Cell Division: Mitosis Unraveled
Organisms like trees, ants, lions, sunflowers, you, and me live and grow. For cells to grow and divide, they will go through the cell cycle to prepare for division by growing and replicating DNA and organelles. The cell cycle ends in M phase, where mitosis takes place, resulting in two identical daughter cells.
If you would like a refresher on the cell cycle and the cell cycle checkpoints, take a look at our previous blogs here:
In this post, we will explore the sub-phases within mitosis and the processes a cell undergoes that ultimately lead to two identical daughter cells.
Interphase and the Preparation for Mitosis
Before we look at the sub-phases in mitosis, it's important to note a few things that happen in interphase in eukaryotic cells. Interphase is composed of three phases: G1 (cell growth), S phase (DNA replication), and G2 (preparation for mitosis). During S phase, the DNA is replicated, ensuring each chromosome consists of two identical sister chromatids. These chromatids are connected by a region called the centromere. Although DNA is replicated, the number of chromosomes in the cell does not change; each chromosome just has an identical copy in the form of sister chromatids.
The S phase and the G2 checkpoint are crucial, as they ensure that DNA is replicated correctly, helping to ensure that each resulting daughter cell contains the same exact genetic information as the parent cell.
When learning about cell division and mitosis, there are many new vocabulary words. Some of these vocabulary words can be found in this post and other. Before we continue, download the Cell Cycle Vocabulary Organizer for free below.
The Phases of Mitosis: PPMAT
Once the cell enters mitosis, it undergoes five distinct sub-phases. Some discussions of mitosis highlight only four phases, using the mnemonic PMAT (prophase, metaphase, anaphase, and telophase). However, we'll explore five phases in detail here: prophase, prometaphase, metaphase, anaphase, and telophase (PPMAT).
1. Prophase
In prophase, the chromosomes found in the nucleus condense into compact structures, making them more easily separable. In the cytoplasm, the spindle apparatus begins to form, which will later help in chromosome movement.
2. Prometaphase
During prometaphase, the nucleolus disappears, and the nuclear envelope disintegrates. This allows the chromosomes to interact with the spindle apparatus. Special proteins called kinetochores, located at the centromeres of each chromatid, attach to the microtubules from the spindle apparatus, preparing for chromosome alignment.
3. Metaphase
The centrosomes migrate to opposite poles of the cell, and the spindle fibers align the chromosomes at the center of the cell along what is called the metaphase plate. The metaphase plate is not a physical structure but a plane where chromosomes align. Before the cell proceeds to the next phase, it checks to ensure all chromosomes are properly attached to the spindle fibers through a process called the spindle assembly checkpoint.
4. Anaphase
In anaphase, the sister chromatids are pulled apart by the spindle fibers and begin moving toward opposite poles of the cell. Once separated, each chromatid is considered an individual chromosome.
5. Telophase
During telophase, a new nuclear envelope forms around each set of chromosomes. The chromosomes begin to decondense, returning to their relaxed, uncondensed state. This marks the near end of mitosis, but there is one final step before division is complete.
Graphic summarizing the stages of mitosis. The stages shown are: Interphase: The cell grows and replicates its DNA. Prophase: Chromosomes condense, and the nucleus dissolves. Metaphase: Chromosomes align at the center of the cell. Anaphase: Chromosomes are pulled apart to opposite ends of the cell. Telophase: Chromosomes reach opposite ends, and the cell starts to furrow. Cytokinesis: The cell divides into two identical daughter cells, and the nucleus reforms.
Cytokinesis: Final Division of the Cell
After telophase, the cell enters cytokinesis, the process in which the cytoplasm divides to form two separate, identical daughter cells, each with its own nucleus and a full set of organelles. However, cytokinesis happens differently in animal and plant cells.
Cytokinesis in Animal Cells
In animal cells, cytokinesis begins with the formation of a cleavage furrow, a pinching of the cell membrane that eventually splits the cell into two. This process is driven by a ring of actin filaments contracting around the cell’s middle.
Cytokinesis in Plant Cells
In plant cells, which have a rigid cell wall, cytokinesis is a bit different. Instead of a cleavage furrow, a structure called the cell plate forms. Vesicles from the Golgi apparatus move to the center of the cell, where they fuse to create a new membrane and eventually a new cell wall that separates the two daughter cells.
Graphic illustrating the process of cytokinesis in animal and plant cells. In animal cells, cytokinesis occurs through the formation of a cleavage furrow, where the cell membrane pinches inward to separate the two daughter cells, which are identical. In plant cells, a cell plate forms in the middle of the cell, dividing the cytoplasm and creating two identical plant daughter cells, each with its own nucleus and organelles.
The Result: Two Identical Daughter Cells
At the end of mitosis, two identical daughter cells are produced, each with the same number of chromosomes as the original parent cell. This process allows for the growth and repair of tissues in multicellular organisms. Proper regulation of mitosis ensures the genetic consistency and stability of all cells within an organism.
The Importance of Checkpoints and Mitosis Errors
Throughout the cell cycle, various checkpoints—such as the G1 checkpoint, G2 checkpoint, and spindle assembly checkpoint—help ensure that each phase of the cycle proceeds correctly. Errors in mitosis or regulation of the cell cycle can lead to serious issues, including uncontrolled cell division, which is a hallmark of diseases like cancer. Proper functioning of checkpoints is crucial to prevent such errors and maintain cellular health.
