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Chloroplast division
Chloroplast division in photosynthetic organisms has been primarily studied in the model plant Arabidopsis thaliana, and the red algae Cyanidioschyzon merolæ. Typically, A. thaliana shoot meristematic cells start out with around 20 proplastids. These proplastids differentiate into chloroplasts upon exposure to light, and divide to produce 100-150 chloroplasts in the mature leaf mesophyll cell.

In single-celled algae, chloroplast division is the only way that new chloroplasts can be produced. Algal cell division is synchronized with chloroplast division, meaning that when an algal cell divides its chloroplasts divide with it, and each daughter cell receives a mature chloroplast. Chloroplast division is carried out by a set of proteins, known as the chloroplast division proteins. Together, these proteins aggregate at the mid plastid and work to constrict the membrane. While some of the division proteins are derived from cyanobacteria, a few are found only in later-diverging plants.

In plants, chloroplast division begins with the assembly of proteins FtsZ1 and FtsZ2 at the mid plastid, forming what is called a "Z-ring" in the stroma.

The Min system manages the placement of the Z-ring. The protein MinD prevents FtsZ from linking up and forming filaments. Another protein ARC3 may also be involved, but it is not very well understood. These proteins are active at the poles of the chloroplast, preventing Z-ring formation there, but near the center of the chloroplast, MinE inhibits them, allowing the Z-ring to form.

Next, the two plastid-dividing rings, or PD rings form. The inner plastid-dividing ring is located in the inner side of the chloroplast's inner membrane, and is formed first. The outer plastid-dividing ring is found wrapped around the outer chloroplast membrane. It consists of filaments about 5 nanometers across, arranged in rows 6.4 nanometers apart, and shrinks to squeeze the chloroplast. This is when chloroplast constriction begins.

In a few species like Cyanidioschyzon merolæ, chloroplasts have a third plastid-dividing ring located in the chloroplast's intermembrane space.

Late into the constriction phase, dynamin proteins assemble around the outer plastid-dividing ring, helping provide force to squeeze the chloroplast. Meanwhile, the Z-ring and the inner plastid-dividing ring break down. During this stage, the many chloroplast DNA plasmids floating around in the stroma are partitioned and distributed to the two forming daughter chloroplasts.

Later, the dynamins migrate under the outer plastid dividing ring, into direct contact with the chloroplast's outer membrane, to cleave the chloroplast in two daughter chloroplasts.

A remnant of the outer plastid dividing ring remains floating between the two daughter chloroplasts, and a remnant of the dynamin ring remains attached to one of the daughter chloroplasts.

Of the five or six rings involved in chloroplast division, only the outer plastid-dividing ring is present for the entire constriction and division phase—while the Z-ring forms first, constriction does not begin until the outer plastid-dividing ring forms.

Regulation
In species of algae that contain a single chloroplast, regulation of chloroplast division is extremely important to ensure that each daughter cell receives a chloroplast—chloroplasts can't be made from scratch. In organisms like plants, whose cells contain multiple chloroplasts, coordination is looser and less important. It is likely that chloroplast and cell division are somewhat synchronized, though the mechanisms for it are mostly unknown.

Light has been shown to be a requirement for chloroplast division. Chloroplasts can grow and progress through some of the constriction stages under poor quality green light, but are slow to complete division—they require exposure to bright white light to complete division. Spinach leaves grown under green light have been observed to contain many large dumbbell-shaped chloroplasts. Exposure to white light can stimulate these chloroplasts to divide and reduce the population of dumbbell-shaped chloroplasts. Copied from Chloroplast

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