Fission (biology)

Binary fission

Organisms in the domains of Archaea and Bacteria reproduce with binary fission. This form of asexual reproduction and cell division is also used by some organelles within eukaryotic organisms (e.g., mitochondria). Binary fission results in the reproduction of a living prokaryotic cell (or organelle) by dividing the cell into two parts, each with the potential to grow to the size of the original.

Fission of prokaryotes

The single DNA molecule first replicates, then attaches each copy to a different part of the cell membrane. When the cell begins to pull apart, the replicated and original chromosomes are separated. The consequence of this asexual method of reproduction is that all the cells are genetically identical, meaning that they have the same genetic material (barring random mutations). Unlike the processes of mitosis and meiosis used by eukaryotic cells, binary fission takes place without the formation of a spindle apparatus on the cell. Like in mitosis (and unlike in meiosis), the parental identity is not lost.

Fragmentation

FtsZ is homologous to β-tubulin, the building block of the microtubule cytoskeleton used during mitosis in eukaryotes. FtsZ is thought to be the first protein to localize to the site of future division in bacteria, and it assembles into a Z ring, anchored by FtsZ-binding proteins and defines the division plane between the two daughter cells. MinC and MinD function together as division inhibitors, blocking formation of the FtsZ ring. MinE stops the MinCD activity midcell, allowing FtsZ to take over for binary fission.

More specifically, the following steps occur:

1. The bacterium before binary fission is when the DNA is tightly coiled.
2. The DNA of the bacterium has uncoiled and duplicated.
3. The DNA is pulled to the separate poles of the bacterium as it increases the size to prepare for splitting.
4. The growth of a new cell wall begins to separate the bacterium (triggered by FtsZ polymerization and "Z-ring" formation)
5. The new cell wall (septum) fully develops, resulting in the complete split of the bacterium.
6. The new daughter cells have tightly coiled DNA rods, ribosomes, and plasmids; these are now brand-new organisms.

Studies of bacteria made to not produce a cell wall, called L-form bacteria, indicate that FtsZ requires a cell wall to work. Little is known about how bacteria that naturally don't grow a cell wall divide, but it is thought to resemble the L-form's budding-like division process of extrusion and separation.

Speed of FtsZ-dependent Fission

Binary fission is generally rapid, though its speed varies between species. For E. coli, cells typically divide about every 20 minutes at 37 °C. Because the new cells will, in turn, undergo binary fission on their own, the time binary fission requires is also the time the bacterial culture requires to double in the number of cells it contains. This time period can, therefore, be referred to as the doubling time. Some species other than E. coli may have faster or slower doubling times: some strains of Mycobacterium tuberculosis may have doubling times of nearly 100 hours. Bacterial growth is limited by factors including nutrient availability and available space, so binary fission occurs at much lower rates in bacterial cultures once they enter the stationary phase of growth.

In archaea

Thermoproteota (formerly Crenarchaeota) possess neither a cell wall nor the FtsZ mechanism. They use a primitive version of the eukaryotic ESCRT-III system (also known as Cdv) to manipulate the membrane into separating, specifically by coming into the middle of the two soon-to-be daughter cells. Euryarchaeota use FtsZ like bacteria do.

Fission of organelles

Some organelles in eukaryotic cells reproduce using binary fission. Mitochondrial fission occurs frequently within the cell, even when the cell is not actively undergoing mitosis, and is necessary to regulate the cell's metabolism. All chloroplasts and some mitochondria (not in animals), both organelles derived from endosymbiosis of bacteria, also use FtsZ in a bacteria-like fashion.

Types of binary fission

Binary fission in organisms can occur in four ways: irregular, longitudinal, transverse, or oblique. For example:

1. Irregular: In this fission, cytokinesis may take place along any plane but it is always perpendicular to the plane of karyokinesis (nuclear division). e.g. Amoeba.
2. Longitudinal: Here cytokinesis takes place along the longitudinal axis. e.g. in flagellates like Euglena.
3. Transverse: Here cytokinesis takes place along the transverse axis. e.g. in ciliate protozoans like Paramecium.
4. Oblique: In this type of binary fission, cytokinesis occurs obliquely. Example Ceratium.

Binary fission means "division into two". It is the simplest and most common method of asexual reproduction.