Who Cares About Centrioles?

Meristematic cells are found in the apical meristem of shoots and roots, as well as in lateral meristems in lateral buds and in the cambia (both vascular cambium and cork cambium).

Meristematic cells are quite different from the "typical" cell you have already learned about. They have thin walls, little to no vacuole, are tightly packed (no intercellular spaces), have immature organelles (proplastids rather than mature chloroplasts), are so small that the nucleus occupies a very large proportion of the total cell volume, and the cells live almost exclusively upon energy from respiration in their mitochondria.

Meristematic cells function almost exclusively in cell division (mitosis plus cytokinesis). This is an important topic. We have discovered that this topic is a weakness in your background based upon our outcomes assessment (Comprehensive Exam) process. The department has decided that cell division must be taught in every course. Here goes!

The Cell Cycle

Cells have a life cycle of cell division, followed by a length of time leading up to the next cell division. This cycle can be depicted as a pie representing the total lifetime of one cell cycle.

The small slice at the top of this pie represents mitosis ending in cytokinesis (more on that below). The rest of the pie represents the time between cell divisions...most of the life of a cell. We call this part of the cell cycle interphase. Inter- means between and so that is an appropriate name for this phase between cell divisions.

Interphase can be divided into three slices of time: G1, S, and G2.

In G1, the cell has just finished dividing and carries out a range of "normal functions" including respiration, transcription, translation, and so on.
In S, the nucleus is actively involved DNA replication in preparation for the next mitosis. Here each molecule of DNA is copied semi-conservatively. The two strands of DNA double-helix are held together.
In G2, the synthesis is over and the cell continues its range of "normal functions."

Cells that do not actively divide (are NOT meristematic) are stuck in either G1 or G2 and their cell cycle does not complete. Human neurons are a good example; collenchyma in plants is another. Meristematic cells, by comparison, rapidly cycle and it is likely that their G1 and G2 phases are rather short. Of course during interphase it is important to make addtional organelles and so on in preparation for the upcoming cytokinesis.

Mitosis means the division of the nucleus. The process has four phases:

Prophase--The DNA and histone proteins coil and condense to form individual chromosomes. These consist of two DNA strands held together at the centromere and surrounded by proteins. They stain heavily with certain dyes; hence the name chromosome. The nuclear envelope will disappear, releasing the chromosomes to the cytosol. Any nucleolus (site of rapid rRNA and tRNA transcription) will likewise disappear. Note: synapsis and recombination are found only in meiosis.
Metaphase--In the cytosol, spindle fibers (microtubules made of the polymer tubulin) form at an organizing center at the poles of the cell. Precisely how this is accomplished is still largely unknown; a likely Nobel prize here for whoever figures it out. At one time centrioles were thought to organize the spinde. Funny thing though, plants don't have them and yet divide just fine! Moreover, destroying centrioles in animal cells does not disrupt cell division either. Conclusion: most of the books are wrong; centrioles are a structure in search of a function as far as we know now. This is a relatively recent conclusion, but there is some evidence suggesting that centrioles are important in flagellum development. Species lacking motile cells lack centrioles and those that do have motile cells have centrioles. However, until this situation becomes clearer with direct evidence, we must minimize discussion of any function for this organelle. However they do form, the spindle fibers from each end of the cell attach to the centromere of each chromosome. The spindle fibers push the chromosomes to the middle of the cells.
Anaphase--The spindle fibers are thought to pull the centromeres apart and drag the (onetime) sister chromatids to opposite poles. This assures that the two cells resulting from mitosis have exactly a complete set of chromosomes. The mechanism for this process is almost completely unknown. Here is another Nobel prize waiting for you. It seems to involve depolymerization of tubulin from the microtubule, but you figure how to shorten a fiber by depolymerization without detaching it from either end and the prize is likely yours!
Telophase--The chromosomes decondense as the nuclear envelope reforms around each cluster at the ends of the cell. The nucleolus may reappear soon.

Cytokinesis, literally cell division, occurs next. In plants this means that a new wall and cell membranes must be laid down between the two re-forming nuclei within the cell. This is accomplished by packing the polymers or at least subunits into vesicles at the Golgi (dictyosome). The vesicles coalesce in a plane between the two nuclei. How do they "know" where to go? Gosh more work for your generation of scientists! The coalescence of the vessicles and polymerization of the wall material between them is usually not complete. Instead, cytoplasmic strands connect between the two cells. These are called plasmodesmata, and are an important feature of plant cells. The importance of these connections through the walls should not be underestimated! Transport in phloem and xylem, and indeed a variety of compounds through and throughout the life of a plant are carried out in these channels.

Slides of each phase of the cell cycle, both electron micrographs and light micrographs are shown in this lecture. When I get some digitizing equipment, I will illustrate this page more thoroughly.

The MLA citation style for this page would be:
Koning, Ross E. "Centrioles What?". Plant Physiology Website. 1994. http://koning.ecsu.ctstateu.edu/plant_biology/centriolehtml (your visit date).

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