Fission (biology)

In biology, fission is a form of cell division in which a single parent cell divides into two or more cells. Fission is a form of asexual reproduction, meaning that reproduction is achieved without the fusion of male and female sex cells.

Binary fission occurs when the parent cell divides into two daughter cells, which are identical or nearly identical to the parent cell. Fission may take place in simple organisms called prokaryotes. A prokaryotic cell does not have a nucleus, which is a part of the cell separated by a membrane that contains deoxyribonucleic acid (DNA). Although prokaryotic cells contain DNA, this material is not held in a separate nucleus. Fission is also possible in eukaryotes, which are organisms composed of cells that have a nucleus.

How Binary Fission Works

Multiple fission, in which the cell divides into multiple parts, is possible in some organisms. However, many simple organisms rely on binary fission to divide and multiply. Under the right circumstances, binary fission allows some organisms to reproduce rapidly. Many kinds of bacteria use this method of reproduction to replicate at astonishing rates. Some forms of bacteria can double their numbers in as little as twenty minutes.

In a prokaryotic cell, several important steps must take place before binary fission can occur. First, the cell will usually increase in size and volume. Next, the DNA of the prokaryotic cell, which is protected by the cell's watery cytoplasm, must replicate. After that, a protein called FtsZ forms a structure known as the FtsZ ring in the middle of the cell. As this happens, the two copies of the cell's DNA and other important cellular materials are divided between the two sides of the cell. The FtsZ ring then tightens around the center of the cell, and a new cell wall forms between the two sides. Finally, the dividing cell wall splits the cell apart, creating two new daughter cells. In most cases, these daughter cells will be genetically identical copies, or clones, of the original cell.

Types of Binary Fission

Among bacteria, there are three types of binary fission: transverse, symmetrical, and asymmetrical. The type of fission that occurs in a cell often depends on the cell's shape. Bacteria cells shaped like rods or ovals follow transverse binary fission, meaning that one of these cells will divide along its long axis. In symmetrical binary fission, the parent cell will produce two daughter cells that are the same size. Cocci, or spherical bacteria, often reproduce through symmetrical binary fission. The third type of binary fission among bacteria is asymmetrical, which results in one daughter cell being bigger than the other.

Fission in Eukaryotes

Fission also takes place in eukaryotic cells, though the process is more complex than the binary fission that prokaryotic cells experience. This is simply because the eukaryotic cells themselves are more complex. Eukaryotic cells contain a nucleus and more organelles, cell parts that have special functions. Although eukaryotic cells still divide in much the same way that prokaryotic cells do, they must undergo a process called mitosis inside the nucleus before cell division can occur. In mitosis, the nucleus of the parent cell divides in such a way that both daughter cells receive the exact number of chromosomes that were present in the original cell. For example, if the parent cell had fifty-two chromosomes, then each daughter cell would also have fifty-two chromosomes.

Mitosis has four phases: prophase, metaphase, anaphase, and telophase. Before mitosis takes place, the cell experiences interphase, during which the organelles in the cell double their numbers, protein synthesis takes place, and the cell's DNA duplicates. During this time, the chromosomes cannot be clearly seen, and the DNA and proteins are loosely arranged as chromatin. In prophase, the chromosomes form as the chromatin condenses. The nuclear membrane also starts to dissolve, the nucleolus disappears, and a spindle structure forms and attaches to the centromeres of the chromosomes, which are chromatids at this point. A chromatid is the paired strands of the replicated chromosome. During metaphase, the nuclear membrane is gone, and the pairs of chromatids arrange themselves along the cell's equator. In anaphase, the chromatid pairs separate into chromosomes, which then migrate to the cell's opposite poles. The final phrase in mitosis is telophase. After the chromosomes move to opposite sides of the cell, the nuclear membrane reappears and the chromosomes uncoil into chromatin. The nucleolus also reforms during this phase.

Once mitosis of the nucleus is complete, the two daughter cells divide during a process called cytokinesis. During cytokinesis, a contractile ring made of protein fibers forms around the cell's equator. This ring pinches the middle of the cell together and splits it into the two daughter cells. This marks the completion of eukaryotic cell division.

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