Week 6

From this week, I learned about cell cycle. Through SCL activity, we studied that Mitosis is the phase of the cell cycle where chromosomes in the nucleus are evenly divided between two cells. When the cell division process is complete, two daughter cells with identical genetic material are produced. Before a dividing cell. enters mitosis, it undergoes a period of growth called interphase. Then, it enters on the mitotic phase which is prophase, metaphase, anaphase and telophase. Then they reach the final process which is cytokinesis where diplood cells were formed.

Then for the second day of class, we touched about totipotency of plant cell. Many somatic plant cells, including some fully differentiated types (e.g. leaf mesophyll), provided they contain intact nuclear, plastid and mitochondrial genomes, have the capacity to regenerate into whole plants. This phenomenon is totipotency, an amazing developmental plasticity that sets plant cells apart from most of their animal counterparts, and was first demonstrated by Steward and Reinert in the 1950s. Often totipotency is revealed when cells or tissues are disturbed or removed from their normal environment and, for example, placed onto artificial media in tissue culture. A differentiated plant cell that is selectively expressing its genetic information can instead initiate expression of the program required for generation of an entire new plant. Many plants have been regenerated from single cells, but not all plant cells are totipotent; some are terminally differentiated, often because of partial or complete genome loss. We can generalise by saying that most plants at most stages of the life cycle have some populations of cells that are totipotent. Totipotency is of course also a property of normal undifferentiated cells, for example in meristems.

The first step in expression of regenerative totipotency is for mature cells to re-enter the cell cycle and resume cell division; a process known as dedifferentiation. This may lead directly to organised development, such as occurs in the epidermal cells of immature hypocotyls of Trifolium (Maheswaran and Williams 1985) where somatic embryos develop (direct embryogenesis), or formation of shoots or roots (direct organogenesis). Alternatively, there may be an intervening callus stage from which organised structures can later be induced to develop which referred to as indirect organogenesis.