Part II: Overall plant shape by cell division

For a root to grow, not only do its cells elongate, but more cells are needed. The multiplication is effected by cell divisions. Let's say that cells in a root can only grow to be a unit long. If all cells are already a maximum 1 unit long, how can the whole root elongate? Each cell divides into two, and the two new cells are then a half unit long. If they, in turn, elongate their maximum 1 unit, the overall root has elongated 2-fold. Guess what: In addition to the internal cytoskeleton's role in elongation, it also plays an indispensable role in the division process.

Generally, a cell requires a nucleus, a compartment where DNA - the genetic material - resides. The DNA (comprising genes) is the blueprint of a cell. It is translated into the building materials required for a cell's operations. Consequently, if a cell is to divide into two cells, a second copy of the DNA is required, and that DNA has to be distributed precisely to the two resulting cells. These two cells are called daughter cells - insofar as they are sisters descended from a previous generation, but there is now no longer a parent; rather, there are now two cells where once there was one (parent).

Two cells in early division (labeled images; 32K gif) Chromosomes are lined for separation into 2 sets by the spindle. The chromosomes (DNA) and the spindle (cytoskeletal scaffold elements termed "microtubules") are visualized by two different fluorescent dyes that produce different images when illuminated with different colors of light. The 2 images were merged by computer.

A parent cell duplicates it DNA then packages it as chromosomes for easy separation of the copies. Mitosis is the process whereby these chromosomes are distributed to the daughter cells. The two sets of chromosomes, in preparation for this segregation, are aligned at a mid-plane of the parent cell. The alignment is carried out by a scaffolding array that is roughly football-shaped, called the spindle. In plant cells, the spindle's "poles" (the tips of the football) are not as pointy, but are more like a football whose ends have been blown open.

One cell, later in division (31K gif) The two chromosome sets are now separated. Note that the cytoskeleton (1) directs them to their respective poles, and (2) keeps apart the poles themselves.

After chromosomes line up at the spindle's equator, the spindle directs the 2 chromosome sets toward respective poles. Other scaffold elements are involved in keeping the poles themselves apart - i.e., maintaining the football's overall structural integrity.

After the chromosomes are separated, two new daughter nuclei can result. What remains to be completed is the second phase of cell division - to construct a new divider between the daughters so they become independent cells.