Eukaryotic cells, which include all animal, plant, fungal, and protist cells, undergo mitosis as their primary method of cell division. These cells are defined by the presence of a membrane-bound nucleus and specialized internal compartments called organelles. Mitosis is a precise, highly regulated process that ensures the accurate distribution of genetic material. This cell division is necessary for all living things to create new cells from pre-existing ones.
Mitosis: The Primary Function in Eukaryotic Cells
Eukaryotes utilize mitosis for fundamental biological purposes, primarily involving growth and maintenance. In multicellular organisms, mitosis allows a fertilized egg to develop into an adult by continually generating new cells. This division also facilitates the replacement of damaged cells or those that have reached the end of their lifespan, such as skin cells.
Mitosis serves as a form of asexual reproduction for single-celled eukaryotes, like yeast or amoebas, creating new, independent organisms. The division results in two daughter cells that are genetically identical to the parent cell. Each daughter cell receives a complete, identical set of chromosomes, maintaining the diploid genetic state. This process is often called an equational division because the chromosome number remains constant.
The Four Stages of Mitotic Division
Before mitosis begins, the cell completes a preparatory phase called Interphase, which is not technically part of mitosis. During Interphase, the cell grows, performs its normal functions, and duplicates its entire set of DNA. This replication results in each chromosome consisting of two identical strands, known as sister chromatids, which remain joined together.
Prophase
The first stage is Prophase, where the duplicated chromosomes condense into compact, visible structures. Simultaneously, the nuclear envelope begins to break down. A structure called the mitotic spindle starts to form from microtubules, with its organizing centers moving toward opposite ends of the cell.
Metaphase
Next is Metaphase, where the condensed chromosomes align precisely along the cell’s central plane, known as the metaphase plate. This alignment ensures that each new cell receives an equal share of genetic material. Spindle fibers attach to a specific structure on each chromosome, connecting the sister chromatids to opposite poles of the cell.
Anaphase
Anaphase immediately follows, marked by the separation of the sister chromatids. The spindle fibers shorten, pulling the now-separated chromatids (considered full chromosomes) toward opposite poles of the cell. This movement results in one complete set of chromosomes moving to each end of the cell.
Telophase and Cytokinesis
The final stage is Telophase, where the separated chromosomes arrive at the poles and begin to uncoil. A new nuclear envelope forms around each set of chromosomes, establishing two distinct nuclei. Following nuclear division, Cytokinesis occurs, which is the physical division of the cytoplasm and cellular components. This involves a contractile ring pinching the animal cell in two, or a new cell wall forming in plants, completing the formation of two identical daughter cells.
Distinguishing Mitosis from Meiosis
Eukaryotes use mitosis for growth and repair in somatic cells, but they also employ a separate, more complex division process called meiosis. Meiosis is reserved for the formation of gametes (sex cells like sperm and eggs) necessary for sexual reproduction. The primary goal of meiosis is to introduce genetic variation and reduce the chromosome number by half.
A cell undergoing mitosis divides only once, resulting in two genetically identical, diploid daughter cells. Conversely, a cell undergoing meiosis completes two successive rounds of division (Meiosis I and Meiosis II). This double division results in four daughter cells that are genetically distinct from the parent cell and from each other.
The resulting cells from meiosis are haploid, containing only one set of chromosomes. This reduction is necessary so that fertilization restores the correct diploid chromosome number in the offspring. Genetic diversity is further achieved in meiosis through crossing over, where homologous chromosomes exchange genetic material.
How Prokaryotes Divide
Prokaryotes, including bacteria and archaea, do not undergo mitosis because they lack a nucleus and complex internal organelles. Their genetic material is typically a single, circular chromosome located in the nucleoid region of the cytoplasm. Their method of reproduction is a simpler, faster process called binary fission.
Binary fission begins when the single chromosome is duplicated, creating two identical circular copies. As replication completes, the two copies move to opposite ends of the elongating cell. The cell then begins to pinch inward at the center, forming a septum or dividing wall.
This process separates the cytoplasm and the two complete sets of genetic material. The final step is the splitting of the cell into two new, genetically identical daughter cells. Since this method lacks the complex spindle apparatus of mitosis, prokaryotes can reproduce much more quickly than eukaryotes.