the metaphase plate. The centromere of each chromosome is attached to spindle fibers originating from the centrosomes, ensuring that the sister chromatids will separate correctly in the next stage.

5. Anaphase

During anaphase, the sister chromatids are separated by the action of spindle fibers, which pull the chromatids apart at the centromere. Each chromatid is now considered a separate chromosome. The chromosomes are then pulled towards opposite poles of the cell by the shortening of the spindle fibers, making the cell elongate.

6. Telophase and cytokinesis

In the final stage of mitosis, telophase, the chromosomes arrive at the opposite poles of the cell and begin to decondense, forming chromatin again. The nuclear envelope reappears around each set of chromosomes, and the nucleolus reappears within the new nuclei. Spindle fibers dissolve, and the cell starts to return to a normal state. During cytokinesis, which occurs simultaneously with telophase, the cytoplasm and cell membrane are divided, forming two daughter cells with identical genetic material.

Key concepts

Cell Division

Cell division is a fundamental process in which a single cell divides to form two or more daughter cells. This process is crucial for growth, development, and maintenance of all living organisms. One significant form of cell division is mitosis, which is responsible for the production of two genetically identical daughter cells. Mitosis ensures that each daughter cell receives an exact copy of the parent cell's DNA, which is crucial for maintaining genetic consistency.

There are several stages in mitosis:

• Prophase: Chromatin condenses to become visible as chromosomes, and spindle fibers begin to form.
• Prometaphase: The nuclear envelope disintegrates, allowing spindle fibers to interact with chromosomes.
• Metaphase: Chromosomes line up at the cell's equator, ready for separation.
• Anaphase: Sister chromatids separate and move to opposite poles of the cell.
• Telophase: Chromosomes decondense back into chromatin, and the nuclear envelope reforms.

During each of these stages, intricate coordination ensures the accurate distribution of genetic material, which is vital for the survival and function of cells.

Chromosomes

Chromosomes are long, thread-like structures made of DNA and proteins that contain the genetic instructions necessary for the functioning and development of all living organisms. Each species has a specific number of chromosomes, which are found in the nucleus of eukaryotic cells.

In humans, for instance, there are 46 chromosomes arranged in 23 pairs. During cell division, chromosomes become highly condensed, making them visible under a microscope. This condensation allows for the precise separation of genetic material.

Each chromosome consists of two identical sister chromatids joined at a central region known as the centromere. Sister chromatids are exact copies of each other, formed during DNA replication. This allows each daughter cell to receive an identical set of chromosomes, ensuring genetic continuity. The alignment and separation of these chromatids during mitosis is critical for genetic stability.

Spindle Fibers

Spindle fibers play an essential role in the process of mitosis by facilitating the movement and separation of chromosomes. These fibers are composed of microtubules, which are protein-based structures that form part of the cell's cytoskeleton. During cell division, the spindle fibers emerge from organelles called centrosomes.

As mitosis progresses, these fibers attach to the centromeres of chromosomes, specifically to protein structures known as kinetochores. This attachment is central to ensuring that chromosomes are accurately pulled apart during anaphase.

• Role in Metaphase: Spindle fibers help line up chromosomes at the cell's equatorial plate, preparing them for separation.
• Role in Anaphase: As the cell enters anaphase, spindle fibers shorten, pulling sister chromatids apart to opposite poles of the cell.

Spindle fibers essentially guide chromosomes throughout mitosis, ensuring that each new cell receives the correct genetic information.

Cytokinesis

Cytokinesis marks the end of the cell division process, where the cytoplasm and cell membrane divide to form two distinct daughter cells. Occurring concurrently with telophase, cytokinesis ensures that each daughter cell receives adequate cytoplasm and organelles to function independently.

In animal cells, cytokinesis involves the formation of a cleavage furrow, a constriction of the cell membrane that pinches the cell into two separate entities. This is often compared to a drawstring tightening around the middle of the cell, leading to division.

In contrast, plant cells develop a cell plate during cytokinesis due to the presence of a rigid cell wall. This cell plate forms in the middle, eventually developing into a separate cell wall that divides the two daughter cells.

This step is crucial for maintaining cell size, distributing cellular components, and ensuring that each daughter cell can survive and thrive independently post-division.